Question about "stretch" method for rod bolts?

Well, I know you will all make fun of me for being so old fashioned. But I have always used a torque wrench to install rod bolts in the past. With these new super-duper Oliver rods and fancy rod bolts I decided to try the "stretch" method. I can't see how you could use anything other than an end wrench to do this (while keeping the indicator on the bolt). I can't get enough leverage with an end wrench to stretch the bolts enough. Do you use a ratchet a little at a time? Or do all of you have much stronger arms and hands than I do? Do I need to look for a long, tall end wrench? Or what?

I think I will break out the old, trusty Snap-On torque wrench and just forget the stretch method :confused:

Rich Krause

Interesting, I've never heard of this method.

I have a feeling it has something to do with achieving proper torque
versus strechting the bolt to a predetermined length to ensure
proper loading.

My guess is that other factors preventing a torque wrench from
applying proper torque to the bolt are eliminated with the stretch
method (dirty bores, lack of lube, etc) ?

Don't mind me...I'm just thinking out loud. I have no idea what
you're talking about, but I'll look damn good if I'm correct. :D

Good question.

Here's ARP's take on it (http://www.arp-bolts.com/pages/tech/fastener.html):
The Stretch Gauge
We highly recommend using a stretch gauge when installing rod bolts and other fasteners where it is possible to measure the length of the fastener. It is the most accurate way to determine the correct pre-load in the rod bolt. Simply follow manufacturer’s instructions, or use the chart on page 13 of this catalog for ARP® fasteners. Measure the fastener prior to starting, and monitor overall length during installation. When the bolt has stretched the specified amount, the correct preload, or clamping load, has been applied. We recommend you maintain a chart of all rod bolts, and copy down the length of the fastener prior to and after installation. If there is a permanent increase of .001" in length, or if there is deformation, the bolt should be replaced. A sample chart is on the following pages.

Here's a link to the catalogue showing the stretch gauge (ARP Part# 100-9941): http://www.arp-bolts.com/media/pdf_files/TO_65-66.pdf

Couldn't find the direct link, but here's a quote from ARP's online catalogue (http://www.arp-bolts.com/media/pdf_files/arp_cat03.pdf) (looks like a micrometer would be fine... and you can alternate tightening / measuring until you get the proper stretch):

Whether measured by stretch or by torque, properly preloading a rod bolt is essential for trouble-free performance. If a bolt is installed without sufficient preload (or pre-stretch), every revolution of the crankshaft will cause a separation between the connecting rod and rod cap. This imposes additional stretch in the bolt. The stretch disappears when the load is removed on each revolution, or cycle. Over time, this cycle stretching and relaxing can cause the bolt to fail due to fatigue, just like a paper clip that is bent back and forth by hand. To prevent this condition, the bolt’s pre-load must be greater than the load caused by engine operation. A properly installed bolt remains stretched by its preload and isn’t exercised by the cyclic loads imposed on the connecting rod. A quality bolt will stay stretched this way for years without failing. The important thing is to prevent the bolt from failing due to fatigue by tightening it to a load greater than the demand of the engine. Protect your bolts—tighten them as recommended.

You can easily monitor the condition of the rod bolts through use of a stretch gauge, or a micrometer for that matter. Prior to installing the rod, measure the length of the bolt in a “relaxed” (untorqued) state. Write this down. You can make up a chart similar to the one shown on this page to properly keep track of the data. When you tear the engine down for maintenance, again measure the length of each rod bolt -- being careful to keep everything in the proper order. If any of the rodbolts have taken a permanent set and have stretched by .001" or longer you should replace the fastener IMMEDIATELY! The stretching is a sure indicator that the bolt has been compromised and taken past its yield point. In other types of bolted joints, this careful attention to tightening is not as important. For example, flywheel bolts need only be tightened enough to prevent them from working loose. Flywheel loads are carried either by shear pins or by side loads in the bolts; they don’t cause cyclic tension loads in the bolts. Connecting rod bolts, on the other hand, support the primary tension loads caused by engine operation and must be protected from cyclic stretching. That’s why proper tightening of connecting rod bolts is so important. See the adjacent charts for recommended stretch and torque.

THE IMPORTANCE OF PROPER ROD BOLT STRETCH/TORQUE...
Friction is an extremely challenging problem because it is so variable and difficult to control. The best way to avoid the pitfalls of friction is by using the stretch method. This way preload is controlled and independent of friction. Each time the bolt is torqued and loosened, the friction factor gets smaller. Eventually the friction levels out and becomes constant for all following repetitions. Therefore, when installing a new bolt where the stretch method can not be used, the bolt should be tightened and loosened several times before final torque. The number of cycles depends on the lubricant. For ARP® recommended lubes, five loosening and tightening cycles is sufficient.


