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I just picked up an '87 KDX 200 for a good price. The PO told me he had spent $1500 last year having the motor and carb rebuilt. It ran good, but it had a abnormally loud rattle in the top end of the engine. I pulled it apart and was glad to see that the cylinder had been re-chromed and it had a new piston and connecting rod.
It had a Pro-X piston and I traced the noise to a loose fitting piston pin. I threw on an old piston and cylinder from a parts bike (was sleeved and .50 over) and the rattle is gone. I plan to replace the piston.
Are Pro-X pistons a POS? Anyone else have issues with them?
The Pro-X piston looks brand new.

I believe pro X has different size, maybe it's the wrong one?
Most use wiseco but pro x has also a good reputation as far as I know.
The "loose fitting piston pin" comment is not good, are you talking about the circlip that hold the sleeve in the piston?

The piston to cylinder clearance is fine. The pin feels tight as you start installing it from the side, but once it is in position in the piston, it moves very freely.
I re-used the upper bearing with the used setup I installed so that is not the problem.
I probably will go with a Wiseco like I put in my 220.

Here is another question: would you replace the standard bore piston in the plated cylinder or re-ring the 0.50 over sleeved cylinder that already has a Weisco?
The consensus on the site seems to be to avoid steel sleeves.
This will be my "friend bike".

"it moves very freely"
Do you mean up and down or form side to side (with the snap rings installed)
BTW, I wouldn't recommend re-using snap rings, I'd get new ones, good cheap insurance.

Check the rod bearing?? Almost sounds like thre isn't one.

The pin feels loose in the piston. It moves side to side and spins.
Typically, they are a tight fit.
Rod bearing was like new and I re-used it during the test.
I wonder if the wrong pin got installed.
Likely it was just a defective piston, if others haven't had issues with Pro-X.

sounds like the wrong pin to me

Sorry, this threw me :

" The pin feels tight as you start installing it from the side"

scheckaet is probably onto something.

Sounds worn to me.... like when they changed pistons they didn't relace the pin.

Calipers would confirm.

Indawoods wrote:Sounds worn to me.... like when they changed pistons they didn't relace the pin.

Calipers would confirm.

Could be. It looked good, but I will check the measurement.
Thanks.

Yeah that's the wrong pin or something else is going on. A pro-x piston should do fine for your 200, of course wiseco will probably last longer. Break in and warm up routines are much more important with the wiseco due to the larger cold clearance required for forged pistons.

Definitely want to get your placed cyl on there instaed of the sleeve. Difference in cooling on an A/C model should be quite different.

Well I broke out the digital calipers last night. The piston pin was dead on. The cylinder actually measured at 65.94mm all of the way around, which is slightly below specs. I will get out the inside micrometer this weekend to verify. As I said, the plating looks good with lots of cross-hatch still visible.

It appears that the problem is the piston itself. The calipers consistently came up with 65.65mm for outside diameter. The difference between the two measurements is well beyond the allowable.
The question is: How is it that the piston is that small? It doesn't look warn or seized.
I know it is good practice to send the piston off when re-plating. I'm a little concerned about ordering a new piston until I get a more accurate measurement of the cylinder. The Pro-X I took out looks to be a cast piston. If I go with a Wiseco forged piston, I'm concerned that it will be too tight.

It is also strange that the noise did not sound like piston slap. It was a constant knock on each stroke which led me to think it was a top or bottom end bearing. The sleeved cylinder and piston I threw on the bike sounds fine, but compression is only 115 psi. I'm thinking of just re-ringing what is on there now.