As you are asking assume you are replacing?
Hi Harry
As you are asking about the size I surmise that you are looking at replacing them. Welch plug is a term that may not be as familiar to your northern CMP cousins where these are commonly called incorrectly freeze plugs. I seen a number of engines that have frozen and cracked over the years but have never seen "freeze plug" even moved. What is kind of funny is if you do a search of the NAPA parts system on Welch Plugs you find the plugs listed, but if you search on the term Freeze Plugs you get no plugs but instead all the different block heaters that go in the plug hole.
These plugs and in particular the one at the rear of the block have been the subject of Service Bulletins both as directly related to Chevy CMPs engines but also for civilian engines because of leak problems generally caused by rust. GM CMP Service Bulletin B-53 6x6 also is about the plug that covers the cam shaft bore as a source of oil leaks in the bell housing. This same Service Bulletin was reissued several times for different vehicles.
Within the last year one of the guys in our MV club had the rear one develop a major leak on his CCKW very quickly emptying the cooling system. Had it been almost any of the other plugs a roadside repair of stuffing the leak would have at least slowed the leak rate but not that location it is also the only one that requires pulling the engine to replace.
Keep us posted on your results.
Cheers Phil
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