Maintenance & Technical > KX500 Original

Not my day :(

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no sub for cubes:
thanks for the replys guys,i did mean to ask what you guys thought caused the damage,the piston is only gouged on that one point.not really other signs of it seizing on the piston,that would make sense that it was caused by the main seals going..id prob be as well as to get the barrel honed? ,measured and buy another piston...just greaves me cos this one still looks brand new!except for the gauge and hotspot on the one exhaust side,the rings still look like new honestly only had the thing running for prob less than 30mins.. ,as i have no acurate gauges to measure the bore myself, can i get an idea from the old piston size? which reads std575ps TD in the centre and on the inlet side reads j4oc2 

BDI:

--- Quote from: martinfan30 on March 15, 2009, 03:07:42 PM ---
--- Quote from: cutting torch on March 15, 2009, 09:05:08 AM ---If most of that scuffing disappeared with a scotch-bright pad rub-down, and the ring grooves were good, I would reuse it.

I have a question, though.... What is the hole above the wrist pin bore for? Is it for pin oiling?

torch

--- End quote ---

Thats a good question. its not really a hole, just a small cavity that may be used to hold oil for the gudgeon pin as you suggest. Just my thought.

--- End quote ---
It's just a hole, that's how wiseco machined the piston to get rid of some weight nothing more. They no longer do that to the pistons for the kx500.

cbxracer30:
A great way I have found to prevent those ugly exhaust bridge scuff are 2 or 3 small holes through the piston right at the bridge. By small I meen like 3/32 shamfured on the outside with a large bit. Put the new piston on the rod {no rings} side cylinder back on and mark the exhaust bridges on the piston from the exhaust hole . I usually start the holes 1/4inch down from the rings and space them 1/4 inch apart. I'll try and post some pictures tomm.

KXcam22:
I would suggest that you polish that one up and replace the rings.  I have run much worse.  I was rebuilding a friends YZ125 which had a center bridge and those holes stock (and the bucket of old pistons that came with the bike were all seized mildly over the holes anyway) -I got him some better oil.  I never required the holes on my K5. Cam.

DoldGuy:
Did you let the motor "warm up" before putting a load on it? All forged pistons expand much quicker than the bore does and will seize ( cold seize ) generally on the exhaust side first / it kills me to hear someone start the bike & then rev the H... out of it.

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