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piston or sprocket

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i recently had the piston replaced in my 2001 yz125. the piston before this one had alot more low end torque but it topped out quick. this new piston has no low end but ALOT of high end. i race arenacross so i really need to low end. do i need to replace the piston?if so with what kind( i know there is an A-D or something)? or would i be better off replacing the rear sprocket? i am a begginer so i could use alot of help!!

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Your piston has little to do with the power band of your bike. It's possible that you are now loosing compression making you think you have lost bottom end. The letter designations of the pistons tell you how much oversize from stock they are. Your old piston should have a letter stamped on it.

For more help in that area you may want to go to a 2 stroke site and get some info on the correct way to measure your piston, jug, ring gap, etc....

On the gearing...your bike comes stock with a 13/51 sprocket set-up, which is pretty good for a 125 in just about any circumstance. If you want a bit more low end grunt, you could go a couple of teeth more on the rear sprocket.

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