If you give up, here's their conversion chart for torque specs (depending on what lubricant you utilize): http://www.arp-bolts.com/pages/tech/images/fasttorq.pdf

:D

This is not exaclty correct but what I do is set my torque wrench about 10 ft-lbs shy of what ARP says is the proper torque spec using their lube and stuff. Then measure the bolt loose with the stretch gage and write it down. Then torque the bolt to the preset torque reading. Then measure the bolt again. if no change increase by 5 ft lbs and remeasure until you get the .001 stretch or whatever your bolts are to be streached to, also make sure you looking at the right bolts on the chart ARP sells lots of different alloy rod bolts and the ones Oliver and the like use aren't always the default ones listed for a SBC.

Once I have the torque setting that yeilds the desired strecth I'll measure all the bolts and torque them all down. check afterwards to make sure they are +\-.001 of the desired reading. I think most are to be streched like .002 to .003.

you don't have tighten these down with the stretch gage in place but if you really want to you could get one of those wrenches with the hole in the middle and hope it has a really long handle and can take the torque setting.

I know the stretch guage method is the best but I've never done it myself. I'm still old tech and torque them 3 times.

Torque to yield is only really good if you can measure the untorqued bolts to see if they're stretched to begin with.

Head bolts on a Cummins N14 are first checked against a length guage to see if any are stretched (never found one yet). They're then torqued down to 220 pounds in a progressive sequence. They're then tightened an additional 1/4 turn. I have no idea what that final torque is but I would guess it's in the 400 pound range. You have to figure that the head bolt must stretch a lot in that final stage. The specs don't give an actual final torque so they'll vary from bolt to bolt unless you do a perfect 90* every time.

With the rod bolts if torque varies by a few pounds, it shouldn't make very much of a difference. Remember before clicker torque wrenches were around people used a spring bar torque wrench and those things couldn't do 2 bolts the same for accuracy.

As for using a box end wrench on the rod bolt, if you need more leverage you could always double wrench it.

I always liked the 3 torque method. First torque sets the bolts and does an initial crush of the bearing. Second torque cleans the threads. Third torque is the final set.

Excellent question rich.

Excellent response Steve in Seattle.

At one time I saw, but never had, a funny looking adapter that went over the rod nut like a box wrench, but bent around over the stretch dial indicator and had another 1/2 square end (aligned with the rod centerline) where you could insert torque wrench.

Just torque, measure, increase torque a few lb-ft and remeasure stretch. Assuming you lube correctly, per APR's recommendation, you'll be able to come very close to your final stretch value by staying a few lb-ft below what you found on the first bolt. This will minimize the number of stretch measurements you need to take. A 3/8-24 stretches about .0001 per degree you move the nut when you get into the stretch zone, so getting the last .004 or so stretch is only about 35-40 degrees of rotation.

Many good rod bolts have centers, or machined conical depressions in the center of both ends to locate the stretch gage or pointed-end micrometers. Otherwise you need a ground flat on each end of the bolts to accurately measure.

Depending on the diameter, material and bolt length, stretch varies from about .005 to .007 on rod bolts we might use but a specific bolt stretch should be held a close as you can get it.

IMO, you need a dial indicator with a minimum of .0005 readings because stretch tolerances are .0004 to .0005. A .0001 dial indicator might be better. It only needs about .010 travel if you set it with an unstretched bolt. Some bolt stretch gages come with a .001 indicator. See Arrow Precision's discussion here, some of which is from ARP:

http://www.arrowprecision.co.uk/assets/arod_care.pdf

Page 6 has a nice chart.

If you are using titanium rods (is that next, rich?), note the lube procedures.

Everyone first torques/stretches their assembled rod/bearings with the rods out of the engine, right? Also note the recommendation to torque/stretch new bolts 5 times to minimize the friction factor if bolts cannot be measured for stretch in the assembled rod. Not a bad idea for all new rod bolts, IMO.

Remember you are not "torquing to yield" rod bolts. You are staying below the yield point because the rod bolts get cycling loads, as was pointed out. If you measure and record the free length of new bolts, and remeasure after the engine is torn down for rebuild, and you find the bolt has permanently stretched as much as .001, it's time to replace the bolts. If you don't have that data, and you are rebuilding a highly stressed engine, replacing the rod bolts is a good idea.


My $.02

Thanks for the replies so far. Very informative! I think I am going to use the method that's worked for me in the past. Prestretch 5 times, then install using the specified lube and a good torque wrench. I may measure the bolts in order to be able to check on stretch at the next teardown. But this would be sort of academic as I replace them each time anyway. OTOH, maybe I don't need to if I check the stretch?

The bolts are ARP "WSP" 7/16" as supplied by Oliver. Funny that ARP doesn't seem to list "WSP" on their web site. Anyone know what material these actually are? When I talked to Oliver they indicated that the bolts would be fine for my application, so I took their word for it. I could have gotten the "L19" bolts but they (Oliver) implied these weren't ideal for a street motor due to possible corrosion and that they just weren't needed.

Rich Krause

There are a few things that i have done...

Although I haven't installed rods yet.. .but on the heads.. I take a pointer and a small hammer and tap all the screws after my last torque run... Then retorque again to the last specs.. Ive gotten out up to 180 degrees more turn at the same lbs of torque...

