Anyway, the 54T chainring and sprockets of 12 and 16 make for a large jump if changing gear. As I was struggling to push the 12T on the flat, I decided that the easiest way to change was to swap the 12 to a 13. A sort through my spares turned up a new-looking 13T smallest shimano sprocket off an 8-speed cassette. When I took the circlip off the Brompton hub and removed the sprockets, the 12T one had 'pointed teeth' (unlike the 13T replacement I'd chosen) and an integral spacer, but it also had a spacer between itself and the 16T sprocket. The replacement 13T spacer had a wider integral spacer than the 12. The 12T along with its separate spacer measured 5mm with the vernier. The 13T measured 4.8mm. With the 13T sprocket in place it looked as though there would be an issue, and after fitting the circlip, as expected the sprockets weren't held tight. Another rummage turned up some thinner spacers (though not thin enough
). I got around this by omitting the black 'spoke protector' and the circlip just went into the groove and held the sprockets in place - not quite as deep in the groove as the original, but it's certainly not going to fly off! Has anyone else experienced this small anomaly? Is the 12T sprocket a 'special' made for Brompton, or possibly off a 9-speed? (although the teeth width is the same as the 8 speed, so that seems unlikely). Some minor adjustments to the derailleur and the road test suggests all is ok, and it changes up and down ok with no apparent 'problem' even though the teeth on the 13T sprocket arent 'pointed' as the original 12T