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rmx250 porting


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hi.. couldn't find many threads on porting... has anyone had a porting job done, or know if there's any room for improvement on these awesome bikes? I had to tear down mine for a crank bearing replacement, and while putting it back together, I broke a significant piece of the back(intake) side of the piston off on accident:bonk: (bout the size of a quarter) I filed it down to round edges and keep from hitting ports and for experimental reasons.. went ahead and finished the rebuild.. well, it worked so great that I couldn't even attempt to keep the wheel on the ground.. such Incredible torque!! seemingly all the way to the top... so it got me curious, what is known about porting for this bike??

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hi.. couldn't find many threads on porting... has anyone had a porting job done, or know if there's any room for improvement on these awesome bikes? I had to tear down mine for a crank bearing replacement, and while putting it back together, I broke a significant piece of the back(intake) side of the piston off on accident:bonk: (bout the size of a quarter) I filed it down to round edges and keep from hitting ports and for experimental reasons.. went ahead and finished the rebuild.. well, it worked so great that I couldn't even attempt to keep the wheel on the ground.. such Incredible torque!! seemingly all the way to the top... so it got me curious, what is known about porting for this bike??

got any pics? :banana:

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What you did was change the intake port timing. When I was with yamaha, I use to change the pistons in the old IT250 to the YZ250. The old IT had two holes in the intake side of the piston that would stress crack across between the holes or it would brake off the entire intake side from 1/2 way down the piston and drop it into the crank. By changing to the YZ 1/2 moon cut piston, it gave the bikes more punch all the way from bottom to top and it eliminated the main reason of breaking the stock style piston.

I just took a look at my 03 RM250 piston and it would mimic the same change that I did with the Yamaha IT. I don't know how your RMX piston is shaped, but if the piston pin, bore and distance from the top of the dome to the center line of the piston pin is the same, then it would be a straight up swap.

Edited by Padgett
Looked at the piston
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  • 1 year later...
  • 7 years later...

I know this thread is ancient...

But I just couldn't resist bumping it to the top....

Holeeeeeeeee Batshit Batman...

" I broke a chunk of piston off the size of a quarter and ran it anyways"

First off... Are all your fingers thumbs and the only tools you own a collection of hammers??

And even that is the case (seems to be so)

Where do make the decision to run it anyways???

My RMX is case reed , so there is no effect from piston on intake port timing... None at all...

So this has to be an earlier model rmx that has the Reed/ intake on the back of the cylinder... But the intake port arrangement should still have lateral port runners that feed the case directly , without regard or dependency for piston/ port timing...

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