CB125S (J) to 145ccm conversion (CB 750 piston)

Hi there!

I got myself a Honda CB125J Italia - I guess its known as CB125S in the US but my Italia comes with a 22mm dellorto carb.
I would like to increase the engine performance and came accross 3 different possibilitys to increase the engine size:

1.) Easy way: i found a complete tuning kit for the cb 125 including cylinder housing + sleeve + 65mm piston + piston rings + piston bolt + sealings. The downside: it costs 300Euro (~390USD), and its the more "boring" way... simple part replacement without learning anything from it.

2.) semi-hard way: Buying 145ccm kit from powroll with sleeve+piston; +machine the housing to fit the sleeve. Downside price+shipping+tool to europe will exceed the price of the "easy way" and thats not worth it.

3.) Hard way and the reason why I opened this thread: Buy a Cb750 piston with 61mm try to get a sleeve somewhere (I actually dont know where), machine the housing to fit the new sleeve...
I am pretty new in messing around with engines and I dont think it is that easy so I have following questions:
- where to find a cb 750 sleeve?
- where can I find the tolerances for machining the cylinder housing? Is there anything I have to take extra care of?
- where do I find the proper sealings for such a big-bore conversion?
- Did anybody actually built in a cb 750 piston? Any experiences?

And last but not least: Do i have to change the head for all three "ways" above? What is happening with the compression ratio?

Thanks in advance for any hints and tipps for a newbe like me.
And please excuse my bad english!

Thanks for your advice Swivel!

As I said the second option is the least one I'd like to take. I will end up paying 500USD for the kit just for small performance boost. Thats to much, thats why i'd like to take the 750piston option or if it is really that difficult to take the 167ccm kit.

Whats so difficult about going with the CB750 piston - Some ppl in other older threads suggested this.
Is it that difficult? Milling and turning is not a problem. But I need some kind of tutorial - where to take care of, tolerances for machining (reamer size), which parts to take and where to get?

Modifying a CB750 piston was all the rage back in the day but it was not a good way to go. Back to back tests always had the Powroll on top for power and torque. There's nothing complicated about machining a jig to hold the piston to cut the crown shape and to modify the valve pockets, but it takes time.

Liners are easy to get. What I did for years was to buy a CB750 cylinder block, heat it and press out the sleeves. Then they can be shortened to whatever length you want and bore the block to take a sleeve and then bore and hone it to finished size.

What do you learn from all that work? Unless you already have a very good idea of the crown shape and compression ratio you are looking for, all that work won't teach you a lot about the subject.

Swivel is correct that 145-150 is the sweet spot for that motor. Above that it chokes and the ports are too small to flow enough gas. It won't rev out and actually ends up not making much more power than stock.

There's more than enough work in building a motor with parts already available and making it work.

Just a crazy thought to throw in the mix, why not get a cb350 twin liner and overbore piston, say a THR 67.5 and go really big? ;D ;D ;D

I know at one point my machinist looked at the GPZ750 piston mod for my 350 and I was almost talked into doing a +3mm forged wiseco gpz750 piston, (the machinist was willing to do the work for 1/2 price because he was interested to see how it would turn out). In the end I opted for a +.75mm overbore 350 piston so that i could get the motor completed for a college class. It never hurts to research other options, just keep in mind the ole' cost to benefit ratio and what your end goals are. If you have the dough to throw at it and then re-do it if you're not happy with the results, hey why not it's your motor.

I like the last idea best
You could probably find an XR200 piston kit?
Crankcase will need machining

Swivel said:

.................... 1960's Honda race bikes had slipper type pistons.....

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Are you sure about that? I agree with the other logic and I agree that slipper pistons are good, but I think that 60s Hondas used pots ie full skirt pistons. At least teh early 6 and Hailwood's RC181 did. So did the CR93 that I re-built and raced and handed back to the owner before I damaged it (Above my pay grade). At least that's how I remember it, but I could be wrong.

I did quite a few 145cc conversions in the 70's.
It's possible to just bore stock liner, but, spigot gets real thin (although I never had one crack or fall off)
You really need a machinist who has done bikes or is willing to learn the differences compared to oversize stuff in auto engines

Swivel said:

I saw an old Yoshimura CB125 slipper piston years ago that was a copy of a Honda race one.Maybe not all Honda race bikes had slipper pistons ,but I'm sure some did.....

teazeron't encourage the guy to get his cases bored,just getting someone to sleeve his cylinder right is enough chance for disaster....