Now.. with rod bolts.. I do know this...

Some rod tightening sequences are TTY (torque to Yield) and can ONLY be torqued once since the bolt "streches" and its a use once type bolt...

The instructions are like this: Torque to xxx lbs then xx degrees more or whatever... when I get home I will check on my alldata... Most rod bolts are TTY... GM started using this from lt1s 96 and up engines...

Hope this helps.

Originally posted by The Highlander
There are a few things that i have done...

Although I haven't installed rods yet.. .but on the heads.. I take a pointer and a small hammer and tap all the screws after my last torque run... Then retorque again to the last specs.. Ive gotten out up to 180 degrees more turn at the same lbs of torque...

Now.. with rod bolts.. I do know this...

Some rod tightening sequences are TTY (torque to Yield) and can ONLY be torqued once since the bolt "streches" and its a use once type bolt...

The instructions are like this: Torque to xxx lbs then xx degrees more or whatever... when I get home I will check on my alldata... Most rod bolts are TTY... GM started using this from lt1s 96 and up engines...

Hope this helps.


Yes, GM uses angle torque readings for installing engine bolts, but that doesn't necessarily make them TTY bolts! I believe that if they are TTY, the instructions say to replace them after one use with NEW bolts, to quote the manual. LS1 shop manual does NOT say rod and main cap bolts are TTY and the only ones they suggest replacing are the main cap cross boltswith a bluilt in sealer in order "To prevent engine block oil leakage".

Rod bolts (LS1) are installed at 15 lb-ft then either 60 degrees more (1st design/single dimple) or 75 degrees more (2nd design/2 dimple). That's equivalent to the total stretch desired. More on the 2nd design because the bodies are slightly smaller diameter so they need to stretch more to get the same clamping load.

Both rod bolts and main cap boots get cycling loads, and IMO are NOT meant to be TTY. Head bolts, on the other hand need to clamp only. Combustion loads are not that high on the bolts, and would probably bend the head casting minutely before they loaded the bolts much more than they are during installation.

FWIW, if you got 180 more turn with a head bolt that had the proper clamping load due to the correct lube or sealant and torquing, you have seriously yielded those bolts. Just my opinion, but that's not something I'd recommend.

Its a trick my father taught me... he used to build 240z's engines back in the day...

180 was ONE bolt....

i tapped them lightly and then retorqued to 70 ft-lbs as the 3rd torque session indicated and I got 30-45 degrees on most and 180 on that one... it has worked so far... I will be using new bolts on my new head install

Originally posted by The Highlander
Its a trick my father taught me... he used to build 240z's engines back in the day...

180 was ONE bolt....

i tapped them lightly and then retorqued to 70 ft-lbs as the 3rd torque session indicated and I got 30-45 degrees on most and 180 on that one... it has worked so far... I will be using new bolts on my new head install

You might discuss that method with a tech @ ARP. They might have some advice.

Good luck.

I think I still have a '68 Datsun shop manual (L-16 here) somewhere in my collection. :)

I tend to follow his advice ;) it has worked whenever I had followed it... unless i TOTALLY know.. that is old school ;) in which case it still applies to many things...

But I will... THe purpose of tapping it is to beat the "friction" forces and thus have the correct clamping force...

Originally posted by The Highlander
I tend to follow his advice ;) it has worked whenever I had followed it... unless i TOTALLY know.. that is old school ;) in which case it still applies to many things...

But I will... THe purpose of tapping it is to beat the "friction" forces and thus have the correct clamping force...

Not to rag on the old Man, because I'm probably older than he, but the reason torque is used to apply load to a bolt is all about friction. If you properly (or don't) lube based on the manufacturer's instructions, a certain torque should result in a given amount of stretch and therefore the desired preload with the friction that exists in the nut/bolt interface with the specified lube or dry, if so specified. Using slipperier lube, or "relieving" the friction once torque is reached merely increases the possible bolt load/stretch you can achieve when you retorque to the spec, and can easily yield the bolt. Once you get much past the yield point, the load starts to drop off with more stretch. It looks a lot like a hp curve if you plot it, and you are operating well past hp peak.

FWIW: A lot of things in nature have that characteristic curve. Airplane wings with lift plotted against angle of attack, as well as tire side force when ploted against slip angle are a couple of examples. You certainly don't want to "stall" your bolt.

Using moly lube vs. motor oil in bolt threads changes the torque necessary to achieve the desired load. Same with lubed vs. dry. Some bolted applications are designed to be assembled dry, like wheel nuts, and some with lube or sealer like some SBC head bolts. It does make a big difference.

My $.02

Originally posted by rskrause
The bolts are ARP "WSP" 7/16" as supplied by Oliver. Funny that ARP doesn't seem to list "WSP" on their web site. Anyone know what material these actually are?

ARP makes Eagles stuff too, and all bolts that are made for a certain manufacture ARP will not sell to you, you hav eto go to the company who makes the rods and get the botls thru them. In an industry as small as the high performance fasteners bussiness there are only a few companies in the world who do this.

