When I ordered my new tappet blocks to put into the big bore kit, I ordered '67 style. I'll admit, I wasn't thinking about all of the mismatching. Had I ordered '71 style, I could have saved some work. However, the '66 cylinder head would have still required a little work.
I mentioned this mismatch the other day, but I didn't take many pictures. Since I've got the top end back off of the engine, it's sharing time!
|This is the new tappet block. It's a '67 style from Morgo. The sealing surfaces are different from the '71.|
|This is where the head interfaces with the pushrod tube. The O ring will seal against the countersunk surface around the bore.|
More on the O ring squish: the ideal amount is .030" to .040". Less than that means that you may not have enough pressure on the O ring to make a good seal. More than that is even worse! Two things can happen if the gap is two big: first, you extrude the O ring and get a bad seal. Second (and worse) you place too much pressure on the head itself and cause it to warp. Once the head is warped, you'll have all sorts of issues that you really just don't want to deal with.
Obviously, if you are worried about keeping an engine in "original" condition, something like this isn't an option. However, if you're goofing around with a combination of years...you weren't worried about keeping it original to begin with.