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Differences between 350 and 500cc open diamond frames


Hoping that someone can advise on the differences between 350 and 500cc pre war open diamond frames.
I am in the process of putting together a pre war bitza from a pile of bits accumulated over the years.
My frame is late 1935 and the engine is a 1937 16H civvy engine.
I'm having issues mating the two (hole size mismatches between crankcases and frame) and general misalignment.
The later may indicate that my frame is out of true or my engine plates are incorrect but in the case of the former it has been suggested that the issue is that the frame is Model 50 rather than 16H or M18.
My frame number is 620xx if that helps.
Richard Norton


Hi Richard,

   If you put pre-war 350 and 500 crankcases next to each other you will see the 350 is physically smaller than the 500 and the 2 bolt holes at the front which fit the front frame bracket are closer together on the 350. You could fit a 500 engine to your frame, but you need to cut the front bracket sides off, make new ones and weld them on. You will need to make your own rear engine plates and there might be problems fitting the mag/dyno. The 350 frame differs from the 500 enough to make this not worth doing. Your best option is probably to get a Model 50 engine and build the bike as that, as fitting the wrong engine in your frame will make the bike a bit of a bodge and make it less valuable. On the other hand if you get a 500 frame, you will be left with the 350 for which there is much less demand. Chris Streather will check your frame number and confirm the model, but there is a search fee for this.


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