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RM piston mod

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Hey guys, been reading a lot lately about doing the RM piston mod in my 2003 cr125 w/ Eric Gorr 134 kit with mid-top port and 7oz flywheel weight.

Eric says the RM piston would make a big difference in the bottom end, and I trust eric completely. The only thing I am wondering is how much will it flatten out my top end? I race harescrambles so I need bottom end, but I would really hate to kill my topend....thats what makes a 125 so fun to ride :smirk:

Bike specs:

2003 cr125

EG 134 BB w/ mid-top port (pump gas)

Boyseen rad valve

pro circuit pipe/shorty

7oz flywheel weight

12/53 gearing

Stock carb/05+ air boot

Opened up airbox (10 1/2 holes)

So how bad of a hit would my bike take on the top end power and over rev??

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why not ask eric? i think he would more or less know the answer to the question.

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I did talk to eric about it, but he was unclear about how it would affect my top end power. I am positive it will help me with the slow speed technical sections, but I really enjoy the pull my bike has up high so I was asking to see if other guys have done this and can comment on what it did to the power curve

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i don't understand, Eric Gorr knows a rm piston will increase the bottom end but don't know what effects it has on top end. Did he just test bottom end with the rm piston.

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i don't understand, Eric Gorr knows a rm piston will increase the bottom end but don't know what effects it has on top end. Did he just test bottom end with the rm piston.

I think what he was saying is that Eric's answer to his question was unclear. IMO, this is an issue with the porting that he chose to get. He got mid/top porting and is trying to trail ride a 125 with it. IMO, it is time to use Eric's option to have porting changed for free. Get the overall power increase port or the low/mid port. There is always a compromise when it comes to these motors, if you put in an RM piston, it is going to increase the low end a bit with the compromise of losing some top end power. No one is going to be able to tell you exactly what that is going to do to your power delivery, you will have to experience it and then decide if you like it or not.

EDIT: IMO I would guess that changing the piston will not make a noticable change in your power delivery. It may show up on a dyno chart but it would take a huge change in power delivery to notice on the butt dyno.

Edited by frdbtr

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