Late 650 flywheel, is it the same as the 750?

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Sep 11, 2009
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I have had a number of 650 engines over the years and all have had "steps" machined into the face of the flywheel opposite the big ends (as per the picture). I've always assumed these steps were for piston clearance at BDC. I recently purchased another 650 engine in pieces and the crank (on the right in the picture) that it came with has a smooth faced flywheel, like an Atlas or Commando one. The whole section of the flywheel opposite the big ends has been machined down. My question is did the later 650s change the pattern of their flywheel machining to be the same as the 750?
Late 650 flywheel, is it the same as the 750?
 
All parts books that I have 61 (thru 68 show the same 650 flywheel PN 23315) and the manxman I worked on had the cut flywheel.

FWIW: The early 650 and very early atlas had rounded crank cheek end profile not the later machined flats as shown on these two.
 
Thanks Dave. I wonder why Norton changed the machining of the crank? It sounds as though the crank I have is usable in a 650 as long as I rebalance it for the lighter pistons.
 
Thanks Dave. I wonder why Norton changed the machining of the crank? It sounds as though the crank I have is usable in a 650 as long as I rebalance it for the lighter pistons.
Only idea that comes as possibility about the cheek machining is to make a more consistant crank cheek mass to improve the balancing.
Yes the crank cheeks could be used on a 650.
I only have 1 650 engine on the bench and it is not coming apart at this time for this issue. I would find it very unlikely that norton built this flywheel with cutouts unnecessarily. The desaxe 750 atlas is a step different than a 650 and I would be more critical when trying to discover the similarities and differences. Try it and let us know?
 
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