After speaking to the cylinder head shop in Surrey their opinion was that iron guides were much harder wearing than bronze, I chose cast guides because a) im tight and b) they are cut down and smoothed into the port so the loads on the guide will be higher combined with the faster wearing bronze might mean my lazy arse would have to change them more often.

They also sell what they call nucleous power guides, they make these from Colsibro, which they describe as being a copper-nickel-silicon alloy. If i was doing it again i would probably use them. Seems Colsibro is used in aviation engines a lot.

They also list a material called Trojan(?) which is even better than Colsibro.

http://cylinderheadshop.com/nucleus-...-power-guides/

Rob,