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Old 03-10-2013, 08:13 PM
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Just one ARP rod bolt doesn't feel right during torquing

I wasn’t going to post about this problem, but I guess I can take the expected ragging and although I am fairly certain I already know what I should do, the fact is I’m not sure, so maybe this is worth posting.

About 2 years ago I rebuilt my FE (broke a piston; timing set wrong as balancer had slipped) and as I was torquing the rod nuts, I came across one that felt funny. It did come up to torque, but seemed to come up more slowly than all the rest. In other words, it took more of a turn to get that last bit of full torque.

For example, if all the other rod bolts took ľ turn to got from 40 ft-lbs to the last step of 50 ft-lbs, the one bolt in question took more like Ĺ turn.

I gave it a good visual inspection, removed the bearing shells to check for grit behind them, tried swapping nuts, but couldn’t find anything wrong. (I didn’t really know much about measuring bolt stretch at that time).

Based on the fact these were ARP rod bolts, only torqued 5 or 6 times, and had 4000 miles on the engine, I decided to continue on with the assembly. Also, when applying torque, it didn’t feel like the bolt was stripping, as the torque did keep building. When threaded fasteners strip, the torque flattens out, and then drops as the threads become more an more damaged. My experience with stretching bolts is limited to removal of rusty stuff, and in those cases, the bolt doesn’t seem to yield at all, but suddenly snaps off. But that’s more of a shear thing than tension.

So, mistake number one; I finished the engine assembly without ever finding out what was going on as well as rectifying the problem. Yes I feel stupid about that now.

Eventually I removed the oil pan (for a variety of reasons) and checked the rod in question. Everything looked fine, and the nuts didn’t move with the torque wrench set to spec. This was after about 1000 miles of operation. I might have hit 6000 rpm a few times, but generally I short shift just having fun on the street.

Mistake number 2: I buttoned it up again without finding out and fixing what might have been wrong. Admitting that makes me feel twice as stupid.

I’ve now driven another 500 trouble free miles and everything is working fine. But I have this nagging thought the bolt might fail.

As I’m still worried, I’m thinking of replacing the bolt, however in the meantime, I did a bit of research and also some rod bolt testing.

I thought I’d share what I found out as it may prove of interest to all of you, but take it with a grain of salt. My measuring technique was without a proper stretch gauge, and I'm not sure if I understand all the theory 100%.

1/ I tested a couple of old (stock) connecting rods had from a 390 parts engine. Interestingly, one of the 4 rod bolts I tightened, felt the same as the mystery bolt in my engine. Basically, the torque came up more smoothly and slowly than the other 3.

This could be explained by the following pie-chart which shows how much of the applied torque is wasted in frictional forces. Basically, if the nut seat on the connecting rod cap and/or the bolt threads were a bit more burnished than the other bolts/caps, this could explain it. A bolt failure analysis I found on the net had this to say:

i. The thread friction coefficient could vary between 0.14 and 0.25.

ii. The under-head friction coefficient could vary between 0.10 and 0.18.

That’s a lot of change from one seemingly identical fastener to another.



2/ Next I decided to try to measure the stretch. It wasn’t easy as I only had digital callipers to work with and these Ford Factory bolts have uneven ends. Here’s what I found out about stretch:

At 40 ft-lbs, there was no stretch that I could detect.

At 50 ft-lbs, the stretch was about 0.004

At 60 ft-lbs, the stretch was about 0.009

Keep in mind, these Ford bolts have a torque spec of 45 ft-lbs and my ARP bolts in the engine have a 50 ft-lb spec.

3/ For the next step I decided to torque to yield. Basically, the torque tapered off around 70 ft-lbs and then dropped to 65 ft-lbs. It stayed at 65 ft-lbs for many turns of the wrench as the bolt stretched. I never did go much further to see if it would break. See below:



ARP states their specs are for a 75% clamp load. You can see that’s quite a ways down from the yield point and on the linear portion of the stress-strain curve.



It appears that when my test bolt stretched it was high up in the yield area, and although it was really stretching, it had not yet fallen off the chart to the point of failure. The test bolt is definitely strong for its size and certainly not brittle.

