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 **** 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.

 

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

 

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/glo...quearticle.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.

 

Thanks, I'll look into it.

Rich Krause

 

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.

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

 

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.