Most all of the black engines (454/502 Mag, etc.) actually used a 2 piece valve. The head and a portion of the stem (about half) is Inconel, and the rest of the stem is SS. They did it to save money, since the Inconel material is so costly. If you get a chance, take one of the exhaust valves and put a magnet to it. The magnet will not stick to the head or the lower portion of the stem, but it will stick about half way up the stem. I'm not sure about the older MIE engines, like the 340.
Eddie

I did a LOT of research on this subject about three years ago with my original build. I bought a set of stock square port heads that came off of a Merc Mag engine - NOT a blue engine. The exhaust valve had "Eaton" stamped on it, but no part number. I tried the magnet trick, and it did not stick to them, but I also had a set of standard stainless intake valves that I know were NOT inconel, and it didn't stick to them either, so you can't really rely on that test. I found an article that said that high nickel alloys like inconel will emit a different spark pattern when touched to a grinder than standard stainless material, but who wants to take a grinder to their inconel valves to find out? :eek:

Finally, someone gave me a number to someone at Mercury Racing tech support, and I was told by them that the black motors had whatever GM shipped them with - probably a regular stainless exhaust valve, but he wasn't sure. He did say that the blue motors were disassembled, blueprinted, and put back together with HP components, including the inconel exhaust valves that were sourced from Manley. So that goes along with what a few others have said. I could see where they might go with a standard stainless intake on the black motors for cost savings, since those were generally not HP applications, and I ran non-inconel valves in my old 454 for years, and it held up fine.

A 1990 340 hp 454 definitely had standard passenger car stainless steel valves in it from Merc. Merc did not disassemble the motor or put anything weird in there.

Some of the HP motors (black, before they painted them blue) got opened up for some special parts and pieces, and all of the blue HP motors got hands-on treatment as well.

Some of the rectangle port Mags of the 1990's got different valves.

Some of the later Vortec motors had upgraded valves.

Inconel is a TYPE of stainless steel that is very heat resistant.
MOST "inconel valves" are two piece.

MC