Piston size?

Hey guys, I'm new on this forum and have my first question. I recently bought a 1989 RM 250 for $350 in great shape but the top end was blown. I took off the head and the piston reads 72.94MM on the top of it but that size isn't the normal size for a piston of that year. As I'v read the pistons that fit that are ( 67.00mm, 69.00mm, 68.00mm, 67.5mm ) can someone help me out im confused thanks.

Your best bet is to measure the bore. Get a caliber, measure it a few times, and see what it really is. 

Someone has done a 295 big-bore on that engine. There are many tuners that do those, getting a piston for it shouldn't be a problem.

is there any bike that takes that size piston so that I can narrow it down on a parts site? because iv been on a few different sites and typed in 72.94mm piston in the search bar but nothing comes up... BTW whats would be the best site for getting stuff

As mentioned it sounds like a big bore kit of some kind has been fitted. The standard bore is 67mm.

The standard piston sizes are graded slightly under that 67mm figure to give correct piston to bore clearance purposes:  In the range of 66.935 - 66.950mm.


One thing to consider is that with a 73mm piston the powervalves will have been modified to suit the larger bore. So if you cannot source the correct piston. Be aware that your current powervalves will not be an ideal fit in a standard bore cylinder should you choose that option.

so a 73.00mm will fit ? because mines a 72.94 ? or are they pretty much the same? this is the first bike iv owned so sorry for all the questions :)

It is very important to have the correct piston to cylinder bore clearance for the engine to run properly and reliably.
The piston ring end gaps also needs to be checked to see if they are within tolerances.
A piston for a 73mm bore will be slightly under that size to give the correct clearance. There may be as many as four different grades of "73mm piston" to be selected from to allow for sizing differences and wear in the cylinder.

You mention the "top end was blown". Is there any damage or scuffing to the cylinder wall, or marks around the ports where debris or the piston rings may have caught the edges?
Also, if the piston came apart then the bottom end may have debris sitting around the crankshaft and reed case areas that could damage a new top end when the engine is next started. It is very important that this is cleaned out properly.


If the cylinder is good, the best thing to do is have it accurately measured so the correct piston size can be selected.
Piston to bore clearances can vary depending on whether a cast or forged piston is used. This is because the two piston types can have different expansion rates, and this needs to be factored in so the clearance is correct when the engine is at optimum running temperature.

It would be best to get the cylinder checked over and measured by someone with experience before ordering a piston.

could i take it to any machine shop to have it looked at? 

Of cour$e. But expect to pay $$$.

thats a big bike for your first.


not sure if you realize the power they have.

I've ridden my friends bike a bunch of times before, This is my first owned..

is there any other numbers on that piston? If so, that would be a p/n, and run it through wiseco. A common practice with big bore kits was to use o/s Honda slugs

There should be a stamping/engraving somewhere on that cylinder that tells who did the work.

8492 Is the only other numbering on the top of the piston

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