Bolt Stretch on SCAT Connecting Rods

[earlymopar]

I have a set of SCAT "I-Beam" connecting rods for a small block Mopar and am looking for feedback on bolt stretch. Interestingly, SCAT recommends using a 2-step torque process (54 lb -ft. initially and a 2nd round to 64 lb-ft.) versus the bolt stretch method. The shop I'm using of course prefers the bolt stretch method due to proven consistency and accuracy which is how I want to go.

The problem is that SCAT only provides a "Not to Exceed" stretch figure (at least for these rods) which is .0046". The difficulty I have is understanding where their "Not to Exceed" figure is as compared to the yield strength of the bolt and how much of a safety factor they used in obtaining their not to exceed figure. In my mind, it would be much more preferable to have a range of stretch such as .0044" to .0046". I just don't like having a not to exceed figure as my only number and not knowing how far under that value I should be with my target in order to still achieve adequate stretch.

[Schurkey]

Does SCAT stretch the bolts before sizing the big-end, or do they torque the bolts before sizing the big-end?

Do it the way SCAT does; or be prepared to verify roundness when you use your preferred method.

[MELWAY]

Problem with scat stuff. Is if you follow their tq numbers only
You end up with at least .001” less stretch and that’s slit if lost clamp load.

[In-Tech]

I would say most of use the stretch calibrated by the fastener company and deal with out of round after.

[earlymopar]

Does SCAT stretch the bolts before sizing the big-end, or do they torque the bolts before sizing the big-end?

Do it the way SCAT does; or be prepared to verify roundness when you use your preferred method.

This is a good question and frankly they didn't say a thing about this so I'll have to ask. The problem with my short block is that it was assembled by another shop earlier and given the way I found other aspects of their work, I doubt they sized. Then at assembly they simply torqued to spec, possibly in increments.

[earlymopar]

Problem with scat stuff. Is if you follow their tq numbers only
You end up with at least .001” less stretch and that’s slit if lost clamp load.

This of course my major concern (well, and not knowing where the Scat not to exceed figure is)..

[earlymopar]

I would say most of use the stretch calibrated my the fastener company and deal with out of round after.

So by taking this path I assume you mean putting the rod in a fixture, stretching the bolts to target and then measuring the big end diameter that results, all done before actual short block assembly. How are you correcting out of round?

[ClassAct]

I’ve heard Tom Leib’s explanation as to why he doesn’t suggest stretching rod bolts. In a nut shell, you can have one bolt at X amount of torque to make it stretch whatever the stretch number is, and the other bolt on the same rod may need Y amount of torque to get the same stretch.

Leib said he would rather both bolts be at the same torque than the same stretch.

I quit stretching rod bolts years and years ago. IMO it’s a waste of time. I have yet to see a failed bolt or a rod get off the crank because the bolt wasn’t stretched.

[In-Tech]

I would say most of use the stretch calibrated my the fastener company and deal with out of round after.

So by taking this path I assume you mean putting the rod in a fixture, stretching the bolts to target and then measuring the big end diameter that results, all done before actual short block assembly. How are you correcting out of round?

Yes, torque has friction variables as you well know when using different lubes and stretch changes depending on the lube. This is basically why new fasteners use a small torque value then a degree method after. Regardless of lube it will "stretch" and "preload" with proper clamping. I am not trying to be argumentative. We can't measure stretch in all fasteners, but if I can, I will measure.

[54chevkiwi]

This is my experience as a cnc machinist but not on engines..,..

I got a machined ready to assemble shortblock with 6” callies compstar rods, 421sbc...
Compstar says 75lb/ft, and says between .004-.005 stretch (i think, card is not with me), 7/16 arp 2000 bolts..

Machinist told me to just use the torque spec, so with the extreme pressure lube i torqued one up to the torque spec in the rod vise and the bolts came up at like .0053 and like .0048 stretch resoectively.. i used a mitutoyo .0001 resolution mic with custom .070 balls on the anvils to measure the bolts before and after...
I snuck up on all of them next and got to 65lb/ft and got the stretch on the low side, around like .0038-.0042 on average, there really was quite a bit between them all, up to .0045... so maybe i might be best to go to 70lbs based on that...
The big ends measured exactly the same end to end with like .0002-.0003 extra at the parting line each time i done anything, so i was confident the shape wasnt changing no matter what i tried..
I also tried stretching them all with a strongarm bar and a .0001 bolt stretch gauge right to the middle of the torque spec, complete disregard to any torque values. absolutely no change in big end shape there either...

