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What is the significance of the SPEC X cast upside down in my block? It is a 641087-LW-12-A-N1-OR-N2 (as near as I can read it), with two freeze plugs.



I can't read the other end of the left side very well, even looking at the real thing. Would these numbers be the mftr date stamp?

Your question is why my M38 survey form asks for folks to tell us about all markings they find on their blocks. You are the first member to mention that marking.

Here is what is normally found there. I assume your block is the later reinforced rear web the same as this block.

My best guess is SPEC X means special run X. There is a possibility that it may be one of a series of late style blocks produced as a test run in Dec51/Jan52 for the M38. What is your block's serial number?

the only number I can find is stamped above the water pump is 36513

You will need to remove that paint with a lifter like DOT 3 brake fluid so you don't rub off the last of the serial prefix. All Willys L134's had a prefix to the 5 or 6 digit serials. They are stamped rather lightly and whenever a machine shop decks the block they get fainter.

Here's a few examples of how faint the prefix can be:








This one is sharper than most.


Here's a brand new factory serial stamp! Note the factory did not paint over the serial with OD!!!

I just assumed the top of my block had been decked and in the process the numbers had been ground off the top, then a new stamp put on the face above the water pump.

I'll do some chemical stripping and see if there's anything under the paint. But it's smooth as a babies butt up there....

I'll check the webbing and your other survey questions, too.

Thanks for the best guess about "run X" in 12/51, 1/52. Very interesting.

Phase 1 of the block serial number quest was inconclusive. Under the paint it was very nicely decked and not even a hint of any letters or numbers. I'll see if I can work some forensic magic on the deck and try to raise the numbers.



ITMT, I can say that there are only two freeze plugs, but only four bolts at the back end. And here is NO ribbing or reinforcement back there either.



There is a reinforcement rib down the back but no freeze plug. And at the back lower right side of the engine I think it reads 12 7 .



I need to correct the casting number, I read it slightly wrong. I belive it reads: 641087-L-W-12-A-N1-CR-N2

Pictures added to the post above. Perhaps even without a serial (hopefully to be raised from the dead, so to speak) we can get a closer idea of the date of manufacture. Any extra information on my "Spec X" block would be greatly appreciated.

I think my jeep will now be named "Special X".

1-What is your engine’s serial number? 36513 (so far, albeit probably incorrect)
2-Engine Casting number (right lower side of block) 641087-L-W-12-A-N1-CR-N2
3-Is there a oval serial space on the upper right front side behind oil filter? NO
4-Does the rear flange of the block have 4 or 10 bolt holes? FOUR
5-Does the right side of the block have 4 or 2 expansion plugs (freeze plugs)? Two
6-Is your timing mark on the flywheel or front crank pulley? unknown

Well if the engine shop that decked the block was the responsible agency for relocating the 5 digit serial then they may have ignored the prefix deliberately.

The serial and the fact the block is a phase 33 641087 rule out any possibility it was part of the Dec51/Jan52 test run of the new style reinforced block.

There are several well known methods used by police to read filed off serial numbers.