Posts
9
Joined
1/30/2017
Location
San Diego, CA
US
Edited Date/Time
9/27/2022 7:50pm
Hello everyone, I am a long time VitalMX lurker, a mechanical engineer and big time fan of the late CR125. Even by today's standards, the last generation CR125 aluminum frame is simply outstanding in terms of handling, suspension and high speed stability. Well the old school KYB suspension components are dated but they can be swapped for newer Showa components for pretty darn cheap these days. The real problem with the CR125 is the motor characteristics from 98-04. After lots of reading, research, conversations with folks connected to Honda and many hours pondering solutions, I think the best bet is quite literally to start from scratch with the cylinder. I am willing to take the time to develop a new cylinder, that will look externally identical to an OEM cylinder however, it will boast a different port layout (kind of). I have been taking data from an 02' CR125 cylinder and an 05' YZ125 cylinder and what I have found is quite astonishing. The port areas and timings are VERY close. The YZ125 does boast a much better bust port in terms of shape and velocity but, the bust port isn't exactly a very effective port to begin with. What the YZ125 does have and what is hard to correct on the CR125 is port shapes and angles that are much more effective. Just glancing at the YZ125 cylinder and you can see that it does a much better job of aiming the charge at the rear of the cylinder and keeping the velocity up, if you know a bit about air flow that is. My goal is to cast a new CR125 cylinder with more YZ125 like port shapes and hopefully correct the weakest part of the CR125. The cases will need to matched to the new transfer layout however, I plan to make plastic stuffers to make it simple. If you look at the 05' up CR125 cylinder and cases, that is exactly what was done...copy the YZ125. If things work out, later I would like to take it a step further and add auxiliary exhaust ports (a la KTM) and see how things go. I have access to CFD software, I can write porting programs easily and make my own testing equipment. I am willing to make the investment for tooling and prototyping but I am curious if others would buy a new cylinder if in fact it proved to be functional and reliable. What would you spend on an old warhorse like that? The prototype cylinders would go to a reputable tuner ie. Dan Lamy, Terry Varner, etc. I would let them sort out those details however, I think I can offer a much better canvas to work with then Honda did. I am also going to do what I can to get the carb inlet straightened out but of course I will be limited by the space available. I do see room for improvement though. As I go through this process, if I do, I will be sharing the my testing methods and technical information. This is for fun, not profit.
The Shop
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The cost of the castings would be prohibitive, to say the least, mush less the machining and testing.
Paw Paw
Pit Row
Would do the same for tm.
I am interested in what you are thinking and happy to work in with you. Below is some history and where I am at on a similar journey, just not focused on the cylinder so much.
I owned a 2002 from new, it was very slow as the mxa articles said, during winter i sent cylinder and head to pro circuit, it totally transformed the bike, it also had PC pipe and silencer and Vforce reeds. That bike got stolen years later.
I now have a 2019 yz125 with PC pipe and silencer and vforce reeds. it is amazing. I also have another 2002 i bought a while back to eventually replace what was stolen and try to get it as close as possible to the yz. I have most of the items commented on this thread to test it all. Most recent i have the same engine work done by Pro Circuit which I am about to try. I also have multiple CR125 cyrlinders and that kx125 pipe and the PC pipe and both PC sliencers and a 38mm car b and Vforce. Oh and a full nut and bolt rebuild.
Sounds like a lot but i just wantt to see what its like compared to the yz at the same time.
I dont really believe the artiicles etc that much as my PC cr125 was great. It may be that it will be good again after i put the new PC cylinder and head on.
Gavan I also have a 02 125 and I and beggin to find any way to get more up out of it. I’ve done airstriker carb, phathead hesd v force reeds, vertex high comp piston, and it still only makes around 30… if you could tell me what you did to ur and ur outcomes that would be awesome!! Thanks, Ben Tuke
Hi Ben,
I have the cylinder back, i can tell you the bike is very easy to ride and close in power to my yz , i do have to confirm that with another rider as maybe i was changing gears better but there was little in it.
I used standard head machined by Pro Circuit and cylinder with vforce 4 reeds, stock carb, Procircuit pipe and silencer.
The porting and head work is not terribly different to the other cylinders i have the big difference is that the cylinder is now decked taking 1mm off and the head is modified to match and allow 0.8-0.9mm squish. Thats a big change in port timing. Jetting was pretty close to standard.
The decking aligns with what eric gorr recommends when he says use a suzuki piston and head work as it gives the same effect but just 0.5mm. This changes the port timing which no other mods you have can mimmmic. It takes an experienced machine shop to deck as cylinder, head is pretty easy. You could talk to pro circuit and know what your getting.
Hope this helps.
Post a reply to: Possible group buy: 98-02, 03-04 CR125 replacement cylinder with corrected port and transfer layout