Piston movement??

[Moto man26]

Hey guys, I was wondering how to know if a piston is moving around too much inside the cylinder(slapping side to side)? I know this means that it's time for a top end rebuild...But what are the noises it should make and symptoms?

Thanks alot!!

[crfjedi393]

Do you have the head off yet? Or are you talking about noises it makes with the engine running?

[RBsRm125]

if there is that much room i dont think you would have a very good compression....if one it all

[pragmatic393]

It makes more sense to check compression, rather than trying to describe a noise. Do a search on here for "compression check".

[Dirtbikindad393]

Originally Posted by pragmatic393

It makes more sense to check compression, rather than trying to describe a noise. Do a search on here for "compression check".

What he said!

[Moto man26]

Hey guys just to mention ...there is nothing wrong with my bike, I'm just asking this question to learn more about it.

[yzernie]

You will notice a gradual decrease in power, especially off of the bottom end. There may also be excessive goop coming out of the breather tube from the power valve. The compression test is a great indicator of lost compression BUT, you would have to know what the numbers were at the start with a fresh piston and rings.

[Faded]

Measuring is the only real way to know, and no I don't mean with a feeler guage... T-Dub did a great write-up on piston-to-cylinder gauging/clearance, you might want to check it out: [Only registered and activated users can see links. ]

[The Baron]

Faded,

When replacing the top end, after measuring the cylinder bore, give me a good idea on how to select piston size. I don't like just ordering a piston for a 2004 RM125 and hoping that it fits okay. Say the bore is 95mm, how do I pick the best sized piston. I know I can order them in 0.25mm increments, so how much smaller than the bore does a guy go on the piston? Basically, what is a good clearance measurement?

[ossagp1]

That is very good. How did I miss it when it was first posted?

Since most don't have those tools, you end up relying on how much plating is missing to determine if the cylinder is still useable

[Faded]

Originally Posted by Baron Von B

Basically, what is a good clearance measurement?

Just like everything else if you're trying to find the 'edge' you need to consider the variables...clearances vary from as tight as 0.0005" (0.013mm) on up.

Generally speaking a clearance of 0.0015" to 0.0020" (0.038mm to 0.051mm) will work just fine.

[The Baron]

Originally Posted by Faded

Just like everything else if you're trying to find the 'edge' you need to consider the variables...clearances vary from as tight as 0.0005" (0.013mm) on up.

Generally speaking a clearance of 0.0015" to 0.0020" (0.038mm to 0.051mm) will work just fine.

Coolio. That was the measurement I was looking for. Thanks brutha.