Scala / Avus / Snowflakes wheel bolt size.

Hi
I'm about to fit a set of 14"" Snowflakes to the Cabby. The bolts that are holding the current wheels on cannot be used because they have a tapered rather than  a radiussed seat. Can anyone tell me what size bolts I need? The thread is M12 but how long should they be?

Cheers
Niels

Standard Mk1 VW alloys like snowflakes just use the bog standard Mk1 bolts which have a thread length of 28mm.

Thanks, have got the bolts