No compression after replacing piston
Posted: 11:34 am Sep 26 2023
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.
![Image](https://i.ibb.co/yqRqCnK/cylinder-shim.jpg)
I can put a 25 mm shim in between the cylinder wall and the piston. I am assuming this is normal?
![Image](https://i.ibb.co/vdhVMs5/cylinder-two.jpg)
Measured the cylinder and it appears to be 66 mm. It appears to be stock?
![Image](https://i.ibb.co/Vj1s9Rt/cylinder-measure.jpg)
I can't account for why I am not getting enough compression! Any ideas?
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.
![Image](https://i.ibb.co/yqRqCnK/cylinder-shim.jpg)
I can put a 25 mm shim in between the cylinder wall and the piston. I am assuming this is normal?
![Image](https://i.ibb.co/vdhVMs5/cylinder-two.jpg)
Measured the cylinder and it appears to be 66 mm. It appears to be stock?
![Image](https://i.ibb.co/Vj1s9Rt/cylinder-measure.jpg)
I can't account for why I am not getting enough compression! Any ideas?