TOPIC:

the pick up would be out so the timing would be wrong.

I'm curious about these numbers. All bikes of the same year have parts interchangeable - same pick up, crank shaft, etc. All built and mounted the same way.
The engine cases and crank shafts are marked to be matched, as an example. But it's all about the required clearances. Connecting rods are marked as well - to keep the same weight in a set of 6 (this can be corrected/fine tuned by some grinding).

So.. The rotors of the same model and year being of the same design - the pick up and timing will not change. So what are these numbers about? If they are balanced gether with one particular crankshaft, certain number should correspond to that particular crankshaft. But I haven't seen any numbers like this on the crankshafts. So, isn't it that these numbers are not important beyond whatever internal reasons the factory marked them (similar to other markings on parts indicating for example a plant where they came from). Wouldn't it be that in this case any same model or marked as fitting other years would be just fine regardless these stamped numbers?

The numbers are part numbers for the manufacturer (Kokusan) as far as I can see there is 3 models, one for the early ones A1,2 and 3 which had a smaller stator so was a smaller rotor, the A4 and 5 had a different part number as the stator was a larger diameter, and the Fuel injected one has a different part number but appears to be the same apart from the keyway has been cut in a slightly different position to alter the position of the magnetic pick up and hence the timing. As for balancing the flywheel it would not need to be matched to a particular crankshaft as it is round and could be balanced the same way you balance a flywheel on a car or even a wheel. You would only have to balance the complete rotating mass on a high revving race engine.

StanG wrote: 1982 A4 for reference.

is there any chance you could post me a picture of your flywheel rotated so it shows me the trigger for the pick ups ( it has a timing mark with the number 2) the reason is the fly wheel I removed has 2 small triggers about 10mm by 8mm and the new one has 1 large plate about 10mm x 70 mm. thanks.

This is the only photo I have at the moment that shows the other side with these points.

Thanks thats what I wanted that one is the same as the one I removed from my bike it has the 2 small metal triggers on it , the one I am replacing it with I think is off a DFI it just has one large trigger I think it will still work OK as the DFI has the same pick ups I think. I have attached a pic of my old rotor next to the new one you can see the different trigger plates on them.