CB125 big bore kit

liomessi019

New Member
I want to get a big bore kit for my 1978 cb125s. What is the maximum cc's i should go up to? I was thinking like 175cc or 250cc
 
liomessi019 said:
I want to get a big bore kit for my 1978 cb125s. What is the maximum cc's i should go up to? I was thinking like 175cc or 250cc

if you really think you can bore a stock 125 motor out to 250... you shouldnt be boring.

and you dont measure by maximum cc's you measure by amount of material removed...
 
I forgot who said it to who, but i think a CB750 piston will fit.

So however much you can over bore a single cylinder on a 750, should be able to do the same to your 125. There's some if's and maybe's involved but it's a start.
 
It's cool, everybody starts somewhere.

Look on eBay for the biggest pistons and rings that can fit your bike. I personally dont recommend trying to squeeze that kind of power from a 125, but it's your bike.

There see other ways to get power from a motor - tuned intake / exhaust, port and polish the head, aftermarket coil, etc. Keep at it man.....those 125's are great scoots
 
Powroll have been kings of the 125 HP race for decades. They aren't cheap compared to some Chinese parts, but it's the place to start.

http://powroll.com/P_HONDA_VINTAGE_100-125.htm


You will need a piston kit plus larger liner. The motor has to be stripped and the crankcase mouth opened out. The liner is removed and the barrel bored to take the new liner which is pressed in and then bored and honed for the new piston. Then you want a cam to help it breath and good port job plus a larger carb and free flowing exhaust.

And that's to get to 145cc. If you add a stroked crank, you get to 175cc.

Other possibilities include CB77 (305cc) 60mm piston or CB750 (61?) or Wiseco CB750 64mm 812 kit or WISECO KZ650 big bore piston. Those will al require some machining to the piston as well as all of the above.
 
Bigest piston you can put in a stock CB125 cylinder is a modified CB750 piston. It gets you a 145cc motor.
Going bigger is a waste of time as the head won't accept much bigger valves, then the valve springs fall into ports if you try and open them too far.
Keep dreaming about a 250 conversion, even with a stroker kit it ain't going to happen.
It would be far easier and cheaper to get an XR200 motor or the 'new' CRF240
 
or a chinese vertical clone. i've got a 250 single in my cb125 frame. got it fired up the other week :)
 
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