RM125 big bore question.

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8/13/2018 1:16 PM

Just ordered the wiseco 4mm over bore (144cc) for my 2005 rm125. Anyone that’s done this did you just get it plated? How much did you allow for the bore clearance and does the stroke change at all to the point where I’d have to be running different gaskets or will the ones that it came with be fine. I believe I heard someone say once they had the 58mm piston in it would hit the cylinder head at TDC. Any other precautions I should take? My other question would be about jetting, what jetting would be best with the big bore. Also have a pro circuit works pipe on the bike and stock reeds. Pretty low elevation where I live.

Thanks in advance.


8/13/2018 2:20 PM

Stroke is not affected by changing the piston. Wrist pin height can change.This causes the dome to be higher up in the cylinder. This is not the case with the RM going to a big bore, but you should always measure before assembly. You will have to bore the cases to clear the piston skirt. Wiseco will give you proper clearances in their instructions.


Tomac and/or Anderson for 2020.....

8/13/2018 2:36 PM

The wiseco flat top 58mm is a direct replacement for the 54mm stock piston once you've bored the cylinder.

You will also need to redo the head and relieve the cases to make room for the bigger piston.

When doing a big bore, because the ports are angled, the big bore effectively lowers the ports and moves the powerband to lower rpm so the person doing the big bore will have to port the cylinder before replating


8/13/2018 6:29 PM

If this helps, it’s pretty easy to modify the cases yourself. You need to tape up the bottom end so no swarf gets in there - I was doing a full rebuild so these cases were bare and I didn’t bother as I blasted them afterwards.

1. You put the 144cc cylinder on and mark the cases with a pen where the piston would contact it

2. Then relieve it with a dremel