125-200cc Piston knocking

Hey guys,

Need help again. My new piston on my WRE125 2008 is making a lot of knocking noise during engine braking. Is this because of a wrong size?

I found 3 sizes of Pro-X piston off the web:

53.94mm
53.95mm
53.96mm

Which one should i use now? Thanks...

what you should use is.....a wrench to tear her down and gauge the bore and piston. then consult the SM for more info (hours, tollerances, etc). no one can accurately diagnose an engine noise over a keyboard, are you sure it's the piston and not the chain/rr caliper slapping while backing into corners due to wheel-hop? dont order parts till you know what you need.

pvduke - How much should you allow for clearance?

Mr X - are they the same piston as wr's? I know where you can get good quality WR 125piston
not sure about Pro - x do you know if they are any good?

The piston to cylinder clearance should be around 0.04mm-0.06mm and absolutely not more than 0.08mm. While best to do this with a bore gauge and calliper, you can measure it using a feeler gauge between the cylinder and piston. Insert the gauge with the piston down to about where the wrist pin hole is. Start with a .05mm gauge. The piston should require a small amount of effort to move it (maybe 4 to 6 Newton’s or about a pound). It should not be able to move under gravity.

The ring gap should also be between 0.26mm-0.6mm.

Also, check to see if there is any radial play in the crank-rod-piston assembly before you remove the piston. To do this, get the piston to TDC (no cylinder of course) then use a small soft wooden wedge between the crank lobe and the case to keep it from shifting. Carefully try to move the piston up and down (not side to side) directly along the line that passes through the wist pin and the center of the crank. You should feel no play at all.

Finally, check the side clearance of the connecting rod in the crank. It should be less than 1mm. Remove the piston and rotate the crank so that you can see the widest part of the gap between the lobes (TDC again). Just use a feeler gauge between the crank and con rod.

Check piston clearance 90 degrees from the wrist pin and at bottom of skirt.

thanks - would be good to see this info in a detailed top end rebuild tips thread
Ive just donme a rebuild and my conrod had some play but couldnt get up and down movement
I am not sure about my piston now as the compression is only up a little to 150 lb - need to check it again after ive ridden it , I will take the head off to check again (seemed like a nice fit though )

i always consult the service manual for service limits and spec's, and, measure according to the manual otherwise the data will be flawed. on cast pistons even if they look ok they hour out. replace them. they are not made to last forever. they have cycle limits and work-fatigue and can fail even though they look fine.

pro-x is a japanese (mostly) made piston to oem spec, cast mostly in that regard. i've never used them as i've used wiseco forged if oem aint avail.