Fitting Parts- when is 40mm not 40mm?

Nothing enhances the value of your classic air-head BMW like stainless pipes and silencers. They also go that nice gun-barrel blue and don't rust. Last weekend, due to go for a run with a friend in this glorious weather, I was up early with my tool box, to treat my R100RS to the nice, shiny stainless system that had been in my shed for weeks.

BMW systems came in two sizes- 38mm and 40mm. Most 1000cc are 40mm, says my supplier, but some were 38mm, so check. I measured mine with a micrometer and the pipes were 38mm diameter so, like a fool, I bought a 38mm system.

Two hours of sweat and WD40 later, I had the old stuff removed, resisting the temptation to just hack it off with an angle grinder… which was just as well, because the new pipes slopped around in the ports by a good 3mm. Cue much Bacofoil, exhaust gum bandage and wicked language, putting the old stuff back on and bodging it up. Even so, it sounded like the Rice Krispies from hell.

Ah, says my supplier. If you have 38mm pipes, you actually need a 40mm system, which has 42mm pipes.The 38mm system actually has 35mm pipes. If the exhausts are original, they will be stamped "38" if you have a 40mm system; and "40" if you have 42mm pipes. If the exhausts are not originals (after 30 years?!!!?), then they won't be stamped at all. Tell you what, you need to check the diameter of your pipes with a micrometer, to be sure….

I have a 12-bore system here – how would you like a look at that, ya b*****ds?

Fitting Parts- when is 40mm not 40mm?