[JRyan]

Mac,

I simply don't know. I've never measured the cc's of a 487X to see how they compared to a 487, but I'm pretty sure the port sizes aren't as different as 461 and 461X's ( I think 7cc's now, and it used to be like 10 cc's different).

The casting dates on one of the 487X heads I have is 1971, so it could be that they were used on California cars which may have had a different EPA requirement for unleaded fuels in the early 70's that would have required the induction hardened seats. I think you'll notice on the head that the exhaust valve spring cup is deeper than the intake. They used the same spring on both int & exh., and just sunk the cup. That might have been one of the reasons for designating the "X". Later heads used shorter springs on the exhaust and the same cup depth.

All speculation on my part, but I'm sure there is someone out there that actually knows.

I'm with Greg on the 993's.

Jerry