Head Bolt/Stud Tech thread...
 

So I wanted to do some testing of the head studs and bolts, to measure stretch, Yield torque, and Eventually breakage, and what torque everything happens at.

I bought a 6" long 7/16" ARP stud (6.9L) and a 7" 1/2" ARP stud (7.3L). I also have some head bolts laying around from my engines.

Tonight I tested out a couple 6.9 bolts, and the 6.9 stud, the results were the following:

6.9 Head Bolt

End-to-End Length @ Torque

5.650" @ 0ft/lbs
5.652" @ 50ft/lbs
5.656" @ 70ft/lbs
5.664" @ 90ft/lbs
Check- No Stretch- 5.650"@ 0ft/lbs
5.664" @ 90ft/lbs
5.674" @ 110ft/lbs
Check- No Stretch- 5.650" @ 0ft/lbs
Yield @ 110-120ft/lbs
Tinsile @ <130ft/lbs

Broke at the minor diameter of the threads. Minor Diameter is .350". If we estimate Yield torque at 115 ft/lbs, with a 7/16-14 thread, that gives us 19,320 lbs of clamp load per stud, and a 201,000psi yield strength.

These are actually pretty damn good bolts...

Now onto the stud...

5.998" @ 0ft/lbs
6.005" @ 50ft/lbs
6.006" @ 70ft/lbs
6.010" @ 90ft/lbs
6.013" @ 110ft/lbs
Check- No stretch- 5.998" @ 0ft/lbs
6.015" @ 120ft/lbs
6.018" @ 130ft/lbs*

*Note torque wrench clicked at 130, but stud permanently stretched .002"-.0025"

I bumped the wrench up to 140, but the stud kept stretching, and I didn't even bother to break it.

The Minor diameter of the stud is still .350", so if we estimate the Yield Torque at 125ft/lbs, that means we can achieve 30,000lbs of clamp load per stud, and a total yield strength of 312,000psi.

The minimum yield strength on a set of 8740 ARP studs is 190-200Kpsi, that means they are severely under-rating their studs as treated. Which is a good thing.

So bottom line, on a 6.9, We can get away with 110ft/lbs on the stock head bolts for a total clamping load of 96,600lbs per cylinder. Or we can get away with 120ft/lbs on ARP chromoly studs for a total clamping load of 150,000lbs per cylinder, over a 50% increase over the head bolts.

I have to get ahold of a torque wrench that will go higher than 150ft/lbs to test the 7.3 stuff, so as soon as I do that, I will update with that info.

If anybody wants to check my math, feel free.

Very Nice....I went 115lbs on my stock 7.3 bolts....Should be in great shape... your going to get a box in the mail in a few days....I will get in touch with you very soon I have some questions for you...

Nice! How do those numbers compare to your expectations of the H13 fasteners that you were looking into? Will you be doing tests on that material too or???

These significantly surpassed my expectations, so Im hoping the H13 studs will do the same. I ordered up 6 extra of each stud size to run the same test on. Its quickly showing that the 1/2" studs will never be a problem, even in ARP chromoly, Im hoping to see even better numbers out of the 7/16" studs...If They can safely be taken to the 150ft/lb range, then I think our head clamping issues will be solved.

I hope you are right... Do you think the more brittle nature of H13 will introduce any problems with breakage compared to the ARP material?

Well it shouldn't if its heat treated correctly. Im going to have the metallurgy shop treat it at the upper end of the range, but it will still be tempered to spec, so I don't see any issues.

We'll see I guess.

so once you find the yield torque of the stock head bolts (mine are 7.3L) does that mean I can tighten up to the safe level anytime, or is it a no-no to tighten head bolts at any time other that during a gasket change?

It would be fine to tighten the bolts as installed, the cummins guys do it all the time.

I think we are going to be surprised at the clamp load we can get out of the 7.3 bolts...

Nice, I'm looking forward to the 7.3L results. It will be good to check and tighten my head bolts when my turbo goes on. Nice work BTW. Thanks

At what point should we concern are selves, could the heads be warped and or damaged by excessive clamping force? :(:-huh:-X19:-X19:-X19

I don't think we will ever get to that point.... Id be worried about cylinder wall deflection more than anything with head fastener load. Eventually Ill get around to building a Rentable Torque plate for these things so we can get machine work done properly.

So I intended to have a 7.3 stud update tonight, I bought a 250ft/lb torque wrench and fully intended to make a 7.3 ARP stud my bitch... However, I was going to do it by drilling and helicoiling my spare 6.9 block for a 1/2" fastener.... I got to the point of needing a 17/32 drill for the helicoil, and I realized that I didn't have one In all my junk.

So tomorrow I will pick one up, and hopefully have time to do all that, and get some results on here.

Of course, I am also thinking about ordering up another stud, and testing two of them to make the results consistent with the 6.9 stud test. Ill probably do that just to be safe, but I want to see what I can actually get away with on a helicoiled 7.3 stud first, that will transfer directly over to my own build.

Hang tight...

While you're experimenting, what would a 5/8 bolt/stud do? Especially one halfway-decent rated.
My calculations indicate that the minor diameter of a 5/8-11 thread is smaller than the major of a 1/2-13, so you could even drill out a 7.3 block to that size... If breaking through into the water jacket etc. wasn't an issue.

I don't think 5/8 would ever be possible. However, 9/16 is worth screwing with. That being said, I think the 1/2" stuff is going to get us where we want. Based off of what the 7/16 stuff has shown, I think the 1/2" stuff is going to be more than anything we need, at least with where we are that...

Of course, those words have never been said before :D....

Ok, Here is a relatively undetailed update on studs for this thread.

1/2" AND 9/16" studs CAN and WILL work in a 6.9 block.

The thread interference going to a half inch fastener is not enough to cause thread failure in the block. I tested a 1/2" stud in a drilled and tapped head bolt hole on the 6.9 block, and I took the half inch stud all the way to yield, which was in between 180 and 190ft/lbs of torque. The threads in the block held.

I also drilled a bolt hole for 9/16" threads, and took the stud all the way to 250ft/lbs, which is where my Torque wrench maxed out. I only got about a half of a turn the whole way up from 80ft/lbs to 250ft/lbs, so the 9/16 stuff does not stretch much at all. The threads in the block held.

The critical issue with drilling the 6.9 block, is the fasteners have to be drilled to 6.9 depth, they CANNOT be drilled to 7.3 depth, or the casting will fail as there is no support below the 6.9 boss depth.

On a side note, I also observed that the 1/2" stud stretched at the top fine threads just below the nut, which is odd, because the minor diameter of the course thread at the bottom is smaller than the minor diameter of the top thread. I can only chalk this up to the bottom threads being rolled and the local compression of the material causing the material to be stronger, or as strong.

I have another 1/2" stud on order, and I plan on testing the 1/2" and 9/16" studs like I did the 6.9 stuff at the beginning of the thread, this weekend, that way we will have numbers to know exactly where we are. I will also test a stock 7.3 head bolt, and it ill be interesting to see of that compare to the 6.9 ARP stud numbers.