Yamaha TRX850 Enthusiasts Forum

Nearing completion of renovation on my TRX I have installed a replacement engine after checking compression and valve clearances. These were better than on the engine that was in the bike when I bought it. When preparing to run it for 1st time today after filling cooling system with water I have 2 leaks even without pressurising the system. \ist one is leak from thermostat housing - plastic cap has deformed and not sealing against o ring. Second and most puzzling leak is from a machined hole on the LHS side of the LH cylinder head. I don't even know what this hole is for but my other engine has the same. Any ideas why water is leaking from this hole?

The thermostat issue is common, they just do that sometimes. As for the machined `hole` I think you are talking about the drain hole for any water that may get down into the spark plug tunnels. If you`ve got water coming from there, FIRST be certain it`s not residual water from the thermostat leak.
I think it might be.

I had the same with the thermostat. Not unusual, with only two bolts for a plastic piece that deforms easily.
I fitted the housing together with liquid gasket. No trouble since.