When I’ve broken (other type) bolts in the past (from loosening), they have just suddenly snapped off. This was due to either corrosion on the threads or under the bolt head. My over-tightening experience has only shown up in stripped threads due to multiple disassembly and reassembly, but these wern't rod bolts either. These rod bolts really stretch, and even the one I stretched a lot, still had some spring back, although no where near its original length.

4/ Based on information from those engine-heads with more experience under their belts than I, tightening an ARP bolt by the stretch spec requires a much higher torque than the torque spec. Some thought is the type of lube used on the bolt threads and under the bolt head must be of a certain type, or the readings wont correlate.

5/ If my ARP bolt was really stretching when I tightened it, I would have either had to be above the yield point, about 67 ft-lbs (if the ARP spec of 50 ft-lbs is 75% of yield), which it wasn’t, or I would be on the downward side of the stress-strain curve and the bolt would have shown signs of stretching, which a visual inspection did not show.

6/ Based on all this, as well as the fact I’ve quite a lot of miles since the re-build, I doubt my installed ARP bolt will fail, although I may be merely trying to convince myself. Whatever I decide to do, I think I’m in the market for a proper stretch gauge.

So my questions are:

1/ Do you think the rod bolt was stretching? Other thoughts might be excessive bearing crush, deformed rod cap or nicked/burred bolt threads.

2/ Based on merely a feeling something wasn’t right, should I bite the bullet, stop running the engine, and start a teardown, or based on the engine lasting 1500 miles so far, and a check at 1000 miles indicating things are fine, just keep driving it?

3/ Has anyone ever had a similar experience tightening rod bolts, particularly ARP, and if so, what did you find out?

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Old 03-10-2013, 08:24 PM
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I'd leave it and run it as is. Stretch is a much easier way to measure the tensile stress the bolt a fully has the. Trying to correlate torque. As you found out torque can be off by a lot for many reasons. Torque is actually relatively meaningless but it's a quick and easy way to guesstimate tensile load and has seemed to work well enough in most applications. Stretch is much more accurate but sometimes good enough is good enough.
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Old 03-10-2013, 10:05 PM
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Its very likely the bolt head wasn't seated in the rod fully. The extra 1/2 turn fixed it. I wouldn't worry about either.
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Old 03-11-2013, 12:14 AM
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You are that pretentious! If you assembled the engine with that fine a microscope, I bet that its fine.

A extra 1/2 turn and you are going to tear it down. That is only one side too. When the bearing cap was fitted there could have been a little more clamp left to do.
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Old 03-11-2013, 05:38 AM
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While I agree with your findings and the comments, you will ALWAYS have this nagging doubt about that bolt and the 'What If' factor. For peace of mind, change it out. Give ARP a call and discuss it with them as well. I had some questions a few years ago when building an engine. The CS tech spent quite a bit of time plus supplied some replacements at a much reduced price
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Old 03-11-2013, 09:02 AM
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Thank-you for your responses. Although I've tried to do some research to convince myself everything's OK, I am still paranoid about it. I'm also lazy and am not looking forward to swapping out the bolt.

Winged oil pan (bolt access isn't the easiest), windage tray (so two gaskets) and all put together with The Right Stuff (tough as nails). Going to be nasty getting the pan off. But nastier still if that bolt ever fails.

With regards to some of the comments, I am sure the bolt had pulled through as far as it could (i.e. it was seated). I tired and retried torquing this bolt, but always with the same result. I even swapped the nuts from the other side of the rod, but no difference. I also tried tightening the problem bolt first in case it was due to too much bearing crush, but no difference.

I've chalked it up to either the surface of the rod cap (under the nut), the bolt is actually failing, or the cap may have been distorted (see pic below).

I am under the impression that a rod bolt goes through more stress during tightening than it ever does while the enigne is running, and that's why I've been taking the chance it's OK.

However, as I worry, I think I'll change it despite the effort. And won't I be ticked off if the new bolt behaves the same way when I tighten it! Then what? LOL.

And good idea about calling ARP. Will be interesting to hear what they have to say.

******

Now here's sort of a pic which could explain things a bit. I suppose it's possible the cap may have become distored a bit and doesn't fit quite right. I exagerated the "gap" in the pic. So it's possible the bolt is merely pulling the gap together when I tighten it. I didn't think of this at the time, or would have checked the rod cap.

