Yamaha TRX850 Enthusiasts Forum

Turns out the pistons are not only melted but also both cracked, also got a buggered head gasket. Diagnosis so far is that it was likely caused by detonation.

The pistons are Cosworth items (I was told Wossner?), and with a good seal returned 225psi

In both of the side on pics you can see scoring and the melted lip.

Waiting for the engine guys to let me know what shape the head, cylinders and valves are in and will go from there. Hopefully everything else is good to go back in and I only need to sort out pistons and a new set of little end bearings.

Heres a shot of the head gasket, and a good pic of the piston crown.

ooh dear - that's a bit of a mess.

And I reckon the detonation diagnosis is about right - must have been running very lean though.

I'd be stripping down the complete engine and wouldn't be at all surprised if you find damage to the rods, the bearing and the crank.

Unusual to see Cossie pistons do that, there aren`t really too many signs on them to indicate them being too hot either. They certainly were not in the motor long looking at the combustion and oil residues and their machining is somewhat rippled anyway, look at the section inside the forging, the gudgeon pin area especially?
I wonder if the material / alloy used by them wasn`t up to the job, but as I say for Cosworth that would be very unlikely / unusual.
Do as Phuk says, check the motor over carefully.
By the way they must have been comissioned `specials` Cossie pistons have Xylan anti friction coating on their skirts.
Images of their pistons:

http://www.cosworth.com/products/perfor ... ons/honda/

http://www.cosworth.com/products/perfor ... d-pistons/

Are the cylinders overbored?

The damage is at the back and hottest part of the engine (the water jackets will be designed to keep the exhaust side cooler allowing a uniform cylinder cooling) If the cylinder wall is too thin by overboring then that kind of damage can occur.

If it hasn't been bored then I'm talking oot my arse..........................

Forgot to say that the pics are f00ked up by my camera... not sure why it's shown up all the wobble/ripples. In the hand/to the naked eye they are rather beautifully made... expensive offerings to the god of speed they will become! The small end bearing on one rod was about to go, bottom end and crank haven't yet been looked at but I will be arranging that in due course.

From what I can tell (judging by pics of peoples overbored cylinders) they are stock bore, but I never got around to measuring them and now I'm some 3500km away from the bike again, hoping my mate kind clear up some room at his place so I can relocate it and have him do the work/deliver bits for me.

Thankfully the girlfriend hasn't been around to have a say in my finances so they bike will be going back together better than ever, I just wish I'd got it tuned properly when I bought it rather than forever putting it off...

They may be cost you a few more dollars but a dyno is well worth it giving you peace of mind that you can find that balance

redkiwi wrote:Forgot to say that the pics are f00ked up by my camera... not sure why it's shown up all the wobble/ripples. In the hand/to the naked eye they are rather beautifully made... expensive offerings to the god of speed they will become! The small end bearing on one rod was about to go, bottom end and crank haven't yet been looked at but I will be arranging that in due course.

From what I can tell (judging by pics of peoples overbored cylinders) they are stock bore, but I never got around to measuring them and now I'm some 3500km away from the bike again, hoping my mate kind clear up some room at his place so I can relocate it and have him do the work/deliver bits for me.

Thankfully the girlfriend hasn't been around to have a say in my finances so they bike will be going back together better than ever, I just wish I'd got it tuned properly when I bought it rather than forever putting it off...

Ahaaaaa, throw that camera away!

So, the damage to that piston in your pictures seems to be adjacent to the middle of the three inlet valves, unusual for detonation issues, you might expect to see that more at the exhaust side but as Dandy quite rightly states the motor will be slightly `hotter` at the rear of the block than at the front.
Obvious when you think of the cooling effects of incoming air while you are riding along.
Where are the cracks in those pistons?

Mincehead wrote:Where are the cracks in those pistons?

TBH I have no idea. To the naked eye they looked all good apart from the melted point - so much so that I thought they could go back in with new rings. Hopefully they'll come back marked out and I'll be able to get some good pics when I'm with the bike again at xmas.

I have to say that I`ve seen pistons in far worse condition than those go back in bike engines too. It`ll be interesting to see where those `apparent` cracks are. Remember to update the thread when you can.

Mincehead wrote:I have to say that I`ve seen pistons in far worse condition than those go back in bike engines too. It`ll be interesting to see where those `apparent` cracks are. Remember to update the thread when you can.

me too! Not worth risking junking the complete engine though