i have a yanmar YSE 12 hp single cylinder engine. it was rebuilt with new bearings,liner ect and new timing gear/face plate which hold the oil pump. Problem is engine was fine for 20hrs then pump was wearing out the bush, and scoring the face plate and oil was seaping into crank case hence losing pressure when warm. ordered new timing housing again and same thing happened. Any clues then email pirate2008@live.co.uk.

andy, August 2008

Hello Andy
I am not that familiar with the workings of the YSE although we have a 10hp in our boat which has been reliable as clockwork for five years.However,I would say you have a fault with what is called the "end float"of the crankshaft.If there are thrust bearings on the crankshaft and I am sure there would be they are probably out of tolerance allowing the crakshaft to float too far and score the items you describe.Try any Haynes or engine manual for "end float" to give a more detailed description.We had this on a VW Golf GTI once.It is quite rare these days.
Regards
Howard

Howard Hancock, October 2008