You forgot to advise him as to whether he should pick a magnetic bolt or a non-magnetic bolt :-).

Actually, this is very important to boaters (living in a corrosion-prone environment). There are cheap steels, claimed to be stainless, that will not work well on a boat -- these are usually 306 -- and will fail after some short time. Then there are those other alloys that are well-suited for a corrosion prone environment, like 316, that isn't magnetic.

Steel, or even stainless steel, by name isn't the answer, especially working around a wet application such as the cooling system.

Anyway, there is always the other complication of electrolysis when you use unlike metals -- that, too, complicates things. Unfortunately, I don't know the exact electronegative value for the particular alloy that Volvo's heads are made from. This would help in picking the right steel alloy, since all these alloys (of Al and Fe) all vary in their value.

But maybe I'm just nitpicking :-) .