Could have happened I suppose. Prior to the rebuild, I did rev the engine a bit too high once... a bit over 6200 rpm and it does have stock rods.

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Old 03-11-2013, 10:30 AM
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Replacing the bolt(s) will also require you to resize the rod again. Since the caps parting surface is cut on an angle, this is obviously a flaw or defect from it being resized to begin with and should also be replaced as well.
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Old 03-11-2013, 11:09 AM
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The FE is very finicky about rods and rod bolts. The rod journals and bearing shells are quite large in diameter while being quite narrow as a percentage of the diameter. This makes it easy to blow the oil wedge out with the disastrous results you'd expect. This is the problem with the FE that the 406 and finally the 427 that could no longer be ignored which caused Ford to explore cross bolted mains, side oiling with priority to the mains thus the rods and finally with the Leman and NASCAR cranks wider journals and bearings and cap-screw rod fasteners. So this is no place for a so-so fastener that may not cause a problem in other engines but one that can easily lead to disaster with the FE.

I back up your through research on plastic deformation of bolts and say YES you have a bolt that is stretching too much for the torque being put on it. And YES torque is a secondary way of measuring the stretch, the stretch being what's important. The assumption, made by test, is that when the friction in the threads is uniform from bolt to bolt (also unstated is the quality of the steel and manufacturing processing against the engineering specification resulted in bolts that actually meet those technical specifications) then a relationship can be made relating an applied torque resulting in proper stretch. But you can see the possibilities where these assumptions fly apart. Measuring bolt stretch gets around the problems inherent with tightening to a torque spec and hoping the inter-thread friction is at least close to the test articles that made the spec. But even measuring stretch is no guarantee that a problem doesn't exist with the bolt continuing to stretch at that applied load. That is if you arrive at a load that gives the proper stretch but that load levels off and the bolt or nut in this case continues to rotate the bolt is continuing to stretch and will neck down as in your picture and will eventually fail at that point.

Your example of the rod cap forming an angle with the shank is a common occurrence with rebuilt rods. It adds a bending load to the rod bolt which is not desirable, one reason why I don't use remanufactured rods. This could give you a similar feel to that of plastic deformation in the bolt from neither the cap or shank mating flat to each other as well as a similar event occurring between the nut or bolt head if using cap screws.

ARP uses a proprietary thread lubricant where most OEMs use motor oil. Anytime you switch the lubricant you must adjust the torque to the lubricant being used. The thread lubricant also needs to be used on the underside of the nut or bolt head where contact with the part to be fastened is made.

When I use a torque wrench I always raise the torque in small amounts 10-15 foot pounds and stop about 5 foot pounds under the end value letting the fastener rest a few minutes then pulling the final 5 foot pounds which shouldn't take much of a turn. Then I let the fasteners rest again, then go back and check each. The torque should read correct with no further rotation. If the fastener moves, the part gets disassembled in rotation and the offending fastener gets replaced. Also in this situation if itís fastener threading into a casting, that threaded hole gets a very close inspection to identify whether the casting itself or its threads are failing.

I certainly would not have continued to close the engine up with a situation you are seeing with that rod bolt, especially with an FE that will see some high RPMS as this engine is not very tolerant of problems in this spot.

Bogie
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Old 03-11-2013, 02:21 PM
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I called ARP, explained the situation, and asked if it were possible I was stretching the bolt when only applying 50 ft-lbs of torque while keeping in mind a stock Ford bolt took 75 ft-lbs to yield and then a constant 65 ft-lbs to keep stretching it.

He said it was probably stretching and I should replace ALL the bolts. He did base this partly on the fact I've had my engine stripped down 3 times. He says some people get away with re-using them up to 10 times, but others feel they need to replace them every tear-down.

The situation gets worse every time I ask a question... LOL.
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Old 03-11-2013, 02:24 PM
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Quote:
Originally Posted by Argess View Post
I called ARP, explained the situation, and asked if it were possible I was stretching the bolt when only applying 50 ft-lbs of torque while keeping in mind a stock Ford bolt took 75 ft-lbs to yield and then a constant 65 ft-lbs to keep stretching it.

