When my M3W came back from its re-manufacture at Malvern last year, I was amazed how tight the belt was. I was checking as they had not bothered to pump up the tyres, I wondered if they had tightened the belt properly. Like a rod of iron is how I would describe it, with maybe less than 1/4" of deflection possible. Considerably tighter than I would have made it before, anyway. My M3W has always clanked at low speed over a rough road both with original chassis' rigid mounted BB and now NVH bushed BB. I found it is quite sensitive on this to rear tyre pressure. Even slightly above 25 psi and the clanking increases a lot.
Wilson
