Yamaha TRX850 Enthusiasts Forum

I checked the valve clearances and a few needed adjusting so i put new shims in set the timing and reinstalled the cams, put everything back together. I started the bike and let it warm up and the motor sounded very quiet. The next day i started it up to take for a ride and again was running very nicley at idle. When i went for a ride i got about 3km away from my house and the bike made some not so pleasant sounds and stopped. When i stripped it back down i found that the damage was quiet severe.

The inlet cam seemed to have ripped into the alloy of the head and i presume seized in there at the time and then the cam chain ripped half the teeth off the crankshaft. Then when i got back to the shed i turned the crank over by hand and by this time the inlet cam was spinning again and i then rammed the exhaust valves into the top of the pistons as the timing was drastically out. The head where the exhaust cam is held had no signs of wear what so ever.

I presume it happened due to no oil and was propbaly just running off the oil i put on it when i reassembled it, which dried up pretty quicky. Has anyone got any suggestions as to why this happened? I can't understand why the the exhaust side was fine when the are fed by the same oil line. So i am now looking at a new crank and valves and i will do the pistons while i am there. Does anyone know of any after market valves and high compression pistons which are made for the trx? I am a bit reluctant to rebuild it if i dont understand what went wrong in the first place!

Sad to hear Your story. Similar thing happened to me, but the engine lasted the whole season without failing completely. When disassembling the engine after the season, I saw severe wear in the head on the inletside. Pure luck the cam didn't seize, I guess. Lasted 8000 kms...

I am not 100% sure as to why it happened, I did the rebuild last winter according to the workshopmanual, with torquewrench etc. What I might have failed to check, was that the inlet cam was rotating freely after tightening the inletcam to the described 10 Nms. Another thing we noticed at the re-build this winter, is that the oil feed holes are quite small on the inlet side, and that the oilfeed at the exhaust side has grooves to let more oil get to the whole "bearing" surface.

Your story is very similar to a lot of engine failures discussed on the German TRX site. I guess it's a combo of lack of oil and tightening too much.

Did You check that You had oil coming out from the tell-hole on the left hand side of the cylinder head before starting the engine?
Did You tighten the cams to 10 Nm, and did You check they rotated freely afterwards?

Hans, I did the torque settings correctly and the cam was moving freely but i didnt check the hole to see if oil was coming out. In saying that if oil was not coming out of the hole what is the likely cause? But still if there was no oil it seems unusual that the exhaust side shows no signs of wear. Did you have to replace your head when yours had wear on it or can it be fixed?

If no oil was coming out of the hole, it means Your head was lacking oil feed. I am nowadays always cranking the engine around on the starter without the sparkplugs until there's oil coming out of the tell-hole, before I put the plugs back and start the engine.

As to why the damage is on the inlet side, I am by far no expert, but have discussed it with my mechanic a lot. His idea is that the difference in the oilfeed groves might be the cause. All similar head damages I have heard about have been on the inlet side. It is a bit weird, as the inlet side have more "bearings" , on the other hand, the "bearing" on RH cylinderside between valve 3 and 4 is really not having much material...

When rebuilding my cylinder head, we improved the oilfeed holes a bit, hopefully this won't cause oilpressure further down to be lowered.

It is possible, and not too expensive, to repair Your cylinderhead. I believe the term is called "line-honing". Just remember this causes the cam's centre to be lowered slightly, i.e. less valve clearance and more slack on the cam chain. This way turned out to be cheaper than sourcing a good 2nd hand cylinder head.

I am however looking for a used cylinder head, partly since the clearances weren't correct, (we had to do some manual polishing afterwards, and I am now a bit scared the clearances are too big and causing problems with the oil pressure...) and partly since we had some problems with the plastic filler in the inlets, so we want to redo the porting in any case.