Piston & Gudgeon Pin Pics and Q's ?

Hey gents, please have a look at my piston, is my bike running rich ? Its an 11' with the marmot woods map, piston has done 86 hours,


Also, the old gungeon pin has a line in the middle, i cannot feel it with my finger but is it starting to pick up ? Should I change the con rod ?


Both parts look normal. Can't tell about AF ratio from the top of the piston, though.

i would change the conrod and circlips and piston and rings. Are your valves in spec? whats the underside of the piston look like? Piston looks about right, lots of carbon buildup though.

i would change the conrod ...

You mean wrist pin, right?

Thanks for the replies and advice guys,

This is a pic of the underside of the piston


Just to clarify I will be doing the piston, wrist pin,timing chain, and valve stem seals as the bike was using oil,

Valves looked great and were still in spec, I tend not to ride the bike up to the limiter so it does not get over revved, I ride with a clean air filter every ride, oil and filter get doen after every race or every other practice ride

so would you say that the oil rich condition was from the consumption of oil vs the marmot woods ignition and FI mapping?

Edited by stroker101

Except for the fact that you say it was using oil, there's no evidence from the piston crown that there ever was such a condition. The carbon "build up" is fairly normal, and the amount one sees on a piston with that kind of time on it will vary a lot based on how the specific engine has been used over that time. Besides, all it takes to burn 200cc per hour is 0.001cc in the combustion chamber each time the engine fires.

The mapping would not have led to any condition that would have caused oil consumption unless it was so rich that it was actually running poorly because of it.

Yeah I meant change the wrist pin. the con rod should be ok.

Hi Stu2

I am having the same problem but my bike started to use oil at the 66hr mark, I have just replaced my topend and timing chain but didnt think i needed to worry about the stem seals (valves are still the same from new and have not moved at all) as i checked the clearences on all the old rings as per the manual and they had all reached or passed the limit as listed in the manual. I dont use cheap brand oil (I am using Motul 300V) as I believe what you put in is what you get out.




I am using avgas 100 leaded fuel in my bike as the fuel from the pumps here in NZ are full of ocean water (salt water) for some unknown reason.

Thanks guys, it was using about 200-300ml in a race, I dont know if its critical, but I dont feel comfortable with the oil getting low, also used castrol power 1 fully synth

thats alot of oil to use in one race... but what you plan to replace should fix all that right up.

Thanks mx54, just to clarify its over a 200km fast gncc type off road race, and the particular race was fast and I had the bike wide oprn and singing in 5th,

Hopefully after the changes it will be good, firing it up tomorrow

Just a quick update...

I have now done 3 hours engine time with the new piston and rings and I am happy to report no oil was lost.

Stu2 I am sure you will be able to report the same after also replacing the valve stem seals on your bike at the same time.

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