I ran my RH350 engine with open valve gear for over 20 years covering about 30,000 miles in that time. The longest trip was 468 miles in a day on the London-Edinburgh run. Typically, I got about 1000 miles from an exhaust valve guide but the minimum was 50 miles to wear which was unacceptable for further use. As Singles Spares Organiser at the time I was able to use, almost exclusively, new old stock valve guides.
At one stage I fitted an 89 carb from a 500 instead of the 76 carb for the 350. This resulted in a lean spot about 1/2 open which resulted in a spate of dropped exhaust valves. In consequence, I got through quite a number of pistons and I have at least 2 still in the workshop, complete with exhaust valve heads embedded in the top of them. Eventually I went back to a 76 carb, having found a replacement body, but my best piston was by this stage somewhat battered where I think 3 valves had broken at the collet groove and dropped onto the piston head. Not pretty, but the piston still worked. It was a slipper type with oil control ring. Then I found another slipper type, virtually identical except for the fact it had 3 plain rings (and was new, and hence undamaged). I fitted this piston. Thereafter, for something like 10,000 miles, there was no need to replace the exhaust guide.
Throughout, I had the oil feed to the inlet guide from the crankcase behind the cylinder. Inlet guides didn't seem to wear.
I fitted a 500cc engine with open valve gear but a piston with an oil control ring. Within 500 miles the exhaust guide (and valve stem) was wrecked. At that point I fitted an enclosed valve head and have had no further problems. One day I will re-fit the open valve head. Before doing so I will change the piston to a 3 plain ring type.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.