This is a good read too on the subject.
http://www.chevyhiperformance.com/techarticles/49258_rod_bolt_torquing_stretch_info/
http://www.chevyhiperformance.com/techarticles/49258_rod_bolt_torquing_stretch_info/
bolts
What i did was researched for myself what happens with torque vs stretch...
I used 6 or 8 rods of different styles, with different size bolts, And some of the rods had used bolts, some new...All the results were the same,,, The recomended torque never produced the correct stretch....
I found that with the larger bolts you can pull them up to the correct torque and only have .003" / .0035" stretch.... I can always tell when i get an engine in my shop to rebuild that came from a shop that only torqued the bolts,because when i remove the rod nuts there is only .003" / .004" max stretch...
As one example, when i was doing this testing i had a rod with 11/32 arp bolts in it. I pulled it up to the proper torque and checked the stretch it was about .0025" I then took the rod apart and re-machined the cap surface where the nut seated and then re-tested, this time it stretched .0045".. I changed nothing but the finish on the cap.... So as you can see many factors can change the amout a bolt stretches when you only torque them..
As a side note when i was doing this testing I also tried different lubes on the bolts including loctite.... I know the guys at ARP will call me stupid but I am telling you I did not see any difference between the loctite and there super duper moly lube, comparing stretch to torque yeild......Loctite is an excellent lube when it's wet..... I don't use loctite on my rod bolts, but it was a old timers trick around here.... I would think because they did not know about stretch back then...
Try this test.... take a eagle rod with a 7/16 L-19 bolt. lube everything up real good with the proper lube, torque to the stated value and then check the stretch.... It isn't even close to the .0065" recomended ( depending on bolt length).........
Keith
What i did was researched for myself what happens with torque vs stretch...
I used 6 or 8 rods of different styles, with different size bolts, And some of the rods had used bolts, some new...All the results were the same,,, The recomended torque never produced the correct stretch....
I found that with the larger bolts you can pull them up to the correct torque and only have .003" / .0035" stretch.... I can always tell when i get an engine in my shop to rebuild that came from a shop that only torqued the bolts,because when i remove the rod nuts there is only .003" / .004" max stretch...
As one example, when i was doing this testing i had a rod with 11/32 arp bolts in it. I pulled it up to the proper torque and checked the stretch it was about .0025" I then took the rod apart and re-machined the cap surface where the nut seated and then re-tested, this time it stretched .0045".. I changed nothing but the finish on the cap.... So as you can see many factors can change the amout a bolt stretches when you only torque them..
As a side note when i was doing this testing I also tried different lubes on the bolts including loctite.... I know the guys at ARP will call me stupid but I am telling you I did not see any difference between the loctite and there super duper moly lube, comparing stretch to torque yeild......Loctite is an excellent lube when it's wet..... I don't use loctite on my rod bolts, but it was a old timers trick around here.... I would think because they did not know about stretch back then...
Try this test.... take a eagle rod with a 7/16 L-19 bolt. lube everything up real good with the proper lube, torque to the stated value and then check the stretch.... It isn't even close to the .0065" recomended ( depending on bolt length).........
Keith
Re: bolts
hey Keith very interesting reading..................... i'm gonna haver to try this out this winter when i build my new motor..!!!
what were your findings on how far off were the torque specs to the bolt stretch like with ARP 7/16 L-19 bolts??? just curious...i know eagle has like a torque spec of 63-67 lbs of tq...........what did you end up with to acheive the stretch??
K-Star automotive said:
hey Keith very interesting reading..................... i'm gonna haver to try this out this winter when i build my new motor..!!!
what were your findings on how far off were the torque specs to the bolt stretch like with ARP 7/16 L-19 bolts??? just curious...i know eagle has like a torque spec of 63-67 lbs of tq...........what did you end up with to acheive the stretch??
i use torque figures, once the bolts tight its tight, making it stretch doesnt make anything any tighter, it only brings the bolt closer to its yeild point as the crank tries to sling the cap off. as long as the fastener is capable of handling the stress and rod caps are tight and the bolts not gonna unscrew itself your all set.
