[TR] Sticking valves?

[Randall]

> My understanding is that iron guides expand less so can use a bit 
> less clearance, but I have no personal experience with those.

Yup, the iron guides expand at the same rate as the head does, so the
opening for the stem actually gets larger when the engine gets hot (to match
the stem getting larger).  With the bronze guides, the bronze expands much
faster than the cast iron, so the guides actually distort and make the hole
smaller as they get hot.

Also, at least what I've been taught in machine work, cast iron doesn't gall
the way that bronze does, and so is less likely to stick even with tight
clearances.

In theory, the bronze guides wear slower, but since you have to start with
much larger clearances, the overall effect seems to be about the same.  I'll
never use them again.

Randall