Re: Silk 98 in Derby Museum - who made the gearbox?

Posted by Garenne on Aug 07, 2021; 4:26pm
URL: http://silk-motorcycles.221.s1.nabble.com/Silk-98-in-Derby-Museum-who-made-the-gearbox-tp5002225p5002227.html

I have had to delve into the Silk gearbox internals more than envisaged when taking on the boxes of bits that constituted the makings of a Silk.   To me the gearbox is a direct copy of the Velocette design with some components interchangeable.   I had known that my bike had a racing history but didn’t realise what that meant until trying to assemble the bits.  It turns out that the gearbox internals were a special order from Silk and had the equivalent of a KTT Velo close ratio set.  This was achieved by replacing the standard 17 tooth sleeve gear with an 18 tooth version.  This took the first gear ratio from the standard Silk one of 2.29:1 up to 2.08:1.   Now I had heard that the standard Silk had quite a high first gear for road work and I had envisaged being able to use this bike two up on our local narrow hilly roads of Wales.  So I had to change it.  I managed to get Velocette drawings of the sleeve gear and it’s mating layshaft gear and discovered that the layshaft gear was identical.   The layshaft gears were easily obtainable from Grove Classics at very reasonable cost.  The sleeve gear however was also the same as the Velo component apart from an increase in length probably to accommodate the Silk clutch withdrawal mechanism.  I did consider trying to get an extension welded on to a standard Velo one which again was obtainable but didn’t quite like the idea.  Luckily I found an engineer making gear sets for classic and modern racing machines.   Looking at all the options available on the internal gear ratios I actually had two new sleeve gears made.  One with 17 teeth which provided the standard Silk ratios and the other with 16 teeth which gave a wider set of ratios equivalent to the old MAC Velos.  Both mating layshaft gears of 27T and 28T had been bought before hand to make sure the right gear form was used.

I have now done 2000 plus miles on the wider set of ratios and I won’t be bothering to try the other set of gears.  I was concerned whether there would be too big a gap between gears but the engine is so torquey that I just don’t notice any problem there.  I am actually surprised that this option wasn’t offered when purchasing these bikes new.  Not having ridden a standard ratio bike it is difficult to assess but I find this bike so rideable and have full confidence of not getting caught out on some unexpected uphill hairpin with these ratios even two up.