I asked a local circle track engine builder i know about this and his opinion was to just go to the torque spec, he said going by the torque spec the bolts arent exactly junk if they go up to .0053, and said the only difference between which way you go (to him) is the difference between doing a few engines a week and a few engines a month.. so a time thing depending on urgency

Im kind of deducing and suggesting that, it may not nessessarily matter being its not a rocket ship, expensive engine yes, but it seems to me like regardless of which way it might be more important to ensure consistency between them all and make sure none of the bolts are duds..
By that i mean, they could be all over the place around that .004-.005 stretch spec when you torque them, even if they upto .0055 it maynt matter, as long as one doesnt end up at say .008 when torqued and they return to pre stretch length when unloaded... but when you use the stretch gauge, the THORQUE value may end up anything from say 65lb/ft up to maybe 80 or 85lb/ft, so, that all over the place instead!

Either way its clear as mud.

Please someone correct me if i have deduced the wrong thing.. ive overthought this subject too..

[54chevkiwi]

I would say most of use the stretch calibrated my the fastener company and deal with out of round after.

So by taking this path I assume you mean putting the rod in a fixture, stretching the bolts to target and then measuring the big end diameter that results, all done before actual short block assembly. How are you correcting out of round?

I think you stick em in a rod vise, cycle the bolts a few times till the stretch near enough repeats at that torque spec they want, THEN hone rods to size (with bolts tightened) to correct the out of roundness, then the size should stay ‘correct’ at that torque spec thereafter...

i think in MY case Above, the guy done just that.. cycled them, left them at 75lb/ft then honed to size, that must be why he told me to torque to 75lbs....
I think nowadays more people than ever have a stretch gauge, but more home guys still Still rely solely on a torque wrench so doing it that way not by stretch errs on the side of safety for the machinist and less complicating it when joe bloggs is assembling it

Scat might be saying “we sized our rods at this torque spec, you might cause an out of roundness and theyll need re sizing if you deviate from this”

[cjperformance]

Check the bolt free lengths. Then using the lubrication you will use for the final assembly tq the bolt till desired/recommended stretch is achieved.
Keep at least a mental note on how far after contact the bolt rotates to achieve tq so you know if something is obviously wrong.
Then if the bolts are accessible verify their stretch.
In the case of a 'not to exceed' stretch number it I would expect to see my TQ number coming close to the do not exceed number.

[Steve Salesky]

The stretch number is what you are trying to achieve. Measuring torque is an indirect method and can be inaccurate since friction can vary a lot based on the threads and lube used. If you are going to torque rod bolts you should put them thru 4-5 torque up cycles first as it wears in the threads and the the method becomes more consistent. Best is to read the stretch directly and know the bolt is properly tensioned. I recall reading on ARP site the stretch number they give is about 80% of the yield point so is not super critical if you are a .001" off. Now this is from memory so I may be remembering wrong, but a little looking should be able to find it.

That all said shop used to work at he found if you cycled the bolts 4-5 times and he used a mix of ARP and oil that found a torque that obtained the right stretch and was consistent. Was something he developed by trial and error. He would check the stretch on two and just torque the rest if they matched up. Never had rod bolt problems.

One other good thing about measuring the rod bolts is if you measure the bolts length before install and write that info down you can measure them at freshen up and know if a bolt is good or bad. If a bolt free length is longer it has stretched and needs replacement.

[earlymopar]

I forgot to mention that I did contact ARP on this since my Scat rods use ARP bolts. ARP said I should defer to using Scats recommendations for either torque or the not to exceed stretch figure. Certainly the rod is another variable in this but It made me wonder what spec these same rod bolts might have in a different model or make of rod.

[BillK]

I forgot to mention that I did contact ARP on this since my Scat rods use ARP bolts. ARP said I should defer to using Scats recommendations for either torque or the not to exceed stretch figure. Certainly the rod is another variable in this but It made me wonder what spec these same rod bolts might have in a different model or make of rod.

Yup, ARP will NOT give you a torque spec for the bolts that come in connecting rods. I have tried in the past with the same answer as you got.