He said it was probably stretching and I should replace ALL the bolts. He did base this partly on the fact I've had my engine stripped down 3 times. He says some people get away with re-using them up to 10 times, but others feel they need to replace them every tear-down.

The situation gets worse every time I ask a question... LOL.
Worse would be the rod putting a new breather hole in the side of the block. That's big time worse!

Bogie
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Old 03-11-2013, 02:44 PM
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Quote:
Originally Posted by Argess View Post
I called ARP, explained the situation, and asked if it were possible I was stretching the bolt when only applying 50 ft-lbs of torque while keeping in mind a stock Ford bolt took 75 ft-lbs to yield and then a constant 65 ft-lbs to keep stretching it.

He said it was probably stretching and I should replace ALL the bolts. He did base this partly on the fact I've had my engine stripped down 3 times. He says some people get away with re-using them up to 10 times, but others feel they need to replace them every tear-down.

The situation gets worse every time I ask a question... LOL.
So the customer service rep told you that you NEED to buy more parts from them... Sounds like he loves his job.

A better response would be- check the stretch. Done.


But if you like dancing around the problem there are plenty of people who will love to sell you some music.
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Old 03-11-2013, 02:50 PM
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Originally Posted by oldbogie View Post
Worse would be the rod putting a new breather hole in the side of the block. That's big time worse!

Bogie
Yes, indeedy!!!

Now there's another question. I see no problem tapping out an ARP rod bolt with the engine in the car and then installing a new one (once the damn pan is off), however there is the thought that rod big-end resizing is required when installing new rod bolts. It was even mentioned earlier in this thread.

Is this true for replacing a rod bolt with an identical one, or is it reserved for when switching from a stock bolt to an after-market bolt?

(suspect I'm gonna cringe when I read the response to this...)
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Old 03-11-2013, 02:56 PM
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So the customer service rep told you that you NEED to buy more parts from them... Sounds like he loves his job.

A better response would be- check the stretch. Done.


But if you like dancing around the problem there are plenty of people who will love to sell you some music.
He did say I ought have checked the stretch the first time I used them and keep a record.

As far as dancing around goes, I've decided to do something so I can sit down and rest. At a minimum I will replace the problem bolt, but mostly so I can worry less.

Stupid as it was to have not rectified the problem when I had the chance, it's hard to believe that particular bolt was yielding when I torqued it. It's more likely due to a different reason, but as I can't identify what that might be, I will have to replace the bolt and see if it acts "normally" when I tighten it.
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Old 03-11-2013, 08:19 PM
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Originally Posted by Argess View Post
Yes, indeedy!!!

Now there's another question. I see no problem tapping out an ARP rod bolt with the engine in the car and then installing a new one (once the damn pan is off), however there is the thought that rod big-end resizing is required when installing new rod bolts. It was even mentioned earlier in this thread.

Is this true for replacing a rod bolt with an identical one, or is it reserved for when switching from a stock bolt to an after-market bolt?

(suspect I'm gonna cringe when I read the response to this...)
The problem isn't with replacing a bolt of like quality, its the actual removing and re-install that upsets and distorts the housing bores.....As Bogie pointed out, the FE rods aren't the beefiest rods out there, and no matter how delicately you try to remove and re-install the bolts, they don't ever re-align as they were before you did because you spread the rod apart where they are the weakest, across the bolts, where the cross section is the thinnest, so they have to be reconditioned to re-establish the indexing of the caps to the rods as well. Believe me, I have recon'd thousands of sets and have tried every possible way to change bolts without having to redo the B/E and its just not possible. Your taking a huge gamble to even try it without checking them either.
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Old 03-11-2013, 09:40 PM
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Originally Posted by oldbogie View Post
Worse would be the rod putting a new breather hole in the side of the block. That's big time worse!

Bogie
You have to lay this out for me. Get me from here to there. How is that going to happen? This was because of less than a 1/2 turn.

When you worked with plasti gauge and you torqued stuff down, how much difference did 1/2 turn make on the clearances? For me the difference could have been negligible.

Argess, supplied a lot of information, but is he over thinking this? I think so.

Why was there a deviation from procedure in reguard to torque steps?
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