After comments in another thread about cast iron valve guides I thought it may be better for a new thread and get some discussion on the subject.
About 8 years ago I did a rebuild that involved new head castings. My local bike shop recommended I run the bronze guides. They don't usually don't work on anything much older than shovelheads but said they worked well in them. I went with them. At first the bike ran OK but after about 3000 miles started skipping very badly after the bike got hot. Engine had obvious valve lash. I could pull over cut it off to cool and in a few minutes hear a definite snap noise. As soon as the snap noise the bike ran fine until it got hot again. I pulled it apart to find the valve stems scarred. The valves had been hanging in the guides. This was the snap noise, valve cooling enough to loosen up in the guide. Also the tin cups were loose under the guides. The fiber washers under the cups had come apart. This also caused a pretty bad oil leak. I went back with more bronze guides reaming them looser. I also used a different fiber washer under the tin cup. The bike has run well since. It now has about 15000 miles on the engine. Only problem is the oil leak. I don't expect this old iron to be leak free but at times it looks like I am spraying for mosquitoes. I put some florescent dye in the oil and ran it to pinpoint the leak. It all seems to be coming from under the exhaust tin cups.
I will most likely pull it back apart soon but wanted some discussion about the material of guides and valves before that.
I was also thinking about the possibility of putting an o ring under the guide to seal the guide to the tin cup. It would take machining a groove on the underside of the guide and a high temperature o ring. Either this or machining the bottom valve spring seat to accept an o ring between the seat and guide where it meets the tin cup.