Originally posted by rskrause
When I talked to Oliver they indicated that the bolts would be fine for my application, so I took their word for it. I could have gotten the "L19" bolts but they (Oliver) implied these weren't ideal for a street motor due to possible corrosion and that they just weren't needed.

L19 bolts are a pain in the ass, but in some instances they are needed. If I remember right MindGame said he was using them in his motor. The corrosion problem is called hydrogen embrittlement. It's from the material used in those bolts, and it can happen if you touch the bolts by hand and and get any sweat or oils or even any water on them. The bolt will only corrode after it is TQ'ed down and has a stretch on it, then it slowly fatugies and breaks. They can be used, and in some instances they should be used, but you have to handle them carefully. Once they are in a motor full of oil then you will be ok. A good place to use them is in a Eagle H beam when you plan on putting 750hp thru them. Good rod bolts are always a plus! but the L19's are not a noivce item.

I have to say that Oliver Rods are probably the best designed connecting rods out there.

Highlander,

About this tapping the bolt thing, I gotta say I would listen to my old man rather than yours. Once you break a bolt from doing this the $$$ lesson will be enough for you not to do it again.


Bret

Here's a good cross-reference thread that was just reactivated in LT1- Tech on just this subject.

http://web.camaross.com/forums/showthread.php?s=&threadid=221187

My post from taner's old thread...

Some random thoughts on bolts.....
Use a torque wrench, but with rod bolts you're looking for stretch, not preload. The manufacturer lists specific stretch requirements for their bolts. Buy a stretch gage and do it right.
On other bolts, use torque to angle (TTA).... a clicker torque wrench can get you in trouble. According to the guys at ARP, friction impedes consistent clamp load values. So you have to overcome so much friction to achieve load..... and friction is hard to predict, even when using specified lubricants. Thus TTA is the way to go because you aren't relying on torque value, but the helix and pitch of the thread to acheive a specific amount of load.
Which brings up another matter..... use the recommended lubricant. Alot of old guys who think they know better, chunk the stuff in the trash and get out the 30W oil...... as my gandpa used to say, "That's plumb stupid!". Look at the R&D that went into the design of those lubricants.
Use torque cycling. Bolts relax once torqued and that amount of relaxation varies. The guys at ARP suggest cycling a bolt to 50% of it's final torque 5-6 times before taking it to final torque... which has a burnishing effect on the threads. That way you're overcoming less friction and increasing the clamp load of the bolt because it will stretch more after cycling.

Just some random thoughts, but ARP put out a good amount of tech here some years back.... at least that's where I remember reading up on this. I do all of the above when I build an engine and I can't say that I know too many other guys who do.... but mine have a habit of staying together for a while...... of course I'm anal too.
Honestly though.... most guys I know, have never even heard of torque cycling."

Not posted to blow myself up... just thought the info would help some people wandering in on this a little. And not to beat a dead horse, but TTA is a big deal for the reasons mentioned. May not mean much in the typical street engine build but I know yours is anything but "typical" Rich.

Oh yeah, buy a rod bolt stretch gage (Tavia etc). Either that or a set of "point" Mics. The gage will be cheaper though.

Good luck.

-Mindgame

Combine dirt farmer with high tech - snug the bolts down then set up your strech gage over a boxend wrench (a GOOD box end wrench). Start tightening until you need some extra force, and you will. I couldn't believe how much I had to pull on that wrench to get 0.0055" stretch. Then slip a nice piece of thick wall tubing (or pipe for this farmer) over the wrench and pull until done. Worked for me.

Originally posted by TimChiaretto
Combine dirt farmer with high tech - snug the bolts down then set up your strech gage over a boxend wrench (a GOOD box end wrench). Start tightening until you need some extra force, and you will. I couldn't believe how much I had to pull on that wrench to get 0.0055" stretch. Then slip a nice piece of thick wall tubing (or pipe for this farmer) over the wrench and pull until done. Worked for me.

Thanks Tim, and Mindgame, and the others who have responded. I have access to a stretch gauge, so I may try it.

I learned from this thread, so thanks again.

Rich

Stretch gauge is great and as Mindgame mentioned a set of mics can also be used. Mics do have the advantage of checking the stretch to .0001 which is great since you need to check them to .0005.

TimChiaretto, that's the way to do it. Gotta love the old pipe method, if it don't turn get a bigger arm on it!

Bret

Guys, most of the OE rod bolts that I'm aware of are torque to yield now. This is done to minimize the preload variation on the engine assembly line. I understand the arguments for staying away from torque to yield for high performance applications, but keep in mind OEs have to balance other things when they design a bolted joint.

For those of you who want a little bit more tech on how bolted joints work, try this one:

http://www.boltscience.com/pages/basics1.htm

Originally posted by 94bird
Guys, most of the OE rod bolts that I'm aware of are torque to yield now. This is done to minimize the preload variation on the engine assembly line. I understand the arguments for staying away from torque to yield for high performance applications, but keep in mind OEs have to balance other things when they design a bolted joint.

