M38A1 valve cover studs

kd4kgx · Post by **kd4kgx** » Sat May 29, 2010 2:43 pm

I have a 1964 Kaiser-Jeep M38A1, USMC model. I broke one of the valve cover studs and am having difficulty finding a replacement or matching my stud to the specs in the 1956-era ORD 9 SNL G-758. The manual gives this spec: STUD, hex, spec, S, 5/16-24NF-3 (1/2) 5/16-24NF-3 (13/32) x 2. However, when I put a 5/16-24NF die on the stud, the die is a fraction too small and actually cuts into the threads. Do the late model M38A1 engines have a different size stud? If so, what is it, and does anyone know a source?

Thanks for any help!

MCS

BullRun · Post by **BullRun** » Sat May 29, 2010 7:50 pm

I have a 1962 USMC M38A1. I will check mine and post the information. There are many little details on my jeep I have found that are different from the earlier M38A1's. This might be another one.

Post by **wesk** » Sat May 29, 2010 9:15 pm

Is your die a quality US made die? Is it a split die or solid die?

kd4kgx · Post by **kd4kgx** » Sat May 29, 2010 9:52 pm

Thanks guys. The dies belong to a mechanic friend and I don't recall the brand name, but assume they are quality. At any rate, they are definitely split--would that make a difference?

If I can find a plain bolt of the same size and thread count I'm going to try to thread it into the head and see if it works. That should tell me something.

BullRun · Post by **BullRun** » Sun May 30, 2010 10:03 pm

Ok, so I looked at a 1952 and 1962 M38A1 and the parts appear identical.

The quick answer is that they are the same and also match the listings in the 1956 parts manual.

The long answer is I had the same thing happen to me when I ran a die over the threads with the metal dust and small shavings coming off. I used a solid professional quality die 5/16-24 NF as required for a 5/16-24 NF stud.

So here are 3 ideas as to what is going on here.

1) After decades of heating and cooling cycles the metal has on a microscopic level "creeped" or expanded outward slightly from it original dimension.

2) Or, the modern tools we have used have a tighter tolerance than the original parts. For instance if the stud had a product variation of + or - .010 to spec. and the modern die has a tolerance of + or - .005 to spec. the shavings are the .005 difference.

3) Different production runs with different out of spec. tolerances similar to #2.

Also, rust, grease and dirt can cause a die to wobble making a clean cut more difficult.

One more thing while listed as a plain nut both of the parts on each jeep had what appear to be ferrules pressed into each side I guess to prevent stripping the threads. And strangely the nuts while appearing identical would not interchange making me think that expansion and contraction has made them (nut and stud) a matched set of parts over time making replacement parts not fit together easily.

I am sure someone has a NOS or replacement stud and nut to fix the problem.

kd4kgx · Post by **kd4kgx** » Tue Jun 01, 2010 8:21 am

Thanks BR. I found some 2-inch 5/16-24NS bolts at the local Tractor Supply store and they fit the threads in my cylinder head just fine. I think you're correct, it's either that the tolerance of the old bolts is different than the modern dies, or else the bolts have expanded over the years. Maybe both. I appreciate the help!