I found something at ARP's web site that warned to make sure the rod bolt is intalled at the exact same torque (or stretch) that the machine shop used when sizing the rods. Sounds like very good advice. LINK

I have the ARP 7/16" 12pt cap screw style rod bolt on one of my engines and it calls for .0052" stretch, (or I think 60ft-lb with ARP lube). I go to 60ft-lb and am getting only .003" stretch. So I end up around 80 to 85ft-lb on my torque wrench for .005" stretch. (Eagle H-Beam rod)

So that lead to this question in my mind....I'm pretty sure my shop just went to 60ft-lb and here I am going to 80ft-lb. So is there any harm in doing that? The answer looks to be "yes".

Here's how I figure:
I install the con-rod on the crank and go to 60ft-lb and see what the bolt strech is...it's only .003" OK not near enough stretch so I end up torquing to 80ft-lb and that gives .005" (technically in spec by ARP). But by torquing more that 60ft-lb I'm also smushing (compressing) the rod cap and distorting it somewhat. Yea I have proper bolt stretch but at the expense of a somewhat distorted rod cap.

Now maybe if my rods were worked at the machine shop with the bolt at 80ft-lb then I'd be fine. But I bet that the machine shop did not use 80ft-lb and I'll talk to them sometime this week about this. (I actually take my torque wrench to the machine shop and match it with their wrench as a cheap means of calibrating).

This post in advanced tech might bring out some engine builders for some comments on this. Tell me what you think. Am I wrong? Somthing I'm missing here?

Summary:
Torque your rod bolts to the exact spec that the machine shop used when sizing them....no matter what ARP or the rod manufacture say as to the torque or stretch spec...because when you torque the rod bolt upon engine assembly it stretches but it also compresses the rod and puts a distortion to it. If it's the exact same clamping force as the machine shop used, then the rod cap should distort pretty much the same and all will be good with the shape of the rod big end. So I guess I'm saying that it's better to pay attention to how the rod was sized than to getting exact, "proper" bolt stretch.

Karl Ellwein

 

This is just an illustration that the two methods give different results. Stretch is more accurate and more reproducible according to all the experts. That said, I still do it the old way.

Rich

 

Remember the old days...? When we didn't know enough to worry about this crap?

 

Yeah, they did seem to run fine before we knew so much! Seriously, if I weren't so old and set in my ways I'd use the stretch method.

Rich

 

HAHA lol.

Only thing I'm thinking about is weren't the bolts already installed and you too them out? So now you're putting them back in and you expect them to stretch the same amount?

I'm no expert but I think the .0052 stretch is for a new bolt, if ARP is saying to install the bolt at the same stretch or torque the matchine shop used, that means don't stretch anymore? I mean I realize it will once you retorque but it will not stretch the same amount again I wouldn't think.

Anyone want to chime in? This is interesting to me as well since I'll be rebuilding the engine soon as well.

 

That is normal using the streach method v torque. Ya will always have to use more torque to get it to the proper streach. Depending on the bolts,I have seen when a bolt is to the proper streach it takes 75-80ftlb to break them loose.
If yours were pulled to 60ftlb it will be allright to streach them to proper stretch or retorque and go a tad more. The proper way is to measure and record the original length and check the length at every tear down. If a bolt is .002 longer than original then it is no good. What a PITA. Can ya see me recording every build and putting in file and checking EVERY bolt in a build,oops the price just went up $500.00

 

Larry,
ARP recommends replacement at .001 permanent elongation.

Keep in mind that ARP also recommends that you take the bolt to 50-75% of final torque before putting it to spec. If you do this and use the ARP lubricant you'll find that your torque "value" is actually a little closer to the typical 65 lbs-ft we use to use.

Thinking back...... I wonder how much clamping load we were losing with the old torque method. Kinda scary IMO.

If your shop didn't stretch the bolt to spec then that's ok. You stretch it to the correct spec and take the rods back so they can hone them to size.

-Mindgame

 

Sorry about the .001 difference.Don't keep up to date on this stuff 'cause I ain't tearing the same engine down every week.They may come back at the end of the season and the bolts are replaced at that time or it's upgraded for more HP.The street engines may never come back.The usually outlast the owner.
It used to be I put sprint car engines together with a Sears flex (bend it until the pointer said the right torque) with no failures using nothing but oil and stock bolts.(scary after using the new wrenches and bolts for a few years) Tight was tight and that was it.Put a few stock rebuilds together with no torque wrench(torque value was two heavy grunts on large bolts and two soft grunts on small ones).

Haven't ever found the stretch and torque values within ten ftlb if ya do the torque and un torque thing like they recommend to break in the threads and lube it still isn't close,now that's measuring them when ya take um loose and it takes a bit more torque to loosen a nut or bolt.

 

I have also put motors together with one of those Sears wrenchs. Last one I remember was a Mopar 440 wedge. Worked great, one of the best runners I ever had!

Rich

 

Yep,makes ya wonder how some things ever survived.

 

hell yes! i remember putting my first bottle SBC together with a china wrench set,black RTV and the auto shop tq wrench, accurate to +- 72.8 ft lbs.ah the good old days.
wait........that was freakin 1995!


sorry ill shutup now........

 

I'm talking '55.Those were the good old days, then.
But if ya wasn't born then '95 will work.

 

I go by ARP's reccomendations with torque, never use rod bolt stretch. Never had a rod come apart at the cap, most customers don't want to pay for the extra time to stretch each rod bolt 3+ times in the process of honing, mockup, building.

 

LOL. good reading all the replys.

I guess the one thing I learned is that I'm going to make sure to tell the machine shop the exact torque when doing the rod checks/resize.

Karl

 

nothing is a substitute for experience in this game. although us young bucks think outside the box and come up with some innovative stuff once in a while.
but for the most part you think you have the fresh combo that will kill em all and somone shows ya dyno results from the same combo from '87 or '75