For those of you who want a little bit more tech on how bolted joints work, try this one:

http://www.boltscience.com/pages/basics1.htm


How about some examples.

LS1 doesn't use TTY rod bolts according to shop manual.

Are you sure you are not referring to torque-angle tightening rather than torque-to-yield? Very big difference.

Rod bolts get many more cycles in an OEM engine than in a high performance engine, but at lesser absolute loads. Stronger bolts take care of the higher loads, but If the OEM bolts are yielded during assembly, that decreases their endurance limit and should cause premature failure.

Interesting link. I must have missed the part about cycling TTY bolts many hundreds of million times.

This page is very interesting concerning OEM tightening of fasteners.

http://www.boltscience.com/pages/glorimertorquearticle.htm

My $.02

I have never heards of TTY rod bolts. That doesn't mean that it isn't being done, but as OldSStroker posted, it wouldn't appear to make much sense. In effect, by saying "when the bolts stretch more than x.xxx replace" is a way of sayng "once they yield, they are no good any more" and must be replaced.

Rich Krause

My LT1 came with TTY head bolts and powder metal rods. I'm sure they were ok for OEM but not good enough for me, so they went in the trash can.

Sometimes, you just have to upgrade.;)

-Mindgame

Snap On makes extra long 12 point box end wrenches with oval shaped handles and a slight offset that makes performing the stretch method much easier.

Originally posted by tjwong
Snap On makes extra long 12 point box end wrenches with oval shaped handles and a slight offset that makes performing the stretch method much easier.

Thanks, I'll look into it.

Rich Krause

Originally posted by OldSStroker
[B]How about some examples.

It's just the way things are done on most engines now. The 5.7L Hemi is one. Most other engines are done this way on the assembly line. For instance, on the 5.7L engine line in Mexico an automatic nut runner first torques the bolt to the snug torque of 20N*m, then it begins the angle turn. As the nut runner turns the bolt through the angle a computer monitors the torque being applied per degree. The change in slope of this line tells the computer when the bolt is approaching yield. If the yield point is reached within a specified torque and angle window the nut runner stops and passes the joint. If either the torque or angle is outside of the acceptance window the joint fails and the operator is instructed to change out the bolts and try again.


Are you sure you are not referring to torque-angle tightening rather than torque-to-yield? Very big difference.

Yes, I'm sure. I know very well the difference.

Rod bolts get many more cycles in an OEM engine than in a high performance engine, but at lesser absolute loads. Stronger bolts take care of the higher loads, but If the OEM bolts are yielded during assembly, that decreases their endurance limit and should cause premature failure.


No, not really. Reread the bolt science article where it mentions having adequate preload to avoid separation of the joint. If you design the rod with adequate stiffness and keep the joint from separating the bolt takes a VERY small percentage of the additional tensile load.

Originally posted by 94bird


SNIP

No, not really. Reread the bolt science article where it mentions having adequate preload to avoid separation of the joint. If you design the rod with adequate stiffness and keep the joint from separating the bolt takes a VERY small percentage of the additional tensile load.

I am not an engineer, but it would seem to me that the rod bolts would have to take the entire tensile load. They are holding the big end together, and nothing else is, right?

Rich Krause

Ah, read the bolt science article. It's all in there. A simple summary is no, the bolt does not take all the tensile load. It takes almost none of the load until all the preload is overcome and the joint starts to separate.

Originally posted by 94bird
Ah, read the bolt science article. It's all in there. A simple summary is no, the bolt does not take all the tensile load. It takes almost none of the load until all the preload is overcome and the joint starts to separate.

This makes no sense to me. If there were no bolt in there the rod would not stay in one piece. So obviously the bolt is taking ALL the tensile load, it might not be stressing it until it surpasses its stretch point, but its taking all the load of the cap wanting to separate.

At least thats what it seems to me

The load the bolt sees is the preload and very little more until the joint starts to separate.

I assumed you meant that it didnt see "much" stress until its past its preload phase. But obvioulsy if the caps are separating, then the bolt is stretching more, so its being stressed higher, so the statement almost makes no sense at all to me.

I think a better statement in my mind would be that the bolts see hardly any stress/fatigue under optimal/normal separating loads from the cap, but if the loads increase and surpass the bolts rated tensile strength/stretch point, it sees exponential stress increases

??????Am I thinking funky, or am I just restating what he said maybe a different way?

Originally posted by 94bird
It's just the way things are done on most engines now. The 5.7L Hemi is one. Most other engines are done this way on the assembly line. For instance, on the 5.7L engine line in Mexico an automatic nut runner first torques the bolt to the snug torque of 20N*m, then it begins the angle turn. As the nut runner turns the bolt through the angle a computer monitors the torque being applied per degree. The change in slope of this line tells the computer when the bolt is approaching yield. If the yield point is reached within a specified torque and angle window the nut runner stops and passes the joint. If either the torque or angle is outside of the acceptance window the joint fails and the operator is instructed to change out the bolts and try again.

