Vacuum limit on Mercruiser/ small block Chevy

[Bushwacker]

Like I said, those aircraft engines run some pretty fancy parts! You only need the nickle valves on the exhaust side. If you can't find Stellite seats, Inconel might be a good second choice, although my experience in gas turbines is that having two hot parts of the SAME material rubbing against each other will create LOTS of wear, so I'd try to find out the exact chemistry of the valve and seat alloys. Some of the nickle alloys also contain lots of cobalt, which is what you want in the valve or seat because cobalt oxide acts like a dry film lube when it gets hot. Looking at my alloy reference book, good old Waspaloy is 13.5% cobalt, Udimet 700 is 15.3%, Udimet 500 is 18%, and Inco 939 is 19%; all have 15-20% chromium too, so they would be very oxidation resistant at high temperature, but none of these are cheap alloys! If you can't get stellite seats, maybe inconel valves with cast iron seats would work ok, as the valve seats won't run nearly as hot as the valves, especially when mounted in an aluminum head. The bronze valve guides are a good idea too, since the wobbly ball-stud Chevy rockers put a lot of side load on the valves. Positive rotators on the exhaust valves would also help. They may be standard on some marine engines.

I'd be inclined to run what you have till it breaks while you're getting another set of heads ready. Connor seems to be getting pretty decent life out of his by observing a 5-7" limit, and he runs with a pretty serious load on his Bahama trips. I'd follow up with him on the riser issue though, as too short of a riser can kill an engine in a hurry!