Measuring ARP Rod Bolt Stretch - Micrometer vs. Stretch Gauge

[Gordo2]

This afternoon I decided to measure the length of my ARP rod bolts before final installation.

Important note - measure your ARP rod bolts before you install them.

Unfortunately, I can't call this a pre-stretch/pre-installed measurement. Prior to today's measurements, I gave the rod bolts to my machinist to use when he reconditioned the rods, so the bolts have been torqued to 40 ft-lb already.

I started out by trying to measure each bolt's length using my Mitutoyo 103-139B metric micrometer (calibrated/zero'd with a 50mm standard prior to measurements).



I was shocked to find my measurements were all over the place - ranging from 69.642mm to 69.775mm - a difference of 0.133mm (~0.005")

Note on micrometer readings: the Mitutoyo 103 is scaled to read to 0.01mm (I believe this is the micrometer's "resolution"). I estimated the 0.00X mm digits.

Shortest rod bolt @ 69.642mm:


Longest rod bolt @ 69.775mm:


These measurements were repeatable (I picked up random rods, re-measured and then checked against previously recorded measurements - spot on).

I assumed that the rod bolts should have pretty close pre-stretch measurements and freaked thinking that my machinist stretched and deformed the bolts when he installed them.

Meanwhile, it seemed odd that my measurements were rather inconsistent. As such, I decided I would see how the rod bolts compared on using my Summit Racing rod stretch gauge.

I used the shortest bolt as my reference and set reference indicator (the black pointer to the right of the needle) on the stretch gauge to the smallest reading I found. The stretch gauge pins measure from the the dimples that are on the ends of the bolts as opposed to the micrometer readings that measure from the bolt faces.



I was relieved to find that I could transfer the rod stretch gauge to all the bolts and come up with the same length. This measurement was repeatable as well.

My conclusions
- Measuring ARP rod bolt lengths using a micrometer can produce misleading results.
- The ARP rod bolt dimples appear to provide the best surface/point of contact to take measurements.
- I recommend taking pre-installed measurements with a micrometer, and recording these measurements, and confirming that the bolts have a consistent length with a rod bolt stretch gauge. This would provide 2 means to check for stretch if you remove and want to re-use the ARP rod bolts at a later time.

- Gordo

[304065]

Gordo, if you want to measure consistently you need to use spike anvils on the mic-- using the flat anvil causes you to measure at a different spot on the bolt head vs. a single spot down in the dimple.

Also, does your mic ratchet? Can you repeat 10/10 measurements on your 50mm standard?

By marking the head with machinist bluing you can label each bolt and measure its relaxed length when you disassemble these-- then compare to the stretched length. This would be a good way to QC the bolts although ARP are very good, I heard once of one stretching like butter when installed. (somewhere here)

Nice thread BTW

[Gordo2]

Originally Posted by 304065

Gordo, if you want to measure consistently you need to use spike anvils on the mic-- using the flat anvil causes you to measure at a different spot on the bolt head vs. a single spot down in the dimple.

Also, does your mic ratchet? Can you repeat 10/10 measurements on your 50mm standard?

By marking the head with machinist bluing you can label each bolt and measure its relaxed length when you disassemble these-- then compare to the stretched length. This would be a good way to QC the bolts although ARP are very good, I heard once of one stretching like butter when installed. (somewhere here)

Nice thread BTW

Thanks for the feedback and additional considerations.

Agree on needing spike or pin anvils to accurately measure rod bolts with a micrometer - the flat faced micrometer contact surfaces can produce consistent but misleading measurements...

The Mitutoyo has a ratchet - I calibrated (zero'd) letting the ratchet click to stop on the 50mm ref - don't think I can hold the ref stable enough to get a perfect 10 for 10 though. I repeated the same method for the rod bolts (consistency of measuring technique).

Not sure I will go as far as bluing, but a great idea - I'm hoping once I button up the engine it's the last time I ever have to look at the rod bolts.

Thanks again,

Gordo

[Gordo2]

Double post deleted.

[Jcslocum]

G,

There is no reason to expect that all of the rod bolts will be the same manufactured length. Each bolt needs to be measured relaxed and recorded. If they are the same length, then that is nice but a coincidence.

The dimple/punch mark raises the surface and makes using the mic with flat ends very difficult.

Temperature changes will make the readings go goofy too. When measuring to such small tolerances someone opens the door and lets the cold breeze in, you want to scream!

[jwasbury]

I measured stretch using a standard mic, without the pointy anvil ends. Yes it was challenging to get consistent results, and it would have been faster if I had the rod stretch gauge or those anvils. My solution was to go slow, take many measurments, use the ratchet stops and record enough repeatable and consistent measurements to be confident about the result. Also used the torque wrench to verify all bolts were at consistent torque levels when I achieved the desired stretch per mic measurments. Finally, since the torque values were substantially higher than the ARP "non-stretch method" instructions, I did a search of these forums and found to my relief that everyone else had the same experience re: torque values and the recommended stretch.

I probably spent the better part of a day assembling the crank and rods.

[304065]

Originally Posted by Jcslocum

G,

There is no reason to expect that all of the rod bolts will be the same manufactured length. Each bolt needs to be measured relaxed and recorded. If they are the same length, then that is nice but a coincidence.

The dimple/punch mark raises the surface and makes using the mic with flat ends very difficult.

Temperature changes will make the readings go goofy too. When measuring to such small tolerances someone opens the door and lets the cold breeze in, you want to scream!

Tell me about it. I get all set up in my metrology lab and then WHAM, somebody conducts an underground nuclear test.



(while the photo is tongue-in-cheek it IS actually the room set up by Mahr-Federal for calibrating their gages!)

[irobertson]

Originally Posted by jwasbury

I measured stretch using a standard mic, without the pointy anvil ends. Yes it was challenging to get consistent results, and it would have been faster if I had the rod stretch gauge or those anvils. My solution was to go slow, take many measurments, use the ratchet stops and record enough repeatable and consistent measurements to be confident about the result. Also used the torque wrench to verify all bolts were at consistent torque levels when I achieved the desired stretch per mic measurments. Finally, since the torque values were substantially higher than the ARP "non-stretch method" instructions, I did a search of these forums and found to my relief that everyone else had the same experience re: torque values and the recommended stretch.

I probably spent the better part of a day assembling the crank and rods.

I did mine with a micrometer as well.
All the first and second measurements were done with the rods lightly clamped in a vise, and I got a really good feel for it with good repeatability. By the time I was doing them on the crank, I had the process down and it went quite smoothly.
The proper stretch gauge probably would have been easier though.

[Gordo2]

Originally Posted by Jcslocum

G,

There is no reason to expect that all of the rod bolts will be the same manufactured length...

Very good point - I guess I would have been more accepting of this fact had I measured the bolts as new/before they were torqued.

Since my first measurements were after they had already been torqued (provided to my machinist) I was a bit concerned that some had stretched and deformed. Meanwhile I was pretty sure he followed the ARP instructions to the T - he used some of the ARP lube and the instruction sheet was well used - greasy ARP lube fingerprints as evidence .

While I agree the bolts will have some amount of length variance (due to thread variations, dimpling etc), I do find it interesting that measuring from the dimples shows that whatever process ARP is using to dimple the bolts - it has pretty tight tolerances that produces a consistent overall length.

Thanks for the feedback folks - good info.

Gordo

[Gordo2]

Originally Posted by jwasbury

I probably spent the better part of a day assembling the crank and rods.

Same - but without the "probably"

- Gordo