Perhaps it is the terminology that confuses me. "Approaching yield" can mean a lot of things. Do you mean that the bolts are loaded beyond their proportional limit where stress-strain is linear, to maybe their elastic limit, where they will still return to their original length when the load is removed, or to their 0.2% offset yield strength point, where they will not return to original length? In other words, that's the accepted definition of "Yield Point". This link explains it better than I:

http://www.brushwellman.com/alloy/tech_lit/aug1999.pdf

If you can get the bolt load precisely to a point just before it yields, and the worst load case it ever sees (when I accidently do a 5-2 downshift instead of a 5-4 and 7500+ rpm results :) ) doesn't exceed the yield point of the bolt, I agree that the joint is almost perfectly designed and executed.

I think that's where you might be going, and why the operator is instructed to change out the bolt and try again because it actually yielded. That part of it bothers me a little. Who's to say the bolt really got changed?

Originally posted by 94bird
Reread the bolt science article where it mentions having adequate preload to avoid separation of the joint. If you design the rod with adequate stiffness and keep the joint from separating the bolt takes a VERY small percentage of the additional tensile load.

Agreed. It's that additional load that bothers me. If you are already at the yield point of the bolt (not just approaching it) when it is installed, will not ANY additional load increase the yield making the bolt longer, thereby reducing the clamping load, and allowing more yield at an even less load? This doesn't sound right to me.

Please point out the page(s) in the Bolt Science link that addresses actually yielding (not approaching yield) a bolt in a joint that takes the cyclic loads experienced by a rod. I couldn't find it.

Bottom line: are you saying that rod bolts are tightened until they actually yield (permanently deform in length) during assembly and the critieria for rejection of the joint is not IF they yield but HOW MUCH they yield?



My $.02

I don't understand why anyone is talking about TTY and torque-angle type bolts. Stretch is for high quality bolts, which is really what we are concerned about, right?
Bolts are like springs: you want to preload the spring a certain amount so it holds the caps on the rods, but you don't want to exceed Young's Modulus for the bolt material so that the spring won't return to its original equilibrium position (then the bolt has yielded and is no good). You also don't want the forces on the bolt due to the engine running to stretch the bolt past the yield point.

As far as what loads the bolt sees, the preload is the only load... until the engine starts running. Then when the crank pin pulls down on the rod cap, the bolts pull down on the rod, which pulls on the piston pin which pulls on the piston which goes down the bore. When all that happens, the bolts see the inertial and friction forces due to the piston and rod, in addition to the preload force. So you want

inertial force + friction force + preload force < yield force

and inertial force + friction force < preload force

so that the bolts won't yield and the caps never separate from the rods.


Right? :)

Originally posted by TheNovaMan
I don't understand why anyone is talking about TTY and torque-angle type bolts. Stretch is for high quality bolts, which is really what we are concerned about, right?
Bolts are like springs: you want to preload the spring a certain amount so it holds the caps on the rods, but you don't want to exceed Young's Modulus for the bolt material so that the spring won't return to its original equilibrium position (then the bolt has yielded and is no good). You also don't want the forces on the bolt due to the engine running to stretch the bolt past the yield point.

As far as what loads the bolt sees, the preload is the only load... until the engine starts running. Then when the crank pin pulls down on the rod cap, the bolts pull down on the rod, which pulls on the piston pin which pulls on the piston which goes down the bore. When all that happens, the bolts see the inertial and friction forces due to the piston and rod, in addition to the preload force. So you want

inertial force + friction force + preload force < yield force

and inertial force + friction force < preload force

so that the bolts won't yield and the caps never separate from the rods.


Right? :)

Sounds right to me.

Rich Krause

I should have known better than to try to give a short explanation in this forum, but I usually find I don't have the time to go through everything with all the questions that pop up.

Yes, many rod bolts are torqued to yield by OEs. The 5.7L Hemi is one, and so is another engine I'm working on now. The spec on the Hemi rod machining print even specifies .025-.127mm of permanent elongation.

I think if you talked to some of the bolt manufacturers out there they could give you many more examples.

Originally posted by 94bird
Yes, many rod bolts are torqued to yield by OEs. The 5.7L Hemi is one, and so is another engine I'm working on now. The spec on the Hemi rod machining print even specifies .025-.127mm of permanent elongation.

I should have known better than to try to give a short explanation in this forum, but I usually find I don't have the time to go through everything with all the questions that pop up.



Thanks for the info. So does that mean that (new) Hemi rod bolts are one-use? FWIW: .001-.005 in (approx) seems like a large tolerance for desired yield. Non-yield stretch tolerances are about 1/10th of that for the really finincky aftermarket suppliers.

Just curious: So why is it you post if you don't have time for the follow ups? Some of us might understand your detailed explanations and learn something which is part of what this forum is all about.

Although IMO this has been a purely academic debate... it has been somewhat interesting.

