paul.jameson wrote: the only thing which matters is to use a 3 plain ring piston rather than one with an oil control ring so that you still get some oil past the rings to lubricate the guide.
I have to wonder if this should be regarded as an old wives tale. For what it's worth I searched the web and all I could find were unsubstantiated assertions that oil in the combustion chamber was beneficial in any way. One thing we do know is oil reduces the octane of the fuel, which is a bad thing. I will be running a modern piston with oil control ring in the 1928 Black Ariel I'm presently rebuilding so I would greatly appreciate references to any credible evidence that oil in the combustion chamber lubricates the guides.
Oil leaking past the rings on the intake stroke wouldn't reach the intake valve stem because air is flowing the wrong way, and it wouldn't reach the exhaust stem for the same reason; plus the exhaust valve is closed on the intake stroke, anyway. Oil that leaked past the rings and vaporized from the cylinder wall wouldn't reach either stem on the compression stroke because both valves are closed. Both valves remain closed on the expansion stroke.
Finally, on the exhaust stroke, the intake valve is closed but the exhaust valve is open. However, the valve lift is only a few tenths of an inch while the guide is ~2" long so any oil that survived the heat of combustion would only reach the bottom ~10% of the valve guide leaving the other 90% untouched. Further, in engines with worn guides oil leaks down
the guide into the combustion chamber so any oil that made it into the bottom 10% of the guide would be pushed back out by the relative pressure difference, not sucked up.