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Speaking of timing lights... how does one connect the typical automotive timing light to a cycle that has a magneto? Where do the leads that would go to the battery go? Or does this sort of light not work, and there is a different version intended for use with magnetos?  And CDIs?

 

Thanks very much guys! Looks like such a swap is beyond what I'd call easy.

 

More directly, my concerns are:
Is the stroke of a 125 and 175 the same?
Is the cylinder-to-case bolt pattern the same?
Is the top-end rod bearing the same?

E.g. In the case of Yamaha AT1 and CT1, all those things are the same. One could change engine size by changing only the top end, and doing so was identical to what the factory did.

I doubt that was a common practice, but maybe it is more common than one thinks.
As another example, a Maico 400 and 440 are the same bottom end, just different bore on the top-end. One can swap top-ends and it will work.

Anyway, what's behind all this?  I've wanted an MC5-175 fora trail bike for a long time. And I might (just maybe) be getting a 125, and I'm wondering about making it into a 175.


 

I think additional confusion is caused by not only switching from Inch to metric tire sizing, but also the metric tire sizing scheme changed recently. I am told the difference is: It used to be that the overall tire width was measured, including the knobs. Now the tire width is measured to the "base" and the knobs are extra.

Here is a link to a tire size conversion chart. Notice there are two columns for Metric sizes. One is th eold, the other the new. Unfortunately, they are not labeled old and new, but you can tell by the relative sizes. The new ratings should look smaller than the old.
http://www.650motorcycles.com/TireChart.html

 

It happened to me. Twice, beore I figured it out. On a 73 Maico I'd had since 1974. It was dormant for 20 years. In 1999 I fired it up. Use pump-super and Maxima. Over the course of 5 months or so I noticed the tank was not hard anymore. It was pliable. It finally got so soft I would not use it. I thought it was the Maxima. I got another 'glass tank. Used a different oil. Noticed immediately that the inside of the tank got slimy. I drained it after each use. Did some asking around and was told it was the modern gas doing it. I switched to a plastic tank. I also switched to race gas, have not tried the race gas in the glass tank, but I suspect it would be okay cuz I see a lot of glass tanks at the vintage mx.
Back in the 70's every one used polyester resins. Epoxy resins don't seem to have the same problem, but are quite a bit mor expensive, so not likely to be used in production environments.

Fibreglass was never a good choice for fuel tanks anyway. It was just the most economical lightweight approach at the time. Plastic is better, even better than metal. E.g. look at US Coast Guard ratings for fuel tanks. Plastic rates better than all the metals. Aluminum is okay for gasoline, but not diesel. Monel is good (anyone want a bronze fuel tank on their cycle?) Black iron rates well too, but must be welded "correctly". Sorry about the digression.

 

Very cool. Thank you Mick.


 

Re circa 1976 / 1977:  Any chance that a KTM 175 top end will fit a 125 lower end (e.g. like early 70's Yamaha 125/175 interchanged) ?

I presume the overall engine mounting would be the same at least, so one could swap an entire engine. True?



 

Frame number:  550 01787
Engine number: 6-5101979

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