Thanks Gardar. I'm not having any trouble with finding the BMW part number - more that when I search for replacement parts, I get an array of options that I cannot confirm will fit.

E.g. - a search for BMW 64557786706 can yield: -

This - https://spareto.co.uk/oe/bmw-64557786706

and this - | Spare Auto Parts New Zealand [2187-ST2] - PULLEY IDLER

and this - http://www.amazon.com/64557786706-Pu.../dp/B00HCO7DJI

What I'm trying to determine is which of the "pulley wheel only" options are the correct fit, hence the question about diameter etc.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)