Basically the 8-bolt engines are late production B20's with the B21 bottom end making an early appearance. So in addition to the obvious difference in the flywheel mount, the crank has different sized bearings on the rod throws, the rods are different on the big end (to match the crank) and on the small end (24mm wristpins just like the B21). And the pistons have 24mm wristpin holes as well.

Being late model B20's 8-bolt engines will (at least originally) have had a low compression F head on them. According to those who know such late production F heads have a little more metal in the right spots and can be worked on more to produce better flow - although stock there is little if any difference.

Big bore rebuils are a little easier on 8-bolt engines because you can just use regular B21 pistons. Although it isn't that hard to have a machine shop modify 6-bolt rods to accept a 24mm wristpin either. I just got my bottom end back from the machine shop and they only charged me $60 to ream the rods and interference fit 24mm wristpins so I could use my 6 bolt rods with B21FT pistons.
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I'm JohnMc, and I approved this message.