Yamaha TRX850 Enthusiasts Forum

Hi

Further to the recent spate of engine heartaches I have a theory that as these engines get older the oil pumps become less efficient to the point that they have worn enough to starve the furthest point of lubrication ( the left hand big end )

I dont know for sure the recommended oil pressure but I guess 50 - 60 psi ?

The oil pressure light fitted at the head union seems to be a bit vague and a long way from the oil pump so Im thinking of trying this arrangement ...........

Question is will it work ? ......... Please let me know if Im about to cock up

I'm still mulling this over since my crank went a week or so ago.

My theory at the moment is that oil pressure to the big ends drops because main bearing clearances become larger as a result of wear at high mileage. This then leads to LH big end failure, this bearing being last in line from the pump. I have not however seen cam bearing damage due to oil starvation or reduced pressure. My thought is that these would be high on the list for early failure/damage as they run directly in the head. I need to look at where the head feed is relative to the pump, balance shafts and crank etc.

I have atleast 0.15mm at the main bearings with an upper limit of 0.1 and a specified 0.02 to 0.038 optimum clearance so quite a large gap.

I will measure these with Plastigauge and also measure the clearances on my pumps and feed back my findings.

Hoping to have found a repairable crank in a complete lump that's for sale locally! Hope it doesn't end up being another 100kg's of scrap metal.

The pressure relief valve in the sump pan is also a mystery to me; I assume that this relieves the main pump not the one that returns oil to the tank. I doubt this would operate if pressure is low as a result of wear so messing with it would have no beneficial effect.

Just measured tip clearance on both my oil pumps after crank failure.

Yamaha spec is 0 to 0.12mm; service limit is 0.17mm

Mine were 0.11 and 0.08mm with very very little scoring or visible wear to rotors in either pump.

Not so sure how to check side clearances.

Seems that pumps are good (engine mileage quite high but unknown) and not the primary cause of big end failure.

Main and big end check next...............gotta watch (recorded) Moto2 first.

Well thats my theory scuppered but I think I will carry on and fit it now Ive got it

I found that the sender wont fit under the engine and the filter housing is all shapes and angles so dad made a little jig and I tapped M10x1.5 ...... its very close through the plug so needs to be very straight

Anyway we now have this ready to fit