Bolt Stretch on SCAT Connecting Rods

[af2]

So what I have done...... Check the bolt length first then on a 7/16 torque to 75 lbs on the engine and see what the stretch is. then you will have a close idea where you are at. Should be within their specs. Using the same lube!!!!!

[Steve Salesky]

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.

Yes they will not give you a spec and will say the bolts are made to the rod mfg specs. Think real reason is they have no control over the rod material or number of threads engaged.

[ProPower engines]

And the other issue that has plagued builders is the bolt lube.
When they changed to the current ultra torque lube in the blue pak from the long time used lube in the white pak
builders every where seen an increase needed in the torque value to obtain the recommended stretch value of the bolts.
We have all or at least most have called ARP tech and asked this question and were just told to torque till the stretch is achieved and call it good.
Now torque wrench calibration can be off and unless you regularly send your wrench out for testing there is still a small margin of error regardless.
The not to exceed stretch value should be compared to the torque value. But the stretch needed to get the bolt into its proper operating elastic range is still not defined by either the rod manufacture or the bolt manufacture which sucks.
I find that if you are within .0005 of the max allowable stretch and not exceeding the max recommended torque value then the bolts are good to go.
If you just compare the torque to stretch they always seem to come up about .0015 short in some cases more depending on the bolt length. The longer the bolt the more stretch for a given torque can be found it seems.
A slicker bolt lube will allow the bolts to stretch more for a given torque load like the lube Callies K1 or Molnar supply with their rods.
Rod material strength is also a factor but given all the info on the rods is supplied to ARP WTF they will not just come out and just recommend a stretch is beyond any of us and really we must learn ourselves what is acceptable and what is not.


What engine are you working on?? That may help with a recommendation here from those that see the same parts on a regular basis

[earlymopar]

Thanks all for your replies and experiences. This is all very helpful. PPE, this is a small block Mopar.

It really sounds like completing the thread burnishing process of 4 torque cycles helps a lot with getting consistency between bolts. Then the torque and stretch figures provided have a better chance of corresponding to each other or at least coming closer. But, it still seems that many who have replied still ended up surpassing the torque spec in order to achieve the stretch spec.

I do like the idea of (after burnishing the threads) experimenting with torque to see what is needed to achieve something close to max stretch. Then setting the torque wrench to that figure and torqueing all remaining bolts from there.
......
A great reply from ProPower.
If you use torque cognitive, you realize the difference between torque and stretch and thereof. I digress even contemplating that others don't.

[Jer73]

viewtopic.php?p=840126#p840126

viewtopic.php?f=1&t=13981

viewtopic.php?f=1&t=12332

https://www.youtube.com/watch?v=sxW7TJTWoOY

https://www.youtube.com/watch?v=uqjHdKkurXs

https://www.youtube.com/watch?v=pCNBw4kSm4Q

I just recently went through this myself putting together a 363ci windsor, used a forged Eagle rotating assembly with ARP 2000 7/16 rod bolts. I have found that the supplied literature's torque value of 75lb/ft only stretched the rod bolts .005in-.0055in. Now the rod bolt stretch spec on the supplied literature was .0063in-.0068in.

I was scratching my head for a while over this. The threads, shank, and under head of the bolt were well covered in the supplied ARP ultra torque lube, torque cycled several times to burnish the threads. I went and bought a brand new digital torque wrench as my twin beam snap-on had been in service for the last 7 years and I was suspect of it's calibration at this point. New torque wrench, burnished threads, properly lubed up, rechecked the stretch at 75lb/ft, and wouldn't you know it .005in-.0055in on the stretch gauge across the board.

So now that I had my variables eliminated all I could do to achieve the desired stretch value of .0063-.0068 was back off the fastener and increase the torque value 5ft/lbs at a time, re torque and re check until I reached my desired value of .0065in. I managed to achieve .0065in on every bolt but the torque values varied between 92.5lb/ft to 98lb/ft.

Please double check your math with a calculator and take your time as things can start to get fuzzy when working late into the evening and after many repetitions; It is very easy to junk a rod bolt or three because it's a bit too late and trying to go a bit too fast .

[earlymopar]

Thanks much Jer73. This sure seems to be a consistent finding; torque and stretch that do not match. Was the .0065" stretch figure also a "not to exceed" or did you have a range (min to max)? I'm just amazed you had to use that much more torque to achieve .001" in additional stretch.

[Jer73]

The maximum stretch value given was .0068in, If any of the rod bolts have shown to have grown .001 in free length un-torqued they are trash. There was no maximum torque / do not exceed value, and no maximum torque cycles in eagles literature. I fudged the math on a few of them and took them to 100lb+/ft and .0072 stretch by mistake. I ordered a new set of rod bolts that night and had to check the big ends on those rods to make sure I didn't bring them out of round when the new bolts came in.

As stated in the links I posted always go off the rod manufacture's specs not the general specs given by ARP as they will be different because the rod manufactures often have the fasteners made to their spec and show their logo on the head of the bolt next to ARP's. Also noted in one of the videos linked, Initially starting the fasteners out at 20lb/ft to seat the cap and then torquing the fasteners in one smooth and slow motion will yield a better stretch reading at a lower given torque value.

https://arp-bolts.com/p/technical.php

Some more reading, the common failure section I found very informative.

[Geoff2]

After reading all of the above, makes me feel better that I am just torqueing the bolts with my trusty torque wrench to the manufacturer's recommended torque value....

