oval top piston -- flat top piston?????

[handleme46]

I recently bought a pro-x piston kit from a local mechanic shop, and when i got home and opened the box, i realized that the piston is a flat top.I dont know what my 96' CR125 came with stock, because it was used, but the piston that was in it when i bought it was an oval top. So i was wondering if anybody knows if it will harm/ or affect my brandly new plated cylinder in any way??? im going to make darn sure its ok before i try it because the plating on that cylinder wiped out two full paychecks....lol thanks in advance

 

[mxneagle]

I don't believe that the piston change will damage the cylinder. However, the piston crown and head shape are related. Obviously putting a flat piston together with a head designed for a crowned one will lower the compression ratio. You have an interesting problem here, did the previous owner use a cylinder and head from an older model? Certain year honda 125's had better power characteristics than others and it's possible that the switch was made.

 

[handleme46]

The bike is actually a 91' model with the 96' year engine, and i checked the serial numbers so i know its the 96' engine beacause i heard it was one of the better year engines.... any other suggestions?

 

[mxneagle]

Is it a 96 engine or a 96 bottom end with a 91 cylinder or some cross match between the years? It is possible that the piston was for a 91 and someone didn't realize it was a 96 motor or intended to raise the compression ratio. You can take some measurements and/or put the piston in and check for clearances and measure compression ratio (you could do this without putting circlips in) I'd do my homework before firing up the bike.

 

[handleme46]

I went and talked to the guy i bought the piston kit from and he said he ordered the piston for a 96' top end. I verifyed this and also verifyed that the top end is the 96' model. I also have recently heard that the pro-x piston that i bought is flat top but keeps the same compression as a dome top because it is a tad longer. The piston that i pulled out of my bike was a wiseco, and was dome topped. So im thinking that its just a difference between brands. Im still waiting on the bottom end, so when i get il try a compression check but i dont know what the original compression was, but it might be in my manual. I also measured my original wiseco against the pro-x, and the pro-x is slightly longer.