2001 cr 125 piston size??????????

7 replies to this topic
  • czapa

Posted December 06, 2009 - 06:34 PM


hi there again
so heres my little strory/problem
i bought used 01 cr 125 few months ago and wanted to have some fun with 2stroke(cause i tried 4 stroke eariler but not too big fan)
i rode it with times and noticed black oil coming in little drips from exaust noticeable on the right side of the swing arm.
my friend told me that i should check the compression on top end and most likely i need to replace the piston with the ring
i did check it and the compression was 75 psi, which is way too low , as far as i've read,
bought the new standard size piston "B" for 2001 cr 125 , put it on , and compression is just the same
while putting it back together i noticed that on the very low left side enginge cover, right under gear lever it says piston stroke 54.5 mm
is this the piston size that i need?cause when i bought new standard piston and ring i tried it in my hands in the cylinder and it felt pretty tight, but i measure it and it was close to 54 mm
what is my problem ??????????????????????????????????
PLEASE HELP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • bikesnwheelers

Posted December 06, 2009 - 06:37 PM


Piston stroke has nothing to do with the piston size. The reason for the oil is it is jetted too rich.

  • czapa

Posted December 06, 2009 - 06:58 PM


So why my compression reads only 75-80 psi? Is it normal ?
What do you think?

  • bikesnwheelers

Posted December 06, 2009 - 07:12 PM


Is your cylinder within spec?

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  • czapa

Posted December 06, 2009 - 07:25 PM


Are you on chat by any chance?
What is within spec?>
i measured it with digital caliper and it reads almost 54mm
so i was hoping that the standard piston b that i bought would do the thing

  • cpr1

Posted December 07, 2009 - 01:57 PM


The standard piston bore size is 54mm. That's what you have in your hands right now. Check the cylinder for excessive wear, such as worn out plating around the exhaust port and at the top of the cylinder.

  • czapa

Posted December 07, 2009 - 05:42 PM


hi , i thought it looks okay and when i try the new piston in the cylinder felt like its pretty tight, but i guess i need a cylinder?

  • jmm18

Posted December 07, 2009 - 06:53 PM


You should meassure cylinder with centesimal presicion and buy a new piston.
For example Honda explain that the correct piston to cylinder clearance is 0.03-0.06, if you cylinder is 53.97 mm you need a 53.92 piston (0.05mm clearance)
Pro X and Vertex produce oversizes piston for worn cylinders (a,b,c,d)

Sorry for my english:thumbsup:

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