It could be a tiny stone or something !!!!! how many times have you seen bits of crap round a spark plug ? and when you take it out the crap falls in to the combustion chamber
yeah, the B12 is very slightly longer but not much. i found that out when i put a B12 swinger in a B6 as the chain was to short enough but I put a smaller rear sprocket on to get a bit more top end and use the chain it had on it.