Not familiar with this engine, but know LPG causes havoc with valve seats. pitting, burning and causes valve seat recession. No chance that the valve clearance has reached its minimum limit ? Looking At AD these are manual tappits, I think it would be worth considering tight tappits.
Fleet customer near us has been running V6 24valve petrol sprinters on gas and is having to fit all new seats at around 60000. They have been that much trouble he is going to diesels. Previous to that he ran the 4 cylinder petrol / LPG units clocking up 1 million Km. They still had valve seat issues with them, but they cost effective.
Cheers Aidan
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