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Yosh pistons were a copy of the CB350 race kit pistons which were slipper design IIRC. I can't find that pic of the originals but they were 66mm and 2 rings. here's the Yosh versions.

I wasn't so much encouraging the OP and adding to the list of "do you really want to do that" items. It's easy to machine head, barrels, liner, cases and piston versus a drop in big bore...... NOT. None of it is impossible, but it's hardly a good investment of time or cash. I'm the guy that uses modified CB350 pistons in a CB77 and that required all the above and then some. Or the 5 speed CB160 with CB500 pistons or the 240cc CB175 with Wiseco-Kawasaki pistons and head all welded up and machined into a different shape. Sometimes you need to push the envelope and other times it's not a good idea. on a 125 it's not a good idea according to people who already tried it.

I do need some 57mm high crown pistons with 14mm pins and super slim rings for high revs in a slipper design for a project I'm dreaming up. I must start looking again or maybe I'll just get them made to my specs and stop messing around.

Best answer I read about for a CB125 was to drop in an XR200 motor

I've always made fixtures to hold piston through the pin boss with a gudgeon pin and locate on inside machined surface for repeatability
I'll have to post a picture one day

Okay,

according to your replys and your arguments it is not the best idea to mess around with the zylinder of an engine.
So the CB 750 option dies...

However - the suggested Powroll option is still the most expensive one, and i am tended to try the 167ccm kit first (in case it fits). I just don't have to hassle with the sleeve with this option. And it is nice to have the original zylinder as spare part in case provincial government likes to check the bike if everything is legal

Concerning the Powroll kit:
- 145cm means still 61mm piston.
- to prevent boring out the std. sleeve a new sleeve is included in the kit
- Powroll says no machining required for 125ccm engines so the outer diameter of the sleeve is still 71mm?
- isn't 5mm wall thickness a bit too thin?
- how is the original liner removed? Heated and pressed?
- how is the new liner inserted... glued?

Concerning the high-comp kit:
Which Fuel do you recommend (ROZ). 11:1 compression doesn't seem to high. A Moto Morini 125H has 11,7:1 and runs with ROZ 95 standard fuel. Powroll recommends 50% racing fuel - thats hard to get here

Thanks for all your replys - I really appreciate your help!

Just do it the cheap way for fun.
Modified CB750 piston, bore stock block, make a copper head gasket.
If you don't have access to a lathe it probably isn't worth the effort though
Static compression is meaningless except for 'bench racing', dynamic compression dictates which fuel you use
BTW, some of the CB125 motors have a cast in sleeve which needs aching out to fit a larger diameter one

Is there a way to tell if my model has a cast in sleeve? At least can I see it if it is dismantled?
Thanks for the hint with the fuel - however it would be nice to know which ROZ (RON) is required when a mixture of 50/50 race+standard fuel is recommended.

KR!

I think the liner will look like it has lugs on it, it's a long time since I've looked at one
If you have motor apart, pic of top and bottom of cylinder should help

Subscribed! Ill be following this one closely. I'm building a 125s and am considering a complete engine rebuild once I'm done with the appearance and realize that I have a cb125. Especially when compared to my cb500.

Also where did you find the "easy way" kit.

http://www.powroll.com/

zachpn said:

Is there a way to tell if my model has a cast in sleeve? At least can I see it if it is dismantled?
Thanks for the hint with the fuel - however it would be nice to know which ROZ (RON) is required when a mixture of 50/50 race+standard fuel is recommended.

KR!

Click to expand...

We keep forgetting that you are not in the US and therefore what makes sense for us may not be the best option for you.

50/50 race gas/street gas is no sort of recommendation. Race gas varies from 89 to 116 octane, so which do they mean and why? What they are doing is to suggest that street gas "may" be slightly too low octane for that combination.

Pressed in liners typically have a large flange on the top to locate them as they are pressed into place and cast in liners typically are retained by upper and lower retaining rings inside the casting so the top steel/iron face is only slightly larger than the bore.

If the liner is say 10mm larger radius at the top that the bore, it's probably pressed in and if it's 2-3mm larger than the bore it's probably cast in place.

Pistons are machined with a small recess in the bottom that located on a jig. To machine a piston, the easy way is to take a bar of aluminum say 65mm OD and machine the top 6mm to be a light press fit into the bottom of the piston. But that piece of metal into the lathe and use a floating center to press on the top of the crown to hold the piston in place as to is machined to a new shape.