No compression after replacing piston

[DenverKdxRider]

Hey Ya'll. Got a 1996 KDX 200 and I am struggling to figure out why I have such low compression after putting in a new piston. Full disclosure, I am a swinging d-ck youtube watching Mechanic, so feel free to make fun and laugh at anytime. I decided to rebuild the top end and put in a standard Stock Bore 66mm Wiseco piston. After putting it in the bike, it seemed to run fine but when I took it out on the trails it had terrible acceleration and performance. I thought it was my jetting and I played around with some jets, but I finally did a compression test, which I should've done the first time, and I noticed the compression was barely breaking 60 PSI. Standard compression range to my understanding is mid 90's to 140 PSI.

I then decided to buy a Pro-X racer 65.96mm +.02mm Oversize "C" Pro-X Piston Kit and put that in, I haven't started the bike but my compression is still barely breaking 60 PSI. When I put in the Pro-x piston, I can put a 25 mm shim in between the cylinder wall and the piston, which I assume is normal. I checked and rechecked the piston rings to make sure they were aligned and still nothing. Also replaced gaskets, sleeve and new surclips. Also tightned all bolts to spec. You are probably wondering why I haven't checked the old piston the bike came with, welp naturally, I can't find it after I moved recently. So both standard bore wiseco 66 pistons do not give me enough compression.

Here's just a top end.


I can put a 25 mm shim in between the cylinder wall and the piston. I am assuming this is normal?



Measured the cylinder and it appears to be 66 mm. It appears to be stock?


I can't account for why I am not getting enough compression! Any ideas?

[bufftester]

I take it you didn't check compression on the old piston before you tore it down? Have you checked the piston ring to groove clearance and ring end gap? Checking cylinder bore with a micrometer is not super accurate, really need bore gauges for that. Either way 66.1mm is the service limit. Stock piston is 65.8mm and your Piston-to-cylinder clearance should be 0.079 – 0.089 mm. If you're getting a 25mm shim in there you got a problem

If it were me I'd probably send the cylinder out to get it replated

[Molly's 70]

Call up these guys.
https://millennium-technologies.com/

[DenverKdxRider]

Called and got an estimate and it is on it's way to Millennium Technologies. Will be a chunk of change but I am done fighting the war and want to get back onto the trails. Will keep you all updated.

[DenverKdxRider]

Quick update. I sent my top end jug off and sent it to Millennium Technologies (MT) to get fixed up. They did a bang up job. I also had them order the correct Wiseco piston and rings from them to be on the safe side.

Welp, I put it all together and did a compression test and compression is around 125 PSI which is well within the range of 95-150ish that the manual says. I had posted earlier that it was the same as before but turns out, I put the rings on upside down (I’m bind as a bat these days)! Super pumped MT was able to solve my problems and I can get back on the trails!

[bufftester]

NIce, pics from the first ride!