KX85 scored cylinder

[IndyMX]

We just replaced the top end on my nephews 2007 KX85 a few weeks ago.

Everything went fine.. no issues with the install.

He heat cycled it as he should and we thought everything was going perfect with it.

He put about 3 hours on the rebuild, and at one point in the day stopped the bike normally. When he went to start it back up, it wouldn't fire. We tried all the basics, new plug, lay it over for a minute.. everything. No fire.

So, I did a compression test, got 60psi (spec is 119 to 185).

Tore the engine down yesterday, and here's what I found...

Can anyone help me understand why this may have happened?

The piston didn't seem to have any issues, it's snug in the cylinder like it should be. The rings were properly measured for end gap before install.

How can a piston rock in the bore so far up if it's tight at the bottom of the bore?

I'm leaning toward an over heat, but I'd think that would cause the whole cylinder to seize, not just the exhaust port.

 

[IndyMX]

One more..

 

[Chili]

No gouge on the piston crown edge? The last (well second last) rebuild we ever did on Troy's last KX80 the pin that the kips valve actuates on snapped and allowed the valve to smack the brand new piston during break in which led to an ugly chain of events.

 

[IndyMX]

The top of the piston has a nice ridge on it, and the rings are scored..

The KIPS seems fine.. I mean, it operates normally.. And it doesn't seem to be protruding into the cylinder at all..

 

[Chili]

Likely is then, on Troy's the valve was stuck after the pin broke.

 

[helio lucas]

is that the plating coming off the cilinder?

 

[IndyMX]

is that the plating coming off the cilinder?

Yea, well, since the piston seems to have dug into the cylinder a couple thousandths, yes it's coming off...

 

[whenfoxforks-ruled]

Stock Kawasaki plating.

 

[helio lucas]

How can a piston rock in the bore so far up if it's tight at the bottom of the bore?

the bore can be wear more on top where the rings pass and at the inlet side where the skirt press hard against the bore. so, even if the piston is tight, it may be ovalised or have some taper.
if not brought up to full temperature it is pretty easy to size a fresh engine since the piston will heat faster than the cylinder.

kawasaki electrofusion plating is known for peeling as soon as the bore has some wear. and to me, it seems the main reason since that spot is a very high wear place.