Hi Richard, Works TT gearboxes usually have normal ‘vintage-close’ ratios, as used on TT Replicas, Sprint Specials, Dirt-Track Models, etc.. Sometimes however I have come across boxes with a lower bottom gear pair. Bear in mind that these vintage era boxes have a solid one-piece layshaft as opposed to the later built-up layshafts. This means that these rare gearboxes must have been very special toolroom jobs. I suspect, but as yet cannot prove, that they were built for the handful of Sidecar TT outfits, and intended to get the relatively much heavier outfits off the line and around slow hairpin bends without burning out the clutch. In 1928 this would have been 28TT1 and 28TT2, built for the 1928 Sidecar TT that never took place due to lack of manufacturer support. In 1929 I believe the same thing probably happened. There was of course plenty of other sidecar racing, so these outfits would have had a life elsewhere ! As I mentioned in a previous posting, the majority of gearboxes in surviving 1928/9 TT machines seem to be actually Dirt-Track gearboxes stamped to look like the Real McCoy. Genuine original TT boxes are VERY rare indeed, and they have hollow mainshafts and layshafts and no high-gear bush. Instead they have a double-row caged roller bearing in a special bolt-on housing. If run with a tight chain these would soon break up, with disastrous results.
In addition, they have a ball bearing on the outer end of the layshaft, instead of the bronze bush used on production bikes.