Top end knocking?? Is this normal?

[joetaco87]

Ok guys, I just rebuilt the entire motor on my 1999 kx250.  New main bearings and seals, new hot rods crank, new piston and rings, new tranny bearings, etc.  Started up in about 4 kicks (awsome) sounds great sounds like its running fine. But at idol and maybe a little less than 1/8th throttle there is a faint knocking(actually more like a clicking) sound which sounds like its in the top end.  i took the spark plug off and rotated the engine slowly by hand and its when the piston reaches the top of the cylinder right as it goes back down.  Is this normal or is it somthing i should look into? Thanks.

[cbxracer30]

How about the wristpin bearing ? new ? or worn cylinder -to much play in cylinder.

[joetaco87]

I think its the piston-cylinder gap, you can see where the plating is worn at the top.  I was planning on having it replated soon i just dont have the cash right now. All the bearings are new in the motor.  Can I ride it like this for a little while?? I dont race, for most part its single track woods riding or powerlines.

[KXcam22]

I am guessing that by gap you are talking piston clearance,. with the noise being the slap as the rod pulls the piston to the other side of the cyl right before the down stroke.  The noise may go away when the engine warms up.  It is a new piston?  Piston slap is not good but for woods riding you should be ok for awhile.  At high RPM is can cause the piston to shatter.  Cam.

[joetaco87]

yea thats what i mean by the gap, everything in the motor is brand new. crank, bearings and seals, piston, rings, etc.  Im pretty sure this bike was raced before i got it.  I bought it for cheap and its was real beat up.  I had planned on having it replated asap im just waiting to pay off my bills lol.  I havn't ridden it yet, my chain and sprockets come in tomm so ill let you know how it goes.

[joetaco87]

I also noticed they have different stock piston sizes (A,B,C)  Do you think getting a "B" or "C"  size piston will give me a little while longer before i have to send the top end out?  The piston thats in there now is "A" size.

[KXcam22]

My understanding is that the A B & C pistons are all different sizes (slighly larger) and are installed to match the manufacturing tollerances of the cylinder.  You could probably run a C in a  worn cylinder that an A is too loose in but I have no direct experience in doing that.  It is possible that you have a C cylinder too, with the A piston being too small. Some measurements will tell the story. Cam.