[cjperformance]

After reading all of the above, makes me feel better that I am just torqueing the bolts with my trusty torque wrench to the manufacturer's recommended torque value....

There's a lot to be said for simply torquing bolts! When things are prepped, clean, lubed and the wrench is correct and used correctly,, well I'll be darned, torquing works!!

[Monza355]

After reading all of the above, makes me feel better that I am just torqueing the bolts with my trusty torque wrench to the manufacturer's recommended torque value....

There's a lot to be said for simply torquing bolts! When things are prepped, clean, lubed and the wrench is correct and used correctly,, well I'll be darned, torquing works!!

And all of these variables makes it a good idea to verify the torque with the stretch gauge

[cjperformance]

After reading all of the above, makes me feel better that I am just torqueing the bolts with my trusty torque wrench to the manufacturer's recommended torque value....

There's a lot to be said for simply torquing bolts! When things are prepped, clean, lubed and the wrench is correct and used correctly,, well I'll be darned, torquing works!!

And all of these variables makes it a good idea to verify the torque with the stretch gauge

And I absolutely agree

[54chevkiwi]

I dont have my engine with me or id simply check...
But, just torqueing it, compared to torque angling it....
If you were to start at the same place and take note of where the tool ends up at the end of tightening, would the tool end up in about the same place with either method..?

In other words, with a 75lb torque for required stretch on a particular bolt, will the tool end up in the same spot from the same start as it would if you were to torque angle that bolt for the same stretch..? Is the angle of rotation the same for each way..?

[Darbikrash]

I just recently went through this myself putting together a 363ci windsor, used a forged Eagle rotating assembly with ARP 2000 7/16 rod bolts. I have found that the supplied literature's torque value of 75lb/ft only stretched the rod bolts .005in-.0055in. Now the rod bolt stretch spec on the supplied literature was .0063in-.0068in.

I was scratching my head for a while over this. The threads, shank, and under head of the bolt were well covered in the supplied ARP ultra torque lube, torque cycled several times to burnish the threads. I went and bought a brand new digital torque wrench as my twin beam snap-on had been in service for the last 7 years and I was suspect of it's calibration at this point. New torque wrench, burnished threads, properly lubed up, rechecked the stretch at 75lb/ft, and wouldn't you know it .005in-.0055in on the stretch gauge across the board.

So now that I had my variables eliminated all I could do to achieve the desired stretch value of .0063-.0068 was back off the fastener and increase the torque value 5ft/lbs at a time, re torque and re check until I reached my desired value of .0065in. I managed to achieve .0065in on every bolt but the torque values varied between 92.5lb/ft to 98lb/ft.

Please double check your math with a calculator and take your time as things can start to get fuzzy when working late into the evening and after many repetitions; It is very easy to junk a rod bolt or three because it's a bit too late and trying to go a bit too fast .

When a rod bolt is torqued, more is happening than just the fastener stretching. An (equal) reaction force is applied to the contact surfaces on the connecting rod. The rod is not infinitely rigid, it compresses (deforms) locally under the head when the fastener is torqued down.

If the torque does not match the specified bolt stretch this extra energy is usually acting to deform the connecting rod contact surfaces, as well as the rod threads. This is the reason ARP defers to the rod manufacturer for stretch, they don’t know what material the rod is made of nor do they know the rigidity of the contact surfaces.

If you are seeing 90+ lb./ft of torque to stretch the bolt when you are expecting 75 lb./ft, take it apart and look closely at the rod under the fastener head. I’ve seen this area Brinell and deform due to the rod being unable to support the fastener clamp load. I’ve sent rods back because of this, specifically Eagle rods.

[FC-Pilot]

Thanks all for your replies and experiences. This is all very helpful. PPE, this is a small block Mopar.

It really sounds like completing the thread burnishing process of 4 torque cycles helps a lot with getting consistency between bolts. Then the torque and stretch figures provided have a better chance of corresponding to each other or at least coming closer. But, it still seems that many who have replied still ended up surpassing the torque spec in order to achieve the stretch spec.

I do like the idea of (after burnishing the threads) experimenting with torque to see what is needed to achieve something close to max stretch. Then setting the torque wrench to that figure and torqueing all remaining bolts from there.
......
A great reply from ProPower.
If you use torque cognitive, you realize the difference between torque and stretch and thereof. I digress even contemplating that others don't.

I had and engine disassemble itself once due to the fact that the rod bolts were not properly burnished. Upon disassembly it was noted that all the rod bolts had very insufficient stretch. The bolts were torqued to the proper torque but not burnished, and therefore the stretch was lacking. Lesson learned for me.

Paul

[XiomarBorjas]

Even though it's been a couple of years since this thread was active, I thought I'd jump in and offer some input. Bolt stretch can be a bit of a puzzle, but it's great that you're looking for consistency and accuracy. SCAT's 2-step torque process might work for some, but if you prefer the bolt stretch method, it's worth pursuing. I understand your concerns about SCAT's ""Not to Exceed"" figure and the lack of a specific range. It would be preferable to have a target range for stretch to ensure adequate performance. Unfortunately, without knowing the safety factor and yield strength of the bolt, it can be challenging to determine the ideal stretch range. By the way, if you need any nuts and bolts for your projects, https://www.scrooz.com.au/ might be a helpful resource to explore. They offer a range of products that could come in handy.