Does remind me of my days at Algor working with engineers. Never a dull moment.:)

-Mindgame

Originally posted by OldSStroker
Thanks for the info. So does that mean that (new) Hemi rod bolts are one-use? FWIW: .001-.005 in (approx) seems like a large tolerance for desired yield. Non-yield stretch tolerances are about 1/10th of that for the really finincky aftermarket suppliers.

Just curious: So why is it you post if you don't have time for the follow ups? Some of us might understand your detailed explanations and learn something which is part of what this forum is all about.

True, it is a very large tolerance. In reality we are typically in the .001-.003" range even with production style cycle times and equipment. Permanent elongation checks are just one of the means of auditing that the nut runner is still operating within spec. The real check on each bolt is the yield point control algorithm on the nut runner.

On the machine line the bolts are just tightened using torque + angle to a point approaching yield. On the engine assembly line they are pushed to yield. You are correct though. The bolts should not be reused if you're doing any work on the engine afterwards.

I know I shouldn't post if I'm not prepared to back everything up, especially in this forum. :cool: Sometimes I just see a topic that interests me and figure I'll drop in and post some info that seems to be lacking in the thread. I came from the aftermarket side of the business but moved to Detroit for just that kind of thing. The aftermarket has their mindset but there is so much to learn in this town working for an OE or supplier. The technology and resources are just in a different league.

By no means can I explain everything I come across, and this is one area where I'm fairly new. My real background is pistons and rings, but many times designing rods (and bolts) comes along with the rest of the power cylinder components.

Let me explain one thing though. The bolt will see a very small percentage of the additional load put on the joint from engine operation. If I wasn't clear on that before, I'm sorry. However, the percentage of the load the bolt sees until joint separation is VERY small. This allows a bolt to be installed into the yield area and still be a very robust joint. We are getting our rod bolts from a local source and are now switching to a torsional yield spec for rating the bolts instead of the tensile strength and hardness. This allows us to make sure the bolts are more consistent in delivering a certain preload. That rating strategy is one used by GM, BTW. The real purpose behind all of this is to get the most consistent preload possible in the cycle time allowed in mass production. We can't take minutes measuring the stretch of each bolt. Since you can, as a private engine builder, measuring stretch will deliver similar results to yield point control using our tools. Here's an example link:

http://www.a1technologies.com/pdf/technicalsectionlink2.pdf

The hydrogen embrittlement issue is one difference the OEs take vs. the aftermarket for critical fasteners. ARP and A1 won't hesitate to harden a bolt above 45 HRc core. At this level the chance of hydrogen embrittlement is very high and the bolt is quite fragile. A bolt this fragile doesn't lend itself well to TTY because the point of no return is so narrow. However, it does allow you to have a very high tensile strength and thus the ability to greatly increase joint preload using the same packaging space as the OE bolts. Chrysler has banned bolts of that hardness (12.9 and above class) across the board, well almost. SRT doesn't always follow DCX's rules. ;)

Good info. I especially like the link. Thanks for posting.

'Tis true that OE engines face a very different "load life" from engines modified from OE to produce multiples of the original hp and rpm high enough to produce multiples of tensile loads in the rods, but with generally the same size parts. Piston g's double in a 350 SBC from 5500 to about 7700.

Do you look at bending in the rod bolts when the big end of the rod "ovalizes" ?

So let me see if i'm understanding this.

You torque to the yield strength and you preload is established. Then you run the motor, and the bolt sees additional loading, albeit small because you have designed a 'hard joint'. The bolt plastically difforms (yields) more from the additional loading, but is still well below the ultimate tensile strength. This yielding reduces the clamping force, but not nearly to the point of seperation where bending and shear forces arise and would cause failure. The additional yielding has effectively strain hardened the bolt and also slightly reduced the preload on the bolt compared to the original TTY value before you run the motor. For the life of the fastener it operates elastically on the new strain hardened stress strain curve. Because you have the same max rpm for the life of the motor the loading will never exceed the yield point established from running it to max rpm the first time. Therefore it behaves elastically on the *new strain hardened* stress strain curve and no additional yielding can occur.

I don't think that your bolt science page adaquately demonstrated this, but thats the way I have resolved this in my mind based on my classes. A requirement for this type of design is that the UTS must be significantly higher than the elastic limit and .2% yield strength (which will be very close) where the initial TTY loading was stopped. If the UTS was too close to the .2% yield strength there would not be sufficient strength to allow the strain hardening to occur from the additional yielding when you run the motor to max rpm the first time.

Maybe that will make sense to someone else, or maybe they will point out where I am wrong. I think the key here is that noone has mentioned the strain hardening and the 'new elastic region' that it creates. :)

-brent

This is a good thread! Thanks for the info all, especially "94Bird" who offers a new perspective.

Rich Krause

Brent, I believe you hit the nail on the head. In searching for some more information to help last night I found another web site. Ajax Fasteners is from Australia but they have a very in depth paper on fasteners on their web site and it presents information in equation format to show what happens with bolted joints. Notice in particular the reference to how bolts react when put into yield on p. 10.

