Lotus Elan

Connecting rod bolt stretch advice needed

PostPost by: 2cams70 » Tue Oct 10, 2017 12:41 pm

I installed a pair of new rod bolts using a stretch gauge and stretched each bolt alternately in 4 stages to 0.0014", 0.0028", 0.0041" and finally 0.0055". They are ARP L19 3/8" bolts and the spec. that came with the rods was stretch to 0.11 to 0.15mm (0.0043 to 0.0059") and DO NOT GO OVER 55 ft/lb.
After stretching them I decided to go over them with a torque wrench set to 50 ft/lb. One bolt didn't move but the other one did. Before assembly all threads and head undersides were coated with ARP lube.
I then backed off the bolts completely with the intention to just stretch them again and forget about the torque wrench. Once I had backed off both bolts however I found that each bolt without tension had now stretched 0.0005" compared to what it was originally. I was surprised and would have expected them to return to the same length they were originally. ARP set a limit of 0.001" stretch before bolts need to be replaced but these were new bolts.

So my questions are:
Is this normal/expected?
For each tightening should I do the stretch twice so that initial 0.0005" baseline stretch is allowed for in the second stretch tightening?
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PostPost by: 2cams70 » Wed Oct 11, 2017 12:20 am

Well, I've been doing a bit of research on those ARP L19 bolts. Saenz supplied the rods with these already fitted. Apparently although they are stronger than the regular ARP 2000 series bolts they are super sensitive to corrosion and hydrogen embrittlement and you are not supposed to clean them with chlorinated solvents such as brake cleaner. Guess what I did - Yes, I gave them a good spray with brake cleaner!! You are even supposed to handle them wearing gloves because body sweat can be a problem.
I'm going to contact Saenz and find out about reverting to ARP 2000 series and whether the rods need to be resized if doing so. Rohan do you have any opinions?
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PostPost by: el-saturn » Wed Oct 11, 2017 5:21 am

here in the alps i never heard of a new ARP rodbolt failing: have you??? just did a little bit of math and it looks like we're far from failure; SO lets, 1st of all, hear about some overstretched arp rodbolts to start with, please!!?? whereafter we may notice that these stretch gauge measurements are unnecessary?? looking forward to receive more knowledge.... sandy
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PostPost by: rgh0 » Wed Oct 11, 2017 10:11 am

ARP 2000 bolts will do all you need in a Twin Cam unless your trying to build a 10000 rpm engine using extra heavy pistons and rods ! Not a combination I would recommend for a Twink. :lol: The L19 bolt material is more for American V8s that have this combination

Light rods ( I use Carillo) , light pistons ( I use JE) and ARP 2000 bolts properly stretched will be good for more than 9500 rpm in my experience.

I have had one ARP 2000 bolt failure many years ago in the first race in a new engine ( a very expensive lesson :roll: ) and it was either a flaw in the bolt or more likely I did not stretch it right when I built then engine as at that time I did not appreciate the need for stretch gauges and was just torquing my rod bolts.

Stretch the bolts to the spec and dont worry to much about the torque. My experience is I normally need a little over 55 ft lbs to get the 5.5 to 6 thou stretch specified for the ARP 2000 bolts.

Some small permanent stretch is to be expected each time a rod bolt is stretch to the spec as you are putting stresses on the bolt very close to the elastic limit. I typically dont stretch ARP 2000 bolts more than twice as I find that can exceed the 0.001 inch maximum they say.

You should be able to use the ARP 2000 bolts in your Saenz rods as dimensionally they are the same I believe

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PostPost by: el-saturn » Wed Oct 11, 2017 3:36 pm

rohan - did you keep that bolt? i can give it to my ex partner: he's a proffesor at one of our univesities and they have all of the equipment necessary to see why it failed! ............................it's free and students need field jobs!! sandy
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PostPost by: RichardHawkins » Wed Oct 11, 2017 8:00 pm

It's 45 years since my last metalurgy lecture, and I thought that once a fastener had taken a permanent set it was not fit for further use, then I remembered creep in things like turbine blades. Is this allowable permanent change in dimension analogous to creep?

I know a bit about corrosion and hydrogen embrittlement (lifting gear in the chemical industry) and generally speaking the stronger the steel the more susceptible to corrosive agents, including things that I would not think of as particularly corrosive.

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PostPost by: Davidb » Wed Oct 11, 2017 9:26 pm

So, I fitted the ARP 2000 rod bolts that came with my rods and torqued them to the required spec-55 lbs I seem to recall-and used the ARP lube-can I check them with a stretch gauge in the future?
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PostPost by: rgh0 » Thu Oct 12, 2017 9:44 am

el-saturn wrote:rohan - did you keep that bolt? i can give it to my ex partner: he's a proffesor at one of our univesities and they have all of the equipment necessary to see why it failed! ............................it's free and students need field jobs!! sandy


Hi Sandy
My storage of failed parts is not that big luckily. It only took 1 minute to dig out the failed bolt from more than 20 years ago. You learn more from your failures than successes so I keep all my failures :lol:

PM your postal address and I will mail you the parts in the photo for examination. The other half of the bolt is still in the rod big end section. When I dug it out I realised it is an "SPS" bolt which is what Carrillo supply with their rods from new, but I understand this is the same specification as the ARP 2000 bolts and visually and dimensionaly they are identical.

