Correct 67 oil drain bolt

[Keith B.]

[Rob M.]

Re: Correct 67 oil drain bolt

There are several post in the archives on this subject. It seems that the JG is incorrect in the 3/4" hex size. I think they are all 7/8" hex up to sometime in the early '70s at which point they change to 9/16 hex. As far as markings go, several manufacturers were available through the years. It would seem that any give model year would have multiple head markings acceptable.

The other item in the description that may be misleading is the head thickness. Some years of the JG state 1/8" which is incorrect. I believe that the 7/8" hex plugs are 1/4" thick.

Maybe others will comment on your specific application.

[Joe L.]

Re: Correct 67 oil drain bolt

There are several post in the archives on this subject. It seems that the JG is incorrect in the 3/4" hex size. I think they are all 7/8" hex up to sometime in the early '70s at which point they change to 9/16 hex. As far as markings go, several manufacturers were available through the years. It would seem that any give model year would have multiple head markings acceptable.

The other item in the description that may be misleading is the head thickness. Some years of the JG state 1/8" which is incorrect. I believe that the 7/8" hex plugs are 1/4" thick.

Maybe others will comment on your specific application.

Rob and Keith-----


I thought the 63-67 oil pan drain plugs had a 7/8" across-the-flats (wrench) size, too. But, it's been a long time since I had any experience with these early drain plugs. As it turns out, there were THREE different drain plugs used over the 1953-67 period, as follows:

1953-56----GM #839079

1957-61----GM #3739264

1962-67----GM #3779764

I think that all of the above have the "large" head size, but it's possible that size slightly differs between them. I do not know just what the differences are between these part numbers, but I feel confident that there are some differences. There was a difference in length between the 1953-56 and later, though, so that may account for that difference.

For 1968, I'm not sure what drain plug was used. I have various reports on this---some say the "large" head size and some say the 9/16" across-the-flats. Perhaps, both were used. The initial part number for the 9/16" head size was GM #3923420 and that appears to me to look like a late-1967 or early 1968-issued part number.

1969-87-----GM #3923420

1988-96-----GM #3921988

The above-referenced are both 9/16" across-the-flats and have a flanged head.

ALL of the above drain plugs, from 1953 through 1996 used for small blocks and big blocks alike (but NOT LT-5) are FUNCTIONALLY interchangeable as all have 1/2"-20 thread. 1953-56 are 3/8" in overall length whereas all 1957-96 are 13/16" in overall length, though.

[John H.]

Re: Correct 67 oil drain bolt

Keith -

You need the Fourth Edition of the JG, which was updated about two years ago. The oil drain plug description was changed to 7/8" across the flats - the 9/16" hex flanged plug appeared later, in '68 or '69.

The JG doesn't mention headmarks specifically, but at least three different headmarks have been seen - "A" in a circle, "TR", and "L".