I need to source a replacement starter motor for my 2012 mk 11 TT Quattro TDI. The present unit is a VALEO one with a part no. 02M 911 024N with an 11 tooth pinion. A replacement VALEO unit from a reputable supplier has 12 teeth and they assure me this will fit and operate satisfactorily. My elementary gearing knowledge says this wont work. Has anyone experience of this or an opinion? David.  

I agree that a 12 tooth pinion would have 11/12ths of the mechanical advantage of an 11 tooth pinion if all other factors are the same, so does the replacement motor have increased torque to compensate?

Until I receive the replacment I dont know its power output, hopefully there will be literature with it. Also the old starter lists a number of power outputs under that part no. All very confusing.

I can't help on that score. Do please let us know the outcome.

How did the new motor work out David?

So far- so good.I've probably had around 100 starts and no horrible noises from the engine bay!! The starter supplier (  starter motor & alternator company limited, in Plymouth) did assure me there would not be a problem and so far so good. However I would still like an explanation of the mechanics of this set up.

Thanks for your interest.  

All's well that ends well, even if a mystery remains.