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what are the difference between these two engine? when & where it was used? TIA...

The MCxxxxx or MCxxxxxx engines were MC serialed engines installed at the Willys factory in M38's. RMCxxxxx and RMCxxxxxx were factory new replacement engines shipped directly to the logistics system needing them. From there they were dispensed when needed to replace in service MC engines on M38's. These MCxxxxx and MCxxxxxx were also installed in power plants and generator sets. They also were replaced when necessary by the RMC engines.

Each of the procurement contracts for these military jeeps usually included the request for certain spare parts and spare engines. There are six known M38 procurement contracts. So there was in all likelihood six requests for factory built spare engines. This is why the RMC serials are found on the 2nd and 3rd series 641087 castings and the final 804380 casting. These included both early 5 digit serials and later 6 digit serials.

Wes K,
bottom pic doesnt have a data plates...
so... this RMC90157 is not her original engine then.
any other infos pls.
again, thanks

Correct. RMC is a replacement new factory M38 engine and would not have come in an M38 from Willys.

Since that serial is newer than MC/RMC74419 it would be a series three 641087 casting with the reinforced rear flanges using an additional 3 5/16" bolts on each side, This block has only 2 freeze plugs on the right side.

Wes...Do you have any idea as to when (years ) those RMC engines where made ???

Reason being I have two jeeps here with RMC engines in them one of which is a M38 and the other is a CJ2a ...

If you visit our data base you can interpolate about when. The serialization does not start over just for the R prefix. For example if Willys pumps out ten MC engines today serials MC74420 thru MC74429 then they start a run of RMC's with serial RMC74430 thru say RMC74530 then they go back to MC production with MC74531.

Gottcha...thanks