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I got an RMC Engine to rebuild for my M38 serial 106678. It has the correct casting number 804380 for the waterproof bell housing but the left side engine mount is different to the pictures in the Miller book. It came from a generator. The tab for the mount faces backwards not forwards. Will this work in the M38 if I just reverse the rubber engine mount? I was rather surprised that the same casting number would have different engine mounts. Also head casting says Willy’s Industrial and casting number is 807763. Would that have been a military application? Hoping I haven’t wasted my money...

I wouldn't say you wasted your money. If you have a M38 engine besides the new one then you have a source for the parts needed to qualify this industrial engine for a M38 installation.

1 - The front steel mount plate on your industrial engine is the early CJ with both mount ears bent aft.


Here's the CJ2A front mount plate.


This is the correct front mount plate for your M38.


You can often find the correct front mount plate for sale alone.

2 - The industrial engine uses a different carb and intake usually. The intake manifold is smaller than the M38's.

3 - The industrial head is also different.

Here's one:

https://www.ebay.com/itm/Jeep-Willys-M3 ... SwyQtVkETD

Thanks for the replies.

Wes, would an industrial engine ever have been put in an M38 using the original M38 carb, manifold and front plate? Also are the M38 and industrial heads different compression ratios? I have original M38 intake manifold and YS carb. Lastly, and thanks for your help, could you estimate the year of manufacture from the serial number?

would an industrial engine ever have been put in an M38 using the original M38 carb, manifold and front plate?

I am sure at some point they may have been. However in your case you have a block with an RMCXXXXX serial number and 804380 Casting #. That block was not produced until 1952. The RMCXXXXX engines were all factory supplied to the US Army as brand spanking new replacement engines. Each batch of RMCXXXXX engines were produced specifically for the US Army's M38 contracts. Thus their purpose when they left Willys were to be replacement engines for the Army's M38. I am sure in the field the Army used them where they needed them. So many of these could easily have ended up in Army generator sets. We haven't even begun to discuss what civilians who purchased 100's of these RMCXXXXX engines from Army surplus sales might have done with them. Now the 804380 casting RMC engine came with the military front mount plate that had rt ear facing aft and left ear facing fwd with a relief in the front of the left ear to allow access to the dual fuel action fuel pump. Earlier casting # civvy industrial engines would have came with the mount left ear aft. The 804380 casting for the industrial engine came with the CJ3A/DJ3A front mount which had the left ear fwd but did not have the relief for the pump or the heavy generator front mount bushing for the m38 generator.

Also are the M38 and industrial heads different compression ratios?

The 804380 Industrial engine came with the CJ3A front plate and 6.48:1 compression.

The 804380 casting enterred production jun 1952 and continued in production thru the end of DJ3A production. So it would be very difficult to nail down a year or month of production. However since Willys blocks were all serialed sequentially no matter what prefix was used you could track down the same or nearly the same basic serial in another prefix and if they have a good data base sort out your engine's date of production.

Thx Wes. I’ll get a correct engine front plate from one of the forums or the internet. My M38 may not be 100% original military correct with the industrial head but it could have happened so I’ll live with it.

Try to remember that the water pump bypass was introduced to the vehicle applications of the head to improve warm up characteristics which addresses performance more than appearance.

wesk wrote:...Willys blocks were all serial sequentially no matter what prefix was used ...

I've read that for MC/RMC blocks... this holds for all of them? J/3J/4R/U/T/RJ/etc.?

All that used the 804380 start with serial 10001 and go from there until production ends irregardless of model jeep and engine serial prefix. Same goes for each of the other castings 641087 and etc.

wesk wrote:All that used the 804380 start with serial 10001 and go from there until production ends irregardless of model jeep and engine serial prefix. Same goes for each of the other castings 641087 and etc.

I did not know that... much appreciated!

My RMC engine is a late 804K replacement and it came with a motor mount for an MB, both ears pointing the same way, it was green and had been there forever. Perhaps installed by the Army? Delivered that way?