rod bolt torque compared to stretch

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steve cowan
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rod bolt torque compared to stretch

Post by steve cowan »

hi guys i bought a set of scat 6'' i beam rods this week they have the bushed small end and these have the 7/16 8740 arp rod bolt
i removed from packet and checked big end with dialbore guage for roundness all good
disassembled and inspected had the necessary moly lube all looks great for a budget con rod
washed and cleaned as i wanted to go through the torque stretch deal to have these rods prepped and logged ready to go when required
i am a backyard hobbiest in the shed having fun and i do not do this for a living
any how i assemble rod back together proper arp lube on threads and under head of bolt
i am looking for 0.0046'' of stretch as per arp specs and scat say they torque there rods to 64 ft/lbs
istart at 40lbs and check stretch,write everything down as i go,torque to 60lbs and i have 3'' stretch
anyway i need to go to 95lbs to get the required stretch which does not surprise me as i have been there before with other rods
anyway long story short i do 4 cycles of torque,check stretch,measure big end of rod,dissasseble rod,check rod bolt relaxed all good,after 4 checking cycles i have the max advised stretch at 65lbs,is this normal because on my rods with 2000 series bolts they need to go to 0.006'' stretch and this takes 90 lbs of tension and it does not change i am sure scat says 70lbs
my other question is are these manufactures leaving themselves open for trouble as on the first cycle i only had half the stretch required for yield at scats required tension
i understand that proper engine builders will over come all that but what about the punter who just bangs them in as good and then spins there motor to infinity,i understand the concept of friction underside of head bolts and burnishing of threads etc as we have discussed this before,i also am 100% confident that arp knows when there stuff breaks,surely under stretch is as bad as over streching a rod bolt,please enlighten me guys and let me know if i am doing the right thing here with my thought process,thanks for feedback :D
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Re: rod bolt torque compared to stretch

Post by user-612937456 »

I recently talked to a scat tech at the 1 800 help line about this concern. The tech guy said that they were advising torque method only because of the excessive stress and distortion placed on the rod material. Lube torque and loosen and retorque. He said that possibly the necessary 90-100 lb torque required to achieve the bolt stretch is too much for the rod. for a 500 is HP engine I no longer have any concern If the application is a more extreme build I will use the bolt stretch method and resize the rod after the stretch torque before final assembly.
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Re: rod bolt torque compared to stretch

Post by modok »

I feel it is OK to finish hone a rod with the bolt at a slightly lower torque, lets say 10-15%, in rare cases 20% would work, because when it has a bearing in it the bearing crush also counteracts the force of the bolt. You want the rod bore to be round when it has a bearing in it, not only when empty.

How significant a difference the bearing will make....... depends on the relative thickness of the bearing shell and rod cap compared to the bolt force, and that varies hugely from one engine to another. But a lot of aftermarket rods designed for lots of clearance for strokers, are actually quite thin with giant bolts, and do stretch a lot, but they can get away with it I assume,(they assume? LOL) being made from stronger alloy rather than mild steel.

It now says "NOT TO EXCEED", so don't exceed. It does NOT say "minimum", it says if you exceed that stretch is when you need to replace the bolt. If you were going to use them once, ok go for the max, but sounds to me like you are tightening and loosening them multiple times so, stay in the safe zone! If you replace the bolts with stronger ones, higher torque, need to check the size, it will change, but how much?
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Re: rod bolt torque compared to stretch

Post by racear2865 »

Give this a thought---when you purchase a new set of rods--Scat, Eagle, Oliver, Crower or??/. How do you think they pulled the bolts--torque or stretch. Give this some thought before you answer, They do hundreds per day. They torque them and then recondition but yet they tell you to set by stretch. Now how round is the rod if you stretch. This is why you should stretch and then check for roundness and recondition if not round. Another point. Loosen and take apart. Restretch and wait until next day and check size(roundness). Do not be surprised.
reed
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Re: rod bolt torque compared to stretch

Post by steve cowan »

thanks guys,glen yes i am tightening and loosening bolts to achieve stretch am i doing this wrong?i have not checked for out of round with bearing in but i will as i have a new set of king bearings to suit,this is why i am asking the question as i am a little concerned with this rod not having a lot of meat around bolt area,i understand that the rod manufacture tensions the rod then resizes as required,scat says they use 64 ft/lbs not stretch to tension there rods,my ways of thinking is to bring the rod bolt to yield and to not excede stretch,i would be a little dissappointed if i had to resize new rods after assembly but i am sure this happens
here is a question that i hope someone can answer-if you take a rod bolt to yield on assembly can a dynamically running engine cause the bolt to over stretch and fail during its duty cycles????? GVX i see you tension your bolts on these type of i beam rods after talking to scat,sounds to me that you had same concerns as myself so thank you for sharing that piece of information,up until 5 years ago i only ever set rod bolts by torque specs only so maybe i am over thinking the stretch deal on rod bolts,my engines are all sbc that make between 460-540hp 7600rpm max with 2000 series arp bolts in h beam rods,this is the first set of aftermarket i beams i have ever had and even though they are fairly cost effective i want to try and do the right thing :?
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Re: rod bolt torque compared to stretch

Post by user-612937456 »

What I would do is call the rod manufacturer and give the part number and ask them there recommendations cuz they actually order the bolts to their specifications ARP bolts are not necessarily one-size-fits-all
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Re: rod bolt torque compared to stretch

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You measure the length of the bolt, then put it in the rod, and tighten it until it's stretched X amount longer. This is good way to make sure they are all the same and take out the factor of friction. Some people probably record all the lengths and then they can see if any of them are yielding the next time. I don't do that myself, but I'm sure some do.

