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So after doing the rear shock rebuild on the 94 kdx200
and cleaning out the KIPS valve... I was about to start on the front shock rebuild.

I figured i was going to replace the piston as well to be safe ( and associated hardware)
Took off the piston and saw this...( already had a new stock piston on hand)

here's my imgur album with pics
[album][/album]
http://imgur.com/a/WH0Op
The streaking you can catch it with your fingernail
The chewedup part i think is from the intake side ( its on side opposite of the arrow on top of piston)

The cylinder wall feels "ok and smooth"

old piston met spec per cyclepedia as did the cylinder ( I need to redo all the measurements to be sure my new piston is ok still unclear on this)

Am I good to go to drop in a new piston? or should I investigate further? ( bottom end)
Have not drained oil, bike is new to me previous owner didn't really take good care of it

black wrote:So after doing the rear shock rebuild on the 94 kdx200
and cleaning out the KIPS valve... I was about to start on the front shock rebuild.

I figured i was going to replace the piston as well to be safe ( and associated hardware)
Took off the piston and saw this...( already had a new stock piston on hand)

here's my imgur album with pics
[album][/album]
http://imgur.com/a/WH0Op
The streaking you can catch it with your fingernail
The chewedup part i think is from the intake side ( its on side opposite of the arrow on top of piston)

The cylinder wall feels "ok and smooth"

old piston met spec per cyclepedia as did the cylinder ( I need to redo all the measurements to be sure my new piston is ok still unclear on this)

Am I good to go to drop in a new piston? or should I investigate further? ( bottom end)
Have not drained oil, bike is new to me previous owner didn't really take good care of it

Can you take any well lit pictures of the cylinder? You are wanting to reuse and us to comment on something that you don't have pictures of. The little flecks of piston went somewhere. Obviously, it was running before and will run again. How well and new is up to you.

yup i'll take some pics of the cylinder.. learning alot more about this bike, spent the last 20 minutes see ing pics of what good and bad cylinders look like.

Those metal pieces went somwhere for sure.
Not looking to reuse the piston in the pic in the comments at all. I have a new one ready to drop in with new rings and such. Just dont know the implications of that "chewed' part" in the old piston. as well as the other wear marks on the piston.. Dont know how to "read" a piston if you catch my drift.

Totally understood on you were not going to reuse the piston that is pictured.

I am guessing you took it apart "just because" and did not hear a grinding noise or anything like that, correct?

Agreed. Bike was new to me. looked abused/ not paid attention too

Wanted to be the good KDX owner and just change the piston to be safe.
It ran fine, no noise or tapping or grinding.

There simply have to be metal fragments in the lower end of this bike. So you have two choices as I see it, (1) remove the engine from the frame, turn it upside down and solvent flush the heck out of the lower end to try and remove any metal or (2) split the cases and do a proper IRAN ( Inspect and Repair As Necessary). Maybe someone else has a better idea. Oh, you could always put it all back together and hope for the best I guess.

Buy a wiesco standard size piston and ring set and put it in there. Flush the case with a oil change and run it! I've changed two pistons on two different e-series bikes that both looked similar to yours. The semicircular groove cut out was sharp and almost looked like it had chipped off, on both bikes. I think those are the stock pistons. I have changed oil in both bikes several times since then and have not noticed any metal shaving or debris in the gear oil.

The crank case/cylinder has nothing to do with the trans oil. Without seeing the cylinder and only looking at that piston I would have it replated. I doubt its ok.

here are some more update pics of the cylinder and one of the piston
[album][/album]
basically the inside felt very smooth, more so after a light rub with a scotch brite pad
there is some "smearing" but its on the bottom of the cylinder not the top and towards the intake side.

i did find a small "hairball" about 3mm x3 mm in a corner very odd, was not magnetic it more than likeley was some detritus from the mouse nest in the airbox and when it was run by the owner.

That looks pretty worn.

I would pull the motor out of the bike and fill it with pre mix. Turn the crank a few times then turn it upside down to flush any trash out. Do this several times. If your piston looks that bad the cylinder definitely isn't "good". I would send it to Millennium and have it replated. It will cost in the ball park of $200 for the replate but you will have a good as new motor when you are done, as long as you know the crank and bearings are good too.