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Discussion Starter · #1 ·
Hi People?

Would I ba able to fit a CB150 - 175 engine (larger displacement copy of CB125) into a CM185 frame?

I have a 1982 CM185 which is fairly beaten up, plus I've been always drawn to the simplicity of a single cyl OHV.
I can buy here tons of bits for the much copied 125 engine, and even 175cc versions are available fairly cheap.

Would it actually fit into the frame?

I have not seen a CB125 from close up, it's rather rare here (just like CM185) so comparing the two by looking at them is difficult. Maybe someone can help me out on this?

Thanks!
 

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They look pretty close, the rear engine mount bolts are close to the same sizes, so that may be easily overcome, the front mount bolts are all 8mm but the twin front mount is much wider on the engine, so maybe just use the single's front mount.

But the frame measurements between the rear mounts are the real question, you can add washers to fill gaps or drill new holes, but how will the drive chain line up and will any part of the frame be in the way?
You may have to try it to know for sure.

https://www.partzilla.com/catalog/honda/motorcycle/1978/cb125s-a/frame

https://www.partzilla.com/catalog/honda/motorcycle/1978/cm185t-a-twinstar/frame
 

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Just from sitting on my couch and thinking of this, the single is going to be a bit taller than the twin. They are approximately equal in displacement. The crank throw of the 185 is 41 mm, the Honda CB125 single is 58 mm. The the crank case deeper and the head taller. Also, the valves for the single will be larger and longer. You could be looking at an engine that is as much as 30 mm taller. I get it about single cylinders simplicity. But a CM185 is a pretty basic bike and doing the swap could be pretty complicated.
 

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Good point there Jim. I took a quick look out to yon interwebs and found this Chinese copy of a CB125 engine with a measured drawing.
CB125 engine dimensions.jpg https://www.alibaba.com/product-detail/Kick-start-CB125-loncin-125cc-engine_60058945477.html

and I also found this copy of the CB250/cmx250 engine with its 53x53mm bore and stroke:
***your CM185 engine has 53 x 41 bore and stroke so that engine may be a bit shorter.

CB250 engine dimensions.jpg https://www.alibaba.com/product-det...c-double-cylinder-motorcycle_60056301380.html


It looks like height may be comparable but the mounts are quite different. That's not to say it couldn't be adapted, but what about chain line?

If I knew more accurately the widths of each frame and each engine, I'd be very curious to see if some simple adapter plates could be made for the rear mounts and a custom made or modified stock mount for the front mount. At that point you may still have to figure out how to get the drive chain lined up, and you may need to figure out a different intake runner between the carb and cylinder head.

I don't mean to encourage or discourage you, but some projects are easier than others.
 

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CB125 rear tire is 3.00 x 16 chain is 428 x 100links front sprocket 15t rear 35t
CM185 rear tire is 3.50 x 17 chain is 428 x 112links front sprocket 15t rear 35t

some difference there but not much
 

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Discussion Starter · #7 ·
Very good points, thank You!

I have thought that the single must be taller... I must take a look at the frame at home to see how much space there is.
As far as I remember, removing the valve cover is already a near-miss thing, so I might run into trouble there.
 

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