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Often customers ask whether the missing keyway on most Powerdynamo rotor is not a problem leading to the rotor slipping on the shaft and why this keyway has been left out.
Advice: we discourage customers to introduce a keyway into our rotor by themselves. Not only will this void warranty, chances are very high that the keyway will be in wrong position and prevent correct timing. We have had already quite a few customers who made this bitter experience and ended up at buying a new rotor from us. 
We ourselves will not machine keyways into rotor at customer demand as the basic problem of high chance of wrong timing is same.
Timing with modern carburants and modern ignitions is different to stock situation and will have to be tested individually in engine what rotor without keywell permit, but a keyway may well prevent.

The keyway/woodruff provision is not so much a means of holding the rotor on the shaft, but a means of guiding the rotor to its correct position for ignition.

The rotor is fixed  by the taper. You will have noticed  how strong that hold is when you pull the rotor off the shaft. If the taper is incorrect or worn, the best woodruff/keyway arrangement will not hold the rotor. The key will be sheared off by the rotor.

In most cases where a customer observes slipping of the rotor on the shaft, incorrect fastening is the cause. The rotor fastening screws might be too long and bottom out before tightening the rotor. In the case of fastening nuts it might be that the tapered end of the shaft protrudes into the nut opening, preventing it to secure the rotor. In both cases some additional  washer is the answer.

The most effective washer is a curved spring washer. We recommend such a washer for our heavy (2kg) rotor

On motorcycles where ignition is originally not on the crankshaft, such as older BMW, there is no provision for a keyway from start and the rotor is not slipping.

Now why do we leave it out?

a) We do not always know exactly where that keyway should be. In quite a few cases we have not access to the complete motorcycle. But even if we would, we would need to change the position of the original keyway (and experiment to find that position) as modern fuels and the high power ignition modify timing (generally less advance needed)
b) Some rotor have more than one application. Making them all to order for specific engines would mean a considerable increase in effort and hence price.
c) The missing keyway gives the customer considerable flexibility in timing which he would not have otherwise.
  

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