The Clausager book has the info you need. The one big clue I'm aware of
is the tach drive. 18G and GA engines (the 3-main ones) had a mechanical
drive for the tach, off the rear of the cam. Later ones have a electronic
Larry Hoy had this to say:
>What is the best way to identify the "vintage" of an MGB
>engine that has no ID plate. This is an earlier engine; the
>block is machined for a generator, not alternator and it is
>a "pre-smog" setup. I really would like to know if it is a
>five main or three main engine. It presently is in a 1964
>car and is running. I know that the 1964's came with a
>three main but this may not be the original engine.
>Any help would be appreciated.
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.