Originally Posted by
rutan around
Taxi cab owners aren't noted for being silly with their money yet they continued to do gas conversions until gas became too expensive and hybrids became available.
And it is well known that gas conversions need hardened valve seats to avoid valve recession. If the engine doesn't have suitable seats installed from the factory, there are people who will build special cylinder heads or install hardened seats into an existing head.
Advanced timing is more likely to damage other components (e.g. the piston, which has much less cooling than the head) than the valve. The exhaust valve is much more likely to be damaged by retarded timing, because there is more heat in the gas while the valve is off its seat. When the valve is closed it is cooled through the seat, when it is open is the time when heat can be a problem.
It is hard to imagine an (advanced) timing or detonation problem that would damage the valve before the much less robust piston.