Some older procedure using only torque, will have you torque and loosen a new bolt twice before final torque. You will find measuring torque and stretch at the same time that they do vary more initially, then after a few cycles they become quite repeatable. when dealing with somewhat unknown stuff, I will experiment with one bolt or two to find what torque it takes to get near the desired stretch, then once I know what that torque number is, I'll check the rods by torquing the bolts to 10-15% less torque.
This way I make sure that I "do not exceed", and can check sizes and bearing clearances and re-size ect, which does sometimes require you to tighten and loosen the bolts many times, and don't have time to measure every bolt every time. Maybe there is somebody who does that, but I've never met them

This works with "torque to yeild" too, finding a torque that puts it slightly below yield so I don't ruin the bolts, which, speaking of that......no I don't think torque to yeild bolts have proved to be more durable in the long run. As far as factories are concerned, I think they don't have time to carefully lubricate and torque a bolt three times........
Last edited by modok on Sat Jul 09, 2016 9:33 pm, edited 3 times in total.
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Re: rod bolt torque compared to stretch

Post by steve cowan »

thanks gvx,i have read that same statement here before,thanks for the reminder,makes alot of sense :P
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Re: rod bolt torque compared to stretch

Post by steve cowan »

thanks glen,good explanation,this was mentioned a couple months ago with a couple of good engine builders so they did not have to check every rod bolt for stretch so saves time,i would think that any rod manufacture would anticipate a person setting there bolts by stretch to the rod bolt manufactures spec so they build there rods to be exceded by torque to reach stretch specs,i am more than likely wrong been there alot :lol:
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Re: rod bolt torque compared to stretch

Post by user-612937456 »

gvx wrote:What I would do is call the rod manufacturer and give the part number and ask them there recommendations cuz they actually order the bolts to their specifications ARP bolts are not necessarily one-size-fits-all
Not a big deal but on an afterthought my reasoning wasn't exactly accurate the manufacturer reasoning for there tightening procedure also has to do with the alloy and architecture of the Conn rod it's self
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Re: rod bolt torque compared to stretch

Post by ozmuscleowner »

I would say are per what scat is saying... if the bolt is stretched to .0046 or greater its junk and you need to replace it.
Reread what Scat has said in step 4 in your picture.
Scat is saying they us the Torque method not stretch method.. I usually measure the bolts and write down the un-stretched measurement, then I torque rods, at a later date when they are removed for engine rebuilding and retorqued again I remeasure them to see if there is any stretch... if the stretch is greater than the .0046 I replace them.. if yours are now stretched beyond that you may actually want to replace them to be on the safe side..
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Re: rod bolt torque compared to stretch

Post by BB70 »

When you are increasing bolt torque to increase stretch, are you just tightening the bolt more or are you loosening it, reapplying lube , and increasing bolt torque by say 5ft/ lbs ?
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Re: rod bolt torque compared to stretch

Post by steve cowan »

hi john,i have only checked on 1 rod so far and yes set stretch gauge to zero in bolt is relaxed
lube threads and under bolt head,first time round went to 40lbs and check stretch 3 thou
back off bolt check and add thread lube retension to 50lbs 0.0030'' stretch
repeat procedure until just a whisker over 4'' but went to 90ft/lbs torque first time round eg first cycle
after each cycle i have checked the bolt and has gone back to zero in a relaxed state,at no time did i exceed 0.0046'' stretch
after cycling the bolts 4 x times i came up with 4 thou stretch at 65 ft/lbs of torque which i thought is where i need to be,tell me if i am wrong
my way of thinking is if i torque the rod bolt to specs 64 ft/lbs torque and the stretch gauge says 0.003'' that to me means it is only clamping 75% of where it should be
glen (modok)went back his 2 posts and i figure thats where i am at,hi craig looking at the bolt chart i see scat torque there bolts and size rod to suit
on my h beams 2000 bolt i stretched to 6 thou,went through the same procedure
i believe the high torque to stretch is friction and burnishing at first then things start to settle down
i checked the rods after each cycle with dialbore gauge but without bearings did not distort rod
if i am doing something wrong please tell me,i think friction is the main player here as i have been amazed over the last few years tensioning cylinder heads with stud kits and finding more clamping force cycling nuts and washers and moly lube,thanks all for your help it is duly noted and respected
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Re: rod bolt torque compared to stretch

Post by ozmuscleowner »

Is it the genuine ARP lube.. it really does work better
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Re: rod bolt torque compared to stretch

Post by steve cowan »

yeah mate in the blue packet got plenty in the toolbox :D :D
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