Michael:

Bronze has a higher expansion coefficent that steel, so the hole in the guide grows faster -- remember, the exhaust valve stem is sodium filled and MUCH hotter than a solid steel valve -- the purpose is to transfer the heat to the head and keep the tulip much cooler at high load. This has nothing to do with insertion pressure, size of the hole in the head, or anything else. Most of the guide, and the portion that actually seals and controls the location of the valve stem, is outside the head. The point is to keep the guide clearance positive under heavy load, otherwise the guide will wear (the valve stem is MUCH harder). Intake guides are steel.

TAke a look at the exhaust valve guides on FastLane -- sure look like bronze to me!

I believe BRASS (not bronze) guides were used in some early engines for machining purposes, but certainly not for long -- wears too fast. All the early ones had poured Babbitt bearings, too (up to the 1950s, anyway).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!