Should I bore or stay stock? rebuilding my CR125

[wexy21]

I have an 02 CR125 and am rebuilding top and bottom.  My cylinder wall and piston are scratched but I didn't lock it up.  The rod needs replaced also.  So I was considering boring to a bigger size.  I'd like to go as big as I can and get as much power out of it as I can.  But if boring it out is going to mean it will take a lot more work and money then I'd rather just stay stock.  I can do everything for about $500 (crank kit, piston, seals, gasket, new cyclinder, etc)  but I would like to just have my cyclinder bored and get a larger piston. 
  If I do bore....is that all I need to do?  Do I need to worry about doing work on the powervalve too?  How about the crank, any work there?  Thanks for the help!!!!

[JohnN]

Welcome to the board!

To bore a cylinder to an oversize requires additional work to your engine, including head work and power valve work.

Yo give you an idea of what's involved check out this site by American tuner Eric Gorr... it will give you an idea of what must be done and approximately how much it will cost.

http://www.eric-gorr.com/supermini144BigBore125service.html

Good luck!

[admiral]

I have an 02 CR125 and am rebuilding top and bottom.  My cylinder wall and piston are scratched but I didn't lock it up.  The rod needs replaced also.  So I was considering boring to a bigger size.  I'd like to go as big as I can and get as much power out of it as I can.  But if boring it out is going to mean it will take a lot more work and money then I'd rather just stay stock.  I can do everything for about $500 (crank kit, piston, seals, gasket, new cyclinder, etc)  but I would like to just have my cyclinder bored and get a larger piston. 
  If I do bore....is that all I need to do?  Do I need to worry about doing work on the powervalve too?  How about the crank, any work there?  Thanks for the help!!!!

you can't bore a stock OEM '02 CR125 cylinder as it is plated. if the plating is scored then you must replate or install a sleeve. the later CR125's models don't respond as well to an increase in bore size. i would recomend staying with the stock bore size and re-ringing  often. depending on how many hours or how hard you ride you can replace the ring a couple or three times before replacing the piston. of course the piston would need to be measured at every re-ring to make sure it is in spec. i have a riding bud who loves YZ125's. he starts the year with a new piston and ring then re-rings two more times a season. this way he can keep the top end fresh, minimize piston wear, and monitor his piston and cylinder wear and can catch any problems before his engine gets trashed. the great thing about 125's when it comes to rebuilding is the cylinder can be pulled off without removing the head thus eliminating the need to buy a head gasket. so for the price of a ring and a base gasket ($35?) you can keep your engine running strong and know whats happening with your jug and piston. big bores on a CR125 will require head work, power valve clearence machining, special head gaskets (non oem).  i am not a fan of aftermarket Honda CR head gaskets. the metal OEM head gaskets are great not so much with the aftermarket ones. i have seen way to many Cometic, Moose, Vesrah, etc. blow on CR125/250's over the years.

[Out of Order]

If displacements not a problem run the biggest piston and you can also stroke the crank to make a 167cc. You have to send the cylinder out to get bored and plated (something you can't do yourself), and if you want to stroke it you have to send out the cases and crank also. Don't forget if your racing, most series only let you bore 2mm overbore, so be careful.

[wexy21]

I've decided to stay stock.  I don't race anymore but it's just hard jumping off of my 450 to my 125.  I would like a little more bottom end (I know, hard on a 125)  but my YZ 125 was a LOT quicker then this bike is.  I'd never trade it but I do want it a lil quicker.  So far I have a SST gold series pipe and turbo II silencer, everything else motorwise is stock. 

[JohnN]

Jumping from a 450 to a 125 will make any 125 feel slow!! LOL

But the 2002 CR125 can be made quite a bit faster by opening up the intake area of the machine... you can read about what needs to be done in this article;

http://twostrokemotocross.com/2009/05/budget-racer-2001-honda-cr125/

I hope this helps guide you in the right direction. A friend of mine did the intake mods recommended in this article and it was a night and day difference in his CR125!

Good Luck!

[wexy21]

That's cool thanks for the help.  I moved from the midwest to the desert here in Cali and although I have to worry about sand....I don't have to worry about rain, mud, dirt, etc that would normally make me hesitant in cutting my box up.  I did that to my first 125 back in the day and Always had that stuff in my airbox!  Out here the sand gets everywhere anyway so I clean the filter more then ever.  Cutting my airbox out here is something that I was planning on doing. 
  Aside from doing that I planned on rejetting, keeping the sst pipe and silencer, and maybe re gearing.  I am almost positive that my reeds are aftermarket.  they are carbon fiber and the gaskets are new...anyone know what brand they could be?  Aside from that above what else could I do to get a lil more grunt? 
  Going from the 450 to the 125 is a huge change...but not in a bad way.  It's slower and doesn't wheely as easy or climb hills at ease.  but It's a LOT more fun on the track anyday!  I can fly on it and not get tired.  But in the open desert I feel really slow:)  (why I got the 450)  Anyway...here are my two bikes.