M38 L134 Short Head Stud

[RonD2]

These first three posts (mine and Wes') were copied by me to this forum topic to not de-rail another members' project thread.


I'm a rookie. Never built a motor before in my life.
My motor is pickled and waiting for me to assemble it.
I had the machine shop install keenserts in all 15 stud holes.

Once I get a round tooit, I'm going to build it using these:
Quantity 15: https://www.armyjeepparts.com/p-3722-cy ... 38589.aspx
Quantity 3: https://www.armyjeepparts.com/p-215-cyl ... a1549.aspx
Quantity 12: https://www.armyjeepparts.com/p-216-cyl ... 49368.aspx

I'm going to set the 12 short studs just high enough to give me 2-3 threads showing above each nut.
Research tells me this is acceptable mechanical practice.
I figure it's one of those things (like Wes often says) that real mechanics know but you can't read in an Army manual.

If it helps:

[RonD2]

Ron,

Not sure what industry standard info you are using. Beware unless it is based on original engineering drawings and reports from Willys you are treading on thin ice. To start with just turning the stud out a few turns to add the needed thread exposure above the nut is dangerous. I have not found an engineering order, or service bulletin/instruction that tells you to do that. Using common sense to sort out issues like this is less dangerous.

For example The current hardened steel 3 1/2" studs have 9 1/2 course threads. The block has 8 1/2 cast iron threads. If you fully seat the stud you will have 1 thread in the anti-freeze. Here is where the smart technician chooses risk levels wisely. He assumes the engineer put 8 threads in the cast iron block so they would provide adequate thread engagement and strength. The more of the cast iron threads you leave unfilled with hardened steel stud results in a weak thread engagement which can and usually does lead to failure of the cast iron threads when torqueing and re-torqueing the stud nuts. You should re-think your approach to thread engagement!

[RonD2]

Thanks Wes,
Wow. That pretty much means that several editions of the ORD9's (G740, G503), the stud measurement sketch you posted, and the vendors (MWM, AJP, etc) who sell 3-1/2 inch studs advertised as correct are all wrong?
Not including 15+ years of posts in this forum and it never surfaced before today?

I don't want to de-rail Don's project thread so I'll open a thread in the Technical Knowledge Base forum.

[RonD2]

I'll open by saying I always appreciate Wes helping me understand issues like this. I'm not a mechanical engineer, and barely maybe a rookie mechanic.

I cannot cite any technical studies or reports on stud applications for the L134 motor or any other motor, and I certainly don't have access to any Willys engineering drawings for the studs, the head, or the block.

I base my decision to set my short 3-1/2 inch studs a little bit shallow because of the following:

1. My block has keenserts professionally installed at a machine shop in all 15 stud holes. The keenserts are fully threaded 5/8-inch deep and they're uniformly set .0025 below the block deck height by the machinist. Both the head and block have been freshly decked.

2. The 3-1/2 inch studs I bought have 3/4-inch long coarse threads. This means the hand-seated stud will protrude 1/8-inch below the bottom of the keensert into the water jacket.

3. Right or wrong, I've read "conventional wisdom" that says stud thread engagement in cast iron needs to be at least equal to or greater than the diameter of the stud in order to fasten and hold torque correctly. Because these studs are 7/16-inch diameter and thread engagement in the keensert will be 5/8-inch, the "conventional wisdom" is met --- with +3/16-inch to spare. Which should be plenty to short-seat the stud (only the amount needed) to achieve the necessary 2-3 threads showing above the nut with no issue holding the torque.

4. Right or wrong, I made several assumptions (I know) that the 3-1/2 inch studs as listed in several editions of ORD9 manuals and advertised as correct by several of our venerable MV parts vendors as "correct", are in fact correct. In order for them to be "correct" they must be slightly short-seated to get 2-3 threads above the nut. I found nothing contrary to these assumptions in searching 15+ years of posts in this and other forums. Maybe I missed it.

5. Remember I'm not a mechanical engineer or mechanic when I say this. It's my understanding of head studs and their torque is that while there's obviously some rotational component to it (the threads rotate), the primary function of the torque on a head stud is vertical "pull out force" (for lack of a better description). This being why full thread engagement is important. I believe I have this by using brand new studs fully engaged in brand new threaded keenserts (not 70-year old cast iron threads). Yes, I know the keensert is threaded into the 70-year old cast iron block.

6. The new nuts I'm going to use (advertised as the original style) shown in the photo above, are fully threaded 7/16-inch tall, so they also meet the "conventional wisdom" for a 7/16-inch diameter stud that I mentioned in paragraph 3 above. I believe that regular nuts aren't as tall as these.

I haven't assembled my motor yet, so the timing is good for another shot at keeping the train on the track. And my good friend Don (over on his project thread) has already identified a Grade 8 stud by brand name and part number that's 3-5/8 inch long. I'll call that "Plan B".

All your experience, expertise, and comments are appreciated. Especially yours Wes. Straighten me out.
Thanks,

[RonD2]

Just reviewed my notes and the machine shop receipt.
I have Timeserts installed in the block, not Keenserts.
If anybody is interested, the machine shop charged $18 each (installed).

