Yamaha TRX850 Enthusiasts Forum

Thought it best to start this as a new thread even though it started elsewhere.

Microscope pics below of middle inlet valve from Sandune51's motor before rebuild.

Question was why do inlet valve clearances sometimes close up? Is it due to plastic deformation of the seat?













Lots of damage on seat area, nothing on the top collet end and a close up of the transition from the valve end and the stem. Not sure how these are made but I thought the stem was a different material grade to the business end?

Superb photos Phil , thanks for posting them.

What a clever piece of kit that camera must be.

I hope this puts to bed the myth once and for all that Trx and Tdm 850 inlet valves stretch.
The photos show really clearly that the valve seat area has become concave seating deeper into the head and slowly reducing valve clearances. I assume this was a material issue that was long ago rectified with the introduction of a revised valve part number suffix.


Mark.

Fantastic photo's

It's great to see this kind of detail. really does tell a story

Thanks for sharing. That means my head is salvageable - just need to fit new valves, although I have a complete late TDM head to fit. Still not got any concrete proof that the TREX and TDM cams are different

Valves are normally 2 piece with the head "spun" onto the stem by friction welding

this is very useful info, my 1996 valves clearances have closed up on the last 2 checks so new valves will be on the cards

Will get some more images today of inlet valve seats from my spare head to see how they compare with Sandune51's.

Hope they are not shot because I was going to use the (already ported) head this winter.

Don't worry the seats in the head are intensely hard, but have a lot of metal.

I've never seen an (cyl head) inlet seat that I would consider to be less than 90+% perfect. Exhaust seats have a tougher time but fare hardly any worse.



Mark.

Well this is just brilliant

Awaiting pics of the seats in the head, hopefully they aren't affected by this and a straight swap of valves is all that's necessary..

Snod Blatter wrote:Well this is just brilliant

Awaiting pics of the seats in the head, hopefully they aren't affected by this and a straight swap of valves is all that's necessary..

Just did mine and the valves were the same. Seats were fine though, so good news, you just need to swap out the valves



This is one of the new inlet valves for comparison

What mileage/kilometrage was that, Mr Rod? And what prompted you to do the work? Do the clearances start to close up more frequently?

Snod Blatter wrote:What mileage/kilometrage was that, Mr Rod? And what prompted you to do the work? Do the clearances start to close up more frequently?

I don't know for sure what the mileage is but best estimate is about 80,000miles.

Yes valve clearances closed up on the inlets over 3 separate adjustments. I was using a 1.50mm shim on that and I believe the original shim would have been about 1.82mm.

I was also using oil, which also prompted the work

my personal view is that it's valve head deformation. fortunately you are going to do something about it before it trashes the head.
Wonder if Yamaha did a recall for shit valves?

Rich wrote:my personal view is that it's valve head deformation. fortunately you are going to do something about it before it trashes the head.
Wonder if Yamaha did a recall for shit valves?

They changed the part number on post 96 models. Didnt do a recall

It is indeed the valves that are the problem, no damage to the cylinder head at all. In fact, and this is bizarre, the shims required for the valve clearances when i put it back together were the 1.82 and 1.86mm shims that were in the bike from factory. Pretty impressive manufacturing tolerances from Yamaha

Just to be pedantic, it's the valve face that is damaged/pocketed - the valve seat is in the head.

4 of my inlets had minus zero clearance at a genuine 9500 miles