I have only ever assembled one head that had the stem seals in the guides, because the knife edge at the top is not needed the set up would suit higher lift cams where more travel is available. However, I personally believe that the original guide set up with the right clearance and sharp knife edge works fine, and I do like the idea of the lubrication of the stem as Bill mentioned. Air cooled engines run way hotter than liquid cooled modern engines, although as Stu has done many miles with nil trouble, then the seal mod seems ok. I would say most early Vincent engines suffered from the oil passageway through the liner to lube the piston skirt was too high, as the later pattern liners have the drilling lower via the machined slot. My guess is that this was actually the main cause of oiling up of spark plugs rather than too much oil in the heads..........I am also a firm believer that the oil returning from the rockers down each pushrod tube aids greatly in lubricating the cams and followers. If you look at how the oil feeds to the cam spindles actually work, the oil flow to each cam lobe is extremely poor........The inner most lobe only gets a "Timed squirt" as the cam rotates past where the hole in the bush aligns briefly with the corresponding hole in the cam spindle..........next time you have a look you will see what I mean. I machine a small radial groove in the inner cam bush to create a constant passageway for a more continuous flow of oil, and I check the depth of the hole in the spindle first from the crank case wall outward so the groove in the cam bush lines up with the hole in that spindle. The outer lobe does not suffer from this issue, as the hole in the spindle delivers oil to the open cavity between the two bushes, where the outer cam lobe is.