One of the things I noticed in this paper was their strong recommendation to not TTY a fastener that will see additional tensile loading during its lifetime. I thought that was kinda humorous given the situation.

http://www.ajaxfast.com.au/PDF/AISCPaperV05.pdf

Yes, we certainly do look at bending load in the fasteners too. As a matter of fact, more modern FEA techniques for connecting rods cycle the rod through a complete combustion cycle every 5 deg. or so to take into account whipping loads. Whipping loads are when the rod is not just in push-pull loading in line with the axis of the rod, but when you are for instance at 15 deg. ATDC, when the beam of the rod is put under loading that would induce bending. When we include those offset loads we can drop our required factor of safety down to 1.2 in certain cases instead of the previouly required 1.5 or better. That means we can get the mass of the rod down quite a bit, and place it just where we need it.

Originally posted by 94bird


Yes, we certainly do look at bending load in the fasteners too. As a matter of fact, more modern FEA techniques for connecting rods cycle the rod through a complete combustion cycle every 5 deg. or so to take into account whipping loads. Whipping loads are when the rod is not just in push-pull loading in line with the axis of the rod, but when you are for instance at 15 deg. ATDC, when the beam of the rod is put under loading that would induce bending. When we include those offset loads we can drop our required factor of safety down to 1.2 in certain cases instead of the previouly required 1.5 or better. That means we can get the mass of the rod down quite a bit, and place it just where we need it.

Is current Hemi designed for any manual trans applications? Auto only prevents overreving, but on a manual, downshifting can lead to very high overspeed which PCM obviously can't control.

Transaxle C5 Vette required a driveshaft "bumper ring" in M6 cars for this reason to keep the whipping d/s from hitting the torque tube at some rpm like 8000+ when the 4-1 "dumb$hit downshift" mode is engaged by the driver. This was an add-on after some "executive" performed the maneuver during a test drive, or so the story goes. Automatics don't have it.

What's the absolute design rpm limit you would use to figure rod loads?

Again, just curious.

There is currently a manual trans application for the 5.7L in trucks. I've never seen one on a lot, but I know we have a flywheel for such an application. :) Anyway, IIRC, the 5.7L has a peak HP speed of 5200 rpm and the overspeed is 5800 rpm. Generally you have a designed shift speed 200-300 rpm above peak HP and an overspeed about 300 rpm over the designed shift speed. Keep in mind the Hemi has an electronically controlled throttle body so things can be done to protect the engine if the driver does something a little stupid. ;)

Originally posted by 94bird
There is currently a manual trans application for the 5.7L in trucks. Keep in mind the Hemi has an electronically controlled throttle body so things can be done to protect the engine if the driver does something a little stupid. ;)

Not really on a manual trans. Over rev of the engine when it's powering the vehicle or free-revving is easy, of course, but mechanically overreving when the vehicle drives the engine due to driver error downshifting a manual was my point. That's not a PCM or throttle body controllable thing. Even Nextel Cup guys occasionally scatter one due to this on a road course.

With a heavy enough hand, you can probably get almost any syncronized manual into a gear, that when you engage the clutch drives the engine to revs WAY above your design. BTDT, but paused during clutch engagement when I noticed the driver's error. :)

My $.02

That's true. Overrevving on a downshift can't really be stopped until we go with DCTs. I drove an Audi with one of those a few weeks ago and it was sweet. The computer would only let you downshift when it thought it was appropriate but the programming was done very well for performance applications. Driving a DCT car will make you think you're Schumacher.

Originally posted by 94bird
That's true. Overrevving on a downshift can't really be stopped until we go with DCTs. I drove an Audi with one of those a few weeks ago and it was sweet. The computer would only let you downshift when it thought it was appropriate but the programming was done very well for performance applications. Driving a DCT car will make you think you're Schumacher.

Maybe it will make you think you are Alanso. :) The "Ren-no" has been getting it DONE at the starts this year. A year or two ago there was talk of twin or double clutches in F1 which may be DCT.

GM's has a demo truck with a 6-speed DCT.

GM DCT (http://media.gm.com/events/productseminar/powertrain/DCTnew.htm)

The THM 400 was/is a clutch-on-clutch 3-speed single shaft automatic, I believe. My guess is that GM will go with 6-speed auto in trucks with clutch-on-clutch, which might be DCT. They showed this in 2002, so 2006 models would be my optimistic guess for production. Note that the diagram in the link is a transaxle. Hmmm, maybe C6 ZX model? It would be a good moderately low production trial for a new powertrain bit. GM has been known to do that in the past.

The sad truth is if GM doesn't move these concepts into production they'll be also rans in the competition. VW & Audi already have DCTs in production in cars that aren't very high priced for their market. Chrysler has now launched MDS on the 5.7L Hemi and is getting very favorable reviews. I just don't understand why GM takes so long sometimes to get things to market.

Wouldn't a Solstice with a DCT be a great novelty? It would finally give GM something to go after and beat the Miata with.