need help with cylinder/piston slap

[TRX_Thumper]

my motor lost all compression i had 80 pounds..pulled it apart the valves were perfect the cylinder didnt have a scratch or bad place one i mean it looks great..no rings broke..no obvious problems...then i took the piston off the rod and stuck it in the cylinder and it was unbelievable how much clearance there was..lots of piston slap..what caused this? the cylinder looks like its never saw a speck of dirt..bought the 400ex new april 20th this year

[TRX_Thumper]

bump

[Dirt]

Pistons always have clearance. I would measure the Bore at top and bottom with set of bore calipers (3 pin) and compare to the service specs. If it is worn from use there should be a ridge at the top of the cylinder and more clearance/larger ID at the bottom of the cylinder.. :D First thing I would of checked is the valve gap, seats and decompression assembly.

[wilkin250r]

If you bought it brand new, then is should still be under warrenty. Why are you taking it apart? I would have taken it back to the dealer as soon as I found out in only had 80psi compression.

[TRX_Thumper]

yeah its still under warranty but that got voided since i raced a midsouth race...its just a small local shop one of there mechanics races and they always show up and watch so..but theres no doubt in my mind piston slap is my problem..last couple rides it would chatter under id say 4000 rpms and i thought it was the cam chain ...i had a race this sunday and i knew i had a problem i just wanted to make it last a little longer...but i took it to the honda shop they checked the compression it was 80 pounds we both agreed i had a broke ring...i knew it would still have more than 80 pounds with a broke ring..but i took the rings off the piston and stuff and the piston just doesnt make a tight fit at all in the cylinder.....but yet the cylinder and piston looks perfect..ive just never saw a cylinder wear away like this on a new honda

[Dirt]

Does it have a ridge at the top and have a larger ID towards the bottom? :scary: Measure it!

[jerry-89-250R]

what type of oil was used in the break-in?

[TRX_Thumper]

well i guys it wasnt my cylinder after all...i took the piston an cylinder to the shop an turns piston has the wear on it...i looked at the piston and it looked ok i thought but it was the problem...the mechanic said i could get away with another stock bore piston and a hone but i just got through ordering a JE416 kit from C&D racing...i bought it new and broke it in according to mototune..so it had honda oil in it....but the reason the piston went down is cause my air filter came off on a motorcross track about a month ago....and i kept riding it and its finally caught up with me..

[dariusld]

my air filter came off on a motorcross track about a month ago....and i kept riding it and its finally caught up with me.. The truth comes out.

[bandit390]

How much time did you spend riding it, after your air filter fell off?

[hondafox440]

So whate exactly happened? The dirt/dust caused the ring to wear down? Or did it make the ring groove in the piston deeper, thus the ring compressed to far or something?

[TRX_Thumper]

i have no idea how long it was rode without the filter...but yeah when i first pulled the motor apart..i thought..that filter musta just been off for a minute or 2 cause i mean cause i saw the cylinder was perfect...and ive never seen a shot piston and a perfect cylinder...so i assumed the piston was ok...but turns out the filter comin off ruined the piston and not didnt so much put a mark on the cylinder...yeah it wore my rings down and was lettin oil by and scorched the piston so much that it fits loosely in the cylinder...but really the piston doesnt look all that bad