Unlubricated valves running out in the open on old motorbikes last a lot longer than 250 hours
IME, they last "about" 200 - 300 hrs in hard use, perhaps longer when mostly paraded.
Heck, even enclosed valve Triumphs tended to need a valve job around 15 - 20,000 mi.
and my experience with VW engines in buggies ,they never had any excessive valve or guide wear...........
If they don't have large ratio rockers, i ccould believe that. If they are indeed hot-rod motors, driven like a hot rod, how many actually make 25K miles before top "refreshment". That is a lot of driving on sand.
Keep in mind, an AC engine is used 100% power for maybe 5-10 minutes on take off, then reduced to between 65% to 75% depending how much fuel the pilot enjoys converting to noise, and is left in that position sometimes for a few hours. Not at all a road driving experience where once up to cruise the car is using maybe 20% power to maintain 65=70 mph with occasional squirrely boosts to 75%, when passing, merging, general animal spirits, etc, and motor sees fairly variable rpm over any multiple hour period.
.incidentally ,the old VW repair manuals used to make of point of setting rockers so the valves would be rotated ......
This is still a factor. Setting up with shims to offset slightly so they rotate.
.a lot of truck engines have positive rotators to keep valve stems straight at operating heat.
Aware of that, no personal exposure or experience.
Mostly with the large ratio rockers, the stems go barrel shaped due to constant heavy side thrust. More or less depending on how they are set up (push rod length modded) when first installed.
I'm going to set my rockers up on the surface plate today, and verify the ratio since i neglected to check the overall lift when removing the heads. I have indicated cam lift directly at .330" - .335" both intake and ex. Someone on another site pointed out that intake lift greater than 25% of valve diameter is wasted and merely goes into extra work at the springs and wear in the valve train.
Graph from homebuiltairplanes site
The intakes on mine are 40mm => optimal lift < 10mm/.394".
I believe currently it has 1.4 ratio rockers which would make lift aprox .465"
If so, plan is to back down to 1.25 ratio rockers which will still have flow benefits of the snappy opening and closing action that over-unity rockers provide.
smt