CR125 VS CR134

Gorr 134 or anyone else that will build you a 134. That bike responds very well to a lil added displacement.

Depends how long you plan on keeping it. I know I would never buy a bike thats been overbored. I think it will be easier to resell as stock, but if you plan to keep it a long time then its really up to you.

Also, before you send out to Gorr, search these forums. A lot of people were posting recently about issues.

A brand new cylinder is almost the same as a rrsleeve, I did it myself this year.

Buy a new oem cylinder ?

I just went through this on my 02 CR125. I pulled my pipe/reeds otherwise stock 02 to do a top end. All looked great... but after close inspection. The exhaust bridge was cracked. After some searching/digging it’s pretty common on the 00-04 cr125s to crack the exhaust bridge. The PV design pushes the water jacket away and also makes the bridge area very thin, thus prone to the cracking. I would think boring the cyl larger would remove even more material and make it more prone to crack. Maybe I’m wrong but I didn’t gamble. Ended up using my 95 HPP cyl and doing a 144cc kit.


Look closely you can see the crack. This cyl has maybe 50-60 hrs max. 100oct gas hp2 premix 32:1. I think a new oem cyl will be your cheapest/most reliable bet if that is what your going for. If not. Go 134 big bore on that damaged cyl.

mx621 wrote:

I just went through this on my 02 CR125. I pulled my pipe/reeds otherwise stock 02 to do a top end. All looked great... but after close inspection. The exhaust bridge was cracked. After some searching/digging it’s pretty common on the 00-04 cr125s to crack the exhaust bridge. The PV design pushes the water jacket away and also makes the bridge area very thin, thus prone to the cracking. I would think boring the cyl larger would remove even more material and make it more prone to crack. Maybe I’m wrong but I didn’t gamble. Ended up using my 95 HPP cyl and doing a 144cc kit.


Look closely you can see the crack. This cyl has maybe 50-60 hrs max. 100oct gas hp2 premix 32:1. I think a new oem cyl will be your cheapest/most reliable bet if that is what your going for. If not. Go 134 big bore on that damaged cyl.

How do you like your 144 setup compared to the old 125 configuration? Anything you don't like about the 144? I have a 2007 CR and am deciding whether to go the big bore route or not.

I haven’t riden it yet. Only a few start ups in the garage. Old man winter has not been nice to us in the north east. So I can’t report.

I'd never buy a used 2 stroke that was overbored. Thats an automatic DQ. That dinky exhaust bridge looks like an overbore disaster waiting to happen and a string of headaches.

Keep it stock. If you want more power get a 250.

What’s funner to ride?? A 134 or 125?

I am runing CR144 that Eric Gorr modified it for me
I chose the low to midrange port because i use it in the woods 70% the rest is supermoto / mx.
Allready 60 hours on it and i love it.
On the other hand i know a guy that blow up hes 144 kit same as mine after few hours
But I would put the blame on him becous he doesn’t mix enough oil for SM use in my opinion

I would personally keep it a 125. I wasn't impressed with my 134. It seemed unreliable and you can't rev it out like a 125. But if you don't rev it out or ride it super hard, it might be okay, the low end will be much better than a 125. But as far as outdoor Motocross goes, I love love love a stock displacement 125.

b_kowalsk wrote:

How do you like your 144 setup compared to the old 125 configuration? Anything you don't like about the 144? I have a 2007 CR and am deciding whether to go the big bore route or not.

We are running an '02 with a HPP 144 on it and a few other mods. It's ridiculously fast. I rode it for the first time in awhile over the weekend and was surprised at how easy it was to ride. I could clear stuff as easy as on my 17 KX250F once I got used to it. With the right set up you can make the CR125 just as fast or faster than just about any other 125 out there...........

Tracktor wrote:

We are running an '02 with a HPP 144 on it and a few other mods. It's ridiculously fast. I rode it for the first time in awhile over the weekend and was surprised at how easy it was to ride. I could clear stuff as easy as on my 17 KX250F once I got used to it. With the right set up you can make the CR125 just as fast or faster than just about any other 125 out there...........

How’s your reliablity on the 144?

I have gone the big bore route with both 134 and 139 versions on my previous YZ125's, reliability was never an issue. However this is my first Honda so I'm unsure of which direction I will go. The Honda is a little more demanding and doesn't have the RPM or rev out quite like my YZ's did but I still really like the way my 07 CR runs.

I think I may have the bike ported and keep it a stock bore 125. Anybody have recommendations for a good tuner? I was always faster on the big bore machines and they were easier to ride but stock 125's are more fun IMO. You can hold a gear longer with a 125 vs big bore and the power just feels more organic.

If you are after pure fun keep it a 125. If you are a little heavier or want to clear jumps, other obstacles and exit corners a little easier go big bore.

Thanks for the help guys!

Thinking I will end up going the big bore route. Any one have any good people in mind for a job like that?

Millennium did my 144 kit. The work looks very professional. Porting looks very targeted and clean, the power valve machining looks surgical. I was hesitant to send it to them as I looked at millennium as more so a machining/plating facility vs a race shop. After a few emails with Tom Morgan, ( technical director at millennium) he convinced me it would be done correctly and work well. I had them do the head for 100oct fuel, Compression is 180psi on my gauge, squish is .041, their spec was .040” hand written on the cyl. So 488$ later it’s done. Only a few start ups in the garage and a little zip down my street is all I have to report. I will say it has ALOT more throttle response and low end power. I felt that instantly riding it down my road.

That being said, if I was looking for a max effort big bore race engine ( I was not ) I would look into HPracedevelopment, Terry Varner, etc.

b_kowalsk wrote:

How do you like your 144 setup compared to the old 125 configuration? Anything you don't like about the 144? I have a 2007 CR and am deciding whether to go the big bore route or not.

Tracktor wrote:

We are running an '02 with a HPP 144 on it and a few other mods. It's ridiculously fast. I rode it for the first time in awhile over the weekend and was surprised at how easy it was to ride. I could clear stuff as easy as on my 17 KX250F once I got used to it. With the right set up you can make the CR125 just as fast or faster than just about any other 125 out there...........

dylanv21 wrote:

How’s your reliablity on the 144?

As good or better than stock . Only mechanical has been when kid put wrong air filter in mid summer the first year we had it .

If Millenium has Tom Morgan running the motor side I will be sending my other 125 cylinder there soon. It needs replating & TM knows his stuff and no mistake

Tom is the technical director at millenium. not the person doing the actual customer work.

I used to race a 2000 CR125 and ended up boring it out to a 134 or 139, I cannot remember, but I had more issues thereafter than I did when it was the original 125.

To this day, it's still not right. Granted it's been sitting for the past 10 years, but is one of those bikes that won't leave. Actually used to be Jeff Willoh's old Arenacross bike! *neat*