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Hi. Just picked up a head for my M38 and the thermostat housing has a threaded square head plug in the top. My other one does not and I haven’t seen any pictures of any that have. Looks like it could be a great help for venting the system when filling but could it be a late modification or maybe civilian? It is painted what looks to be the correct shade of green but that doesn’t mean much really. Any ideas?

Sounds like you have a head off a Willys engine used in a PE-95 Generator Unit. Can you post a picture?

M38 engine thermostat housing only has the raised boss.


Here's a PE95 engine with the wide housing and the temp gauge sender on top.

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Here's another photo showing the housing and sender alone.

Housing doesn’t look like the PE95 one. It’s exactly like the top one and two others that I have but the top boss is threaded to take a plug like the one by #4 spark plug in the top picture. All the casting marks on the head match the late M38 head marks in the M38 green book.

The correct M38 head is a 800376 casting. The M38 version has the raised boss at the front vertical surface drilled and tapped for a pipe thread fitting that accomodates the water pump bypass. The civilian version of this head does not have that raised boss drilled and threaded. The thermostat housing was raised boss top center only with no hole or threads. You should post a photo of what you have so we can offer something better than a wild guess at what it is.

Housing doesn’t look like the PE95 one. It’s exactly like the top one and two others that I have but the top boss is threaded to take a plug like the one by #4 spark plug in the top picture. All the casting marks on the head match the late M38 head marks in the M38 green book.

And does this odd thermostat housing head have the open, threaded water pump bypass port at the front end?

Hi Wes. The head does have the raised boss drilled and tapped with a threaded spigot in place for the water pump bypass so I’m sure it is the correct head for an M38. The thermostat housing has A1192 then a gap then 1 stamped on it running from near the flange and going towards the dome. The bolts are new so I have no way of knowing if this housing belongs with this head.

Well then the solution to your problem is quite simple. Take the thermostat housing from your old head which is correct for the M38 and put it on your new head and take the unusual housing from your new head and place it on the mantel or Ebay.

Usually a M38 cylinder head when purchased brand new or used came by itself with no accessories bolted to it, so essentially you have been fortunate to not only find a replacement head, you also have a very unusual housing to discuss the next few years with other jeep folks.

A-1192 is the correct part number for the Willys MA, MB, M38, CJV35, CJ2A, CJ3A, 4x4-63, Wagons 2WD & 4WD, and Industrial L-134. It may be time to guess that just maybe someone over the last half century took the liberty to drill the boss and tap it for a pipe plug or some other specialty application they had envisioned for it.