Piston blow by with low ring wear?

[NZ500]

I've just pulled the cylinder off my 1990 kx500 and all measures up pretty good (appears to have a recent sleeve and piston still well within spec). However the piston has blow by, mostly on one half of it and the rings are only 0.45-0.49mm in cylinder. I know they're getting a bit worn but I wouldn't have expected to see that blow by already?

Some possiblities I've thought of:
Old rings replaced without cleaning piston
Sleeve replaced without replacing piston and rings
Wear on exhaust side of cylinder?

The bore still has quite evident hone marks on it with only minor wear spots around ports.

Anything I should be concerned about/checking while I'm at it? I was thinking of a very light hone and some new rings. The previous owner supposedly knew nothing about the motor (including that the bottom end is shot).

[Big Daddy J]

This is just my 2 cents, but if your bottom end is shot, your piston and con rod angles are probably all over the place, which would back up the wear around the ports, I would guess it is v-shaped tapering from the edges of the ports towards the center of the ports going south (towards the crankcase). Anyhow let's address what you stated in your question : Old rings replaced without cleaning the piston, buy a new set of rings to match your piston, take one of your old rings, break it in half, clean the grooves on your piston using the sharp edge of the broken ring, be carefuful not to remove any piston material. Take the piston off of the con rod,don't loose the needle bearings! Put the new piston ring into the bore, then use the piston to push the ring down into the bore so that it is square with the bore, measure from the piston skirt to the cylinder base, this needs to be spot on, lay a straight edge across the skirts, your measurements should be within .010 of an inch.Carefully pull piston out so as not to disturb the ring(s), then measure the end gap of the piston rings. Check against factory specs. If it ain't right, gap it until it is. Put piston back on con rod with bearings but no rings,oil it up, oil cylinder up, put cylinder back on motor as if you were reassembling it,pull stator cover off,slowly roll motor over until piston stops moving up, mark piston position with magic marker,roll engine over until piston stops moving down,mark piston position with magic marker. measure roughly in between these two marks, put piston head at this mark and measure the skirts with a feeler gauge. This will tell you if the piston is too small for the bore.  As far as the wear on the exhaust side of the cylinder, you will need to take a snap gauge and mic and measure radially around the cylinder above and below the exhaust ports. But even so, if your bottom end is "shot" Your piston is going to be doing the "hula" whilst going up and down the bore. Hope this helps

[NZ500]

Thanks for your reply. The engine is completely stripped now. The difference in ring gap size from the top of cylinder (at position where clymer book says to measure) and where the most worn part is (at bottom of cylinder in middle of intake ports) is only about 0.05mm. Sorry I didn't have the cylinder with me when I worte original post and mentioned the wear as being on exhaust side not intake.

My piston measures 85.91mm and bore is 86.02 when measured 90? to piston pin. On same axis as pin it looked closer to 86.03.  That gives me a piston to bore clearance of  0.11mm or 4.4 thou which I'd say is pretty decent?

So unless the piston wasn't cleaned it must have been the piston wandering like you mention. Just surprised me a bit at first because I've had bikes with more wear and no blow by. The main bearings feel fine, all of the wear is in the crank (ignition side) and it's obviously been worn for a while without being fixed....

[jonny500]

i agree with the advice given so far on this thread. however in my experience the bigest killer with big bore 2 strokes are vibrations. if your bore has a liner then it is cheap to re-bore as aposed to plating. if this was my bike a would do every thing while it is apart. conrod, mains + seals, piston, and rebore. i belive you get a knock on effect. if one thing is slightly wrong it can have a bad effect on the rest on the engine. you dont want to spend presious money on new parts just to have them worn out though vibartions from other defective parts. beside you will get more hours from an engine that is in good trim so i consider it more ecconomical this way. and also less vibrition and an engine in good order = MORE POWER. but every one is on a budget so i suppose its what you wana spend now.
best regards
john

[NZ500]

yeah there is a shop that replates about 50 miles away but i think i'd prefer to have the sleeve and know what im getting. I thought this bike was going to be any easy fix. has a snapped kick starter shaft and broke the inner clutch/engine cover. but needs quite a few repairs. going to rebuild some power valve parts. thinking of doing the exhaust sleeve after seeing that on here recently too. one of the gears in inner clutch cover has been machined out and had a bronze bush installed, going to replace that bush. if i make any new valve guides i'll post some pics and measurements of them. was thinking of making steel left and right guides. I want everything to be spot on before it goes back together so it lasts a long time.

Yeah you're right about the vibrations. I know a good reconditioning shop that does bike cranks for a lot of local bike shops. going to get them to do my kx crank and make sure its spot on. They did my 03 ktm sx200 crank a few months ago and ground down the worn journal, hard chromed it and reground. What you mentioned happened to that bike. I put a new wiseco prolite piston in it and within 10 hours bottom end was gone. was running high compression though with 0.9mm squish. I really dont know how the piston didnt smack the head. but the piston was rooted from bearing particles (I presume) and put a used one back in it.