The machinist has not got to work on the heads yet but while I was there he shined a light in the ports and it appears that I have bent valves on 6&8. I think I mentioned that when I took off the first head there was some ugly oxidation even though we ran oil through when winterizing. When I pulled off the second head the plugs were nice an oily like they should be. Also the detonation damage did not look as bad.

The valve guides on an auto engine are set tighter than on a marine engine. That may have caused them to stick and get wacked by the piston. The Merc 525 uses a very similar head. Hone the valveguides to the 525 specs since you have it apart already. Check your carb jetting. If you are getting preignition with 9.6 compression and 93 octane and 32 degree timing, something else is wrong. I'm going to guess it's running lean and the fact that #8 is the worst reinforces that. Also, Merc has run 140 degree thermostats in their carb engines for years, don't go hotter. If you have preignition or detonation, try to cool everything. Cool the air going into the carb by adding vents or a scoop. Cool the cylinder heads by cooling the water going thru them.