small block timing cover 1970 and up

[str12-340]

I have a stack of small block timing covers stashed and someone local needs one (currently has a mix of 69 and 72 parts with no way to see the timing marks). I pulled out the stack of 70 and up covers that I have and they have 2 very different seals in the lower hole: a smaller hole in the ones date coded 72, and a much bigger one in the ones date coded 78. First - will these covers interchange as long as you have the right seal? Does anybody know why and when they changed the design?

 

[Newbomb Turk]

Yes, they interchange if they are not magnum covers.

The later one you can install the seal from the front.

 

[townsend]

Timing cover #2951671 was used on 1970 thru 1977 small blocks. You had to remove cover to replace seal.
A new cover #3769964 used in 1978 and later LA small blocks. Seal could be replaced without removing cover.