Stepped camshaft key

[Vespa]

I was into my 2001 ST4 engine recently and found that there is a 'stepped key' used on one of the camshafts. The explanation I got was that when the factory makes the camshaft/cams assembly, and it is put into the engine, if the cam timing is going to be off then they use stepped keys of various degrees to bring the cam timing back into range.

Thinking about this, my understanding is that the camshaft keyway has been cut in the wrong location so that if you use a standard key, then the cams are at the wrong degreee of rotation. Further thinking about it, I wonder if what this comes down to is that the timimg 'dot' is now in the wrong location and that the cam timing will be wrong if using this dot?

Anyway, what blows my mind is that in this day of CNC machining, how can a keyway get milled in randon locations.

Or am I missing the reason behind the stepped key??

cam norris
kingston, Ontario

[jon snadden]

The cam in question was originally intended for use in a bike with a different timing spec. On the ST4, an offset key simply allowed the factory to use an available camshaft without retooling. The dot on the cover will only line up if you use the offset key. If you're replacing the key, make sure you use the correct one as the factory produces keys with several different offsets ( to allow for tuning).
If you're really concerned about accurate valve timng, consider replacing the stock pulleys with adjustable ones.