540Malibu said:
That is not entirely accurate........K-Star covered the reasons that torque values are mostly useless, but the stretched fastener is assuring that the "clamp load" is correct, which for instance on rod bolts, affects the cocentricity of the rod big end bore when assembled.
The rod bolts must be installed and clamped to the correct load to check or correct a rod big end measurement, it needs to be the same when assembled for service.
Is it nit-picking?
Just a bit maybe, but then again, I found a huge discrepancy between the stretch and torque methods, and I just have to say that I got lucky for 20+ years that I didn't have any engine failures, but the bulk of my engine building was mild street or O.E. rebuilding, not all out race engines.
Do what you're comfortable with, but I'll never leave it to chance anymore.
Re:
torque readings can be misleading and that is one of
several areas that need to be correctly assembled..
George
any engine builder worth his salt uses stretch figures,Supe said:
torque readings can be misleading and that is one of
several areas that need to be correctly assembled..
George
Re: Re:
I work for an engine builder part time and he's also been my best friend for 30+ years. I've learned quite a bit over those years. One thing I've learned is that stretch and torque are 2 completely different animals. When I put the prop on my outboard, I torque it to 55 pounds. Your not going to stretch a prop shaft at 55 pounds, but you dont have to worry about anything being perfectly round either. When I do a set of rod bolts, we "cycle" them until the torque number repeats. Take a 3/8 rod bolt, measure it relaxed and then torque it to just 20 pounds and see how much it stretches. Alot of times, it wil stretch .001-.002 + or -.
Heres an example I've seen recently.......on a good set of rods with 7/16 ARP bolts, the "directions" had a torque spec of 80# if my memory serves me correctly. After torquing the rods to 75 pounds, I checked the stretch and had reached my desired .005-.006 of stretch. Loosened them and torqued again, I had reached my stretch spec at 70pounds. loosened them and retorqued and again, reached the desired stretch at 70 pounds. 70 pounds was the torque spec, not the 80 they called for.
So is it splitting hairs? Probably, but it does make a difference.
http://cgi.ebay.com/ebaymotors/ROD-BOLT-STRETCH-GAUGE-CHEVY-FORD-NEW-PROFORM-_W0QQcmdZViewItemQQhashZitem3364ca28a5QQitemZ220734302373QQptZMotorsQ5fAutomotiveQ5fTools
Well said GeorgeSS402 said:
I work for an engine builder part time and he's also been my best friend for 30+ years. I've learned quite a bit over those years. One thing I've learned is that stretch and torque are 2 completely different animals. When I put the prop on my outboard, I torque it to 55 pounds. Your not going to stretch a prop shaft at 55 pounds, but you dont have to worry about anything being perfectly round either. When I do a set of rod bolts, we "cycle" them until the torque number repeats. Take a 3/8 rod bolt, measure it relaxed and then torque it to just 20 pounds and see how much it stretches. Alot of times, it wil stretch .001-.002 + or -.
Heres an example I've seen recently.......on a good set of rods with 7/16 ARP bolts, the "directions" had a torque spec of 80# if my memory serves me correctly. After torquing the rods to 75 pounds, I checked the stretch and had reached my desired .005-.006 of stretch. Loosened them and torqued again, I had reached my stretch spec at 70pounds. loosened them and retorqued and again, reached the desired stretch at 70 pounds. 70 pounds was the torque spec, not the 80 they called for.
So is it splitting hairs? Probably, but it does make a difference.
http://cgi.ebay.com/ebaymotors/ROD-BOLT-STRETCH-GAUGE-CHEVY-FORD-NEW-PROFORM-_W0QQcmdZViewItemQQhashZitem3364ca28a5QQitemZ220734302373QQptZMotorsQ5fAutomotiveQ5fTools
I just used the stretch method for the first time on some hard-core 7/16" MSA capscrews that were an upgrade on my Howards Cams rods.