My examination of the failure shows it was a fatigue cracking failure which suggests fluctuating load and thus inadequate stretch. The bolt failed in the third race of the engine. After about 1 hour of full load full revs (9000rpm max) running. There is some indication of a flaw from where the fatigue crack originated from so potentially the bolt had a flaw which triggered the failure.

failed-rod-bolt.jpg and


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PostPost by: rgh0 » Thu Oct 12, 2017 9:50 am

Davidb wrote:So, I fitted the ARP 2000 rod bolts that came with my rods and torqued them to the required spec-55 lbs I seem to recall-and used the ARP lube-can I check them with a stretch gauge in the future?


You can always release and re-torque a rod bolt and measure the stretch as you re-torque them with a stretch gauge. What you don't know is how much they have stretched since new if you did not measure and record their original free un-stretched length with a micrometer and then compare their current free un-stretched length to that.

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PostPost by: rgh0 » Thu Oct 12, 2017 9:54 am

RichardHawkins wrote:It's 45 years since my last metalurgy lecture, and I thought that once a fastener had taken a permanent set it was not fit for further use, then I remembered creep in things like turbine blades. Is this allowable permanent change in dimension analogous to creep?

I know a bit about corrosion and hydrogen embrittlement (lifting gear in the chemical industry) and generally speaking the stronger the steel the more susceptible to corrosive agents, including things that I would not think of as particularly corrosive.

Richard Hawkins


Yes you are right in that once a high stressed a fastener takes a permanent set its should not be reused. The issue is what do you define as a permanent set. ARP define that as greater than 0.001 inch. No material is perfectly elastic so a very small amount of set is acceptable over a couple of installations which is what thew ARP allowance of 0.001 inch is all about.

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PostPost by: 2cams70 » Thu Oct 12, 2017 10:49 am

Another thing with the stretching procedure. How do you normally stretch the bolts? - A little at a time progressively and alternately for each pair of bolts or do you do the full stretch on each bolt at a time? I notice on the Arrow Precision website that they recommend you first tighten each bolt to 20ft/lb. Then back one bolt off completely and put the stretch gauge on it and zero the gauge. Then tighten the bolt in one go to the full stretch figure (0.0055 to 0.006") for 3/8" ARP 2000 bolts. Once this is complete back off the other bolt and repeat the procedure.

This would seem to me to be a very convenient method because you can zero the gauge for one bolt and work on it until it's done rather than alternating between two bolts having a different zero datum point. Saenz instructions on the other hand were a little sparse - they just recommended stretching the bolts in 3 steps to the recommended figure.
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PostPost by: rgh0 » Thu Oct 12, 2017 12:05 pm

I stretch the bolts in 3 stages and do both bolts to the first, then both to the second and then both to the final.

I snug the bolts up to set them and then set the stretch gauge and torque to around 20 ft lbs for both and check the stretch, then to 40 ft lbs and check the stretch and finally to the torque required to get the 0.0055 to 0.006 inch stretch.

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PostPost by: RichardHawkins » Thu Oct 12, 2017 7:29 pm

Rohan,

Thanks for the advice.

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PostPost by: 2cams70 » Thu Oct 12, 2017 11:24 pm

rgh0 wrote:I stretch the bolts in 3 stages and do both bolts to the first, then both to the second and then both to the final.

I snug the bolts up to set them and then set the stretch gauge and torque to around 20 ft lbs for both and check the stretch, then to 40 ft lbs and check the stretch and finally to the torque required to get the 0.0055 to 0.006 inch stretch.

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Rohan


That's roughly how I did mine too. I thought the Arrow method was interesting though and it simpler and quicker. They are a very reputable company, not sure how it translates to rods made by other manufacturers though.
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PostPost by: rgh0 » Fri Oct 13, 2017 8:40 am

2cams70 wrote:
rgh0 wrote:I stretch the bolts in 3 stages and do both bolts to the first, then both to the second and then both to the final.

I snug the bolts up to set them and then set the stretch gauge and torque to around 20 ft lbs for both and check the stretch, then to 40 ft lbs and check the stretch and finally to the torque required to get the 0.0055 to 0.006 inch stretch.

cheers
Rohan


That's roughly how I did mine too. I thought the Arrow method was interesting though and it simpler and quicker. They are a very reputable company, not sure how it translates to rods made by other manufacturers though.


I never like asymmetrically loading any bolted joint as you do it up and given the crticality of the big end join I am happy to spend the extra time :D

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