[jake138]

With a perfect stud in a perfect thread hole, the first 6 threads are doing almost all of the work:

http://www.gizmology.net/nutsbolts.htm

Not withstanding further discussion on the thread percentage or the quality of those first 6 threads in Don's block and unknown torques and retorques, I would have to say that if backing out the studs from the bottom of their threaded holes one revolution weakens the threads to the point where they strip out of the block, then those holes would have needed Helicoils anyways.

One extra thread should not be the breaking point of threaded hole, and if it is, that means the first seven threads were already too far worn to be trusted anyways.

The article I linked also discusses the fact that the threads left above a nut are due to the first threads on a stud being improperly formed. The article also states that the average fine threaded nut has 8 threads, which means that by the rule stated that the first 6 threads do most of the work, the last two threads in the nut are not contributing much clamping force, and that by leaving the nut flush with the end of the fine thread on the stud, you are not sacrificing any strength.

[RonD2]

The article I linked also discusses the fact that the threads left above a nut are due to the first threads on a stud being improperly formed. The article also states that the average fine threaded nut has 8 threads, which means that by the rule stated that the first 6 threads do most of the work, the last two threads in the nut are not contributing much clamping force, and that by leaving the nut flush with the end of the fine thread on the stud, you are not sacrificing any strength.

Thanks Jake.
Interesting, but I guess I read it differently where he says:

"It is often said that two threads must be exposed above a nut. The reason for this is that the first two threads of a bolt are often poorly formed, and may not engage the nut properly. If they're not doing their share, the other threads in the nut will be overloaded, and the nut may strip."

Maybe I misunderstand something, but it sure looks like he's contradicting himself.

[jake138]

It's pretty common knowledge in every aerospace manufacturing environment I've been in that Helicoils, timeserts, or whatever you use, are much stronger than the base hardware threaded directly into a mating component. It's easy to visualize as well:

Imagine the thread size of your Timesert that's threaded into your block, let's say the thread size is 1/2"-13. Imagine that you have a stud that size holding your head to your block, and compare that threads strength to the 7/16"-14 threads you would have originally: which is stronger? Obviously the larger diameter threads with the same length of thread engagement is going to be stronger. Larger threads have more contact area and can hold more force.

Now, instead of actually using a 1/2"-13 stud, you have a Helicoil or similar installed that decreases your threaded hole down to 7/16"-14 again. The external threads of the Helicoil have the strength of the larger threads, and now the smaller stud is engaged into the Helicoil.

Virtually every threaded hole that I can recall seeing on any aluminum and magnesium aerospace housings were designed to have Helicoils installed from brand new.

[jake138]

[Maybe I misunderstand something, but it sure looks like he's contradicting himself.

I would say it's contradictory depending on the pitch of the thread you apply this to. By his own calculations, a fine threaded nut has an average of 8 threads whereas a course threaded nut only has 5. Losing 2 from 8 still leaves you within the range of "only the first 6 do most of the work", but losing 2 from 5 leaves you with a weak thread.

[RonD2]

Not worth the chance for trouble to me.
I'm shooting for 2-3 threads showing above the nut.

I couldn't agree more about timeserts.
They're truly a modern marvel for a 70-year old flathead motor.

After the machinist inspected my block and told me that about half the stud holes needed love, I told him to put inserts in all 15 no questions asked.
Best money I ever spent.

[jake138]

I'd post of picture of my engine if I could but it's still in the shop. Every single stud in my block extends at least a thread above the nut and I wouldn't change a thing. I agree that it's best to make sure you have at least that much above the nut, but if even three or four of my studs were a little short, I wouldn't sweat a thing.

[RonD2]

I'd add this if I'm not mistaken (again), but when he's says "6 threads" and "8 threads" here and there I think he's making reference to t.p.i. (threads per inch).

So, if you have a stud made with less than an inch of threads (like the short stud topic of discussion here with 9/16-inch of thread), then you don't really have all the 6- or 8-threads he's talking about. It's proportional.

[jake138]

Threads per inch or TPI, also called pitch, is the second number after the fractional major diameter in a given thread size. A 7/16"-20 thread has a major diameter of 7/16" which can be measured with calipers or a micrometer, and the pitch of 20 threads in one inch of length. Seeking a fine threaded nut with 8 threads, the thickness needs to be 8/20ths of an inch, which simplifies to 4/10ths or .400".

If you have 9/16ths of stud sticking up above head, you have .562" of stud length. Subtracting .400" for the nut, that leaves .162" sticking above the nut.

Now this is the method you'll use if you ever need to tap a hole to a specific depth. We know our pitch is measured in Threads Per Inch. That being said, if you divide 20 threads into 1 inch, 1/20 is .050". This means that quite literally, one full revolution of the nut will move the nut .050" closer to the head. So if you ever need to tap a hole to a specific depth, divide one inch by the pitch, and then divide your thread depth by that amount, and that tells you how many revolutions your tap needs to make in order to achieve a specific thread depth.

Going back to our .162" sticking up above the height of a .400" thick nut, .162" divided by .050" that means we have just over 3 revolutions or threads sticking up.

[RonD2]

Great explanation (even this old Jarhead got it)......thank you O' Wizard!

[jake138]

Glad I can impart what little I know