The recommended torque was 72ft/lb and .0084" stretch, but when I measured stretch after torquing to 72ft/lb, it was only about .004"!
So I checked with the pros on Speedtalk.com (excellent engine building site) and they recommended:
1) Use ARP moly lube
2) Tighten each capscrew (or rod bolt) to torque 5 times to burnish the threads
3) Then, preferably on one smooth stroke, try to tighten to the stretch value
I had a hard time with Step 3 as I couldn't hold the gauge steady and pull a long breaker bar at the same time. But after a couple time you learn to get pretty close with one pull, then finish with one small adjustment. You have to measure each capscrew before using it and record the length. Then if you go over by much more than maybe .0012"-.0015, you need to loosen the capscrew fully and check the measure against the original length - if it is over the original length by more than ~.0006" it means they have been stratched past the yield point and must be thrown away. I ended up ruining one capscrew by overstretching. And doing this is a lengthy process.
But spending $10 for another capscrew is mice nuts compared to a blown-up motor because of a rod bolt failure.
By the time I finished, all 16 capscrews were within the tolerances for installed stretch, and I'm confident that my bottom end is put together solidly.
Jim
Build pictures: http://home.mindspring.com/~jim_fisk/thegreyghost
The recommended torque was 72ft/lb and .0084" stretch, but when I measured stretch after torquing to 72ft/lb, it was only about .004"!
So I checked with the pros on Speedtalk.com (excellent engine building site) and they recommended:
1) Use ARP moly lube
2) Tighten each capscrew (or rod bolt) to torque 5 times to burnish the threads
3) Then, preferably on one smooth stroke, try to tighten to the stretch value
I had a hard time with Step 3 as I couldn't hold the gauge steady and pull a long breaker bar at the same time. But after a couple time you learn to get pretty close with one pull, then finish with one small adjustment. You have to measure each capscrew before using it and record the length. Then if you go over by much more than maybe .0012"-.0015, you need to loosen the capscrew fully and check the measure against the original length - if it is over the original length by more than ~.0006" it means they have been stratched past the yield point and must be thrown away. I ended up ruining one capscrew by overstretching. And doing this is a lengthy process.
But spending $10 for another capscrew is mice nuts compared to a blown-up motor because of a rod bolt failure.
By the time I finished, all 16 capscrews were within the tolerances for installed stretch, and I'm confident that my bottom end is put together solidly.
Jim
Build pictures: http://home.mindspring.com/~jim_fisk/thegreyghost
Lots of interesting info, theories & research in this one. At least one poster stated he uses a stretch gage for AFTERMARKET hardware. I thought that said more than anything else about whether torque or stretch is the supposed "right" or "wrong" method. I can all but guarantee that any "modern" assembly line GM (& pretty much ANY other brand) factory production engine out there was popped together with an automated torque wrench... It's worked for them for billions of units for decades...
Is rod stretch more accurate? Absolutely. Is it 100% necessary? Maybe not. Is measuring torque completely inaccurate. No. Should it be ignored as a viable alternative method? Also no.
Remember, you can overtighten a bolt, too. Especially if you're not real experienced with the stretch method. I would say applictaion, preference, experience all play a part in the choice. It makes sense to me to PROPERLY use a stretch gage for true high end performance & race stuff. It also makes sense to me that it would be more or less overkill for a garage builder putting together something with stock bottom end parts. They can definitely safely use a torque wrench.
Tommy
Is rod stretch more accurate? Absolutely. Is it 100% necessary? Maybe not. Is measuring torque completely inaccurate. No. Should it be ignored as a viable alternative method? Also no.
Remember, you can overtighten a bolt, too. Especially if you're not real experienced with the stretch method. I would say applictaion, preference, experience all play a part in the choice. It makes sense to me to PROPERLY use a stretch gage for true high end performance & race stuff. It also makes sense to me that it would be more or less overkill for a garage builder putting together something with stock bottom end parts. They can definitely safely use a torque wrench.
Tommy
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