....and then I stopped looking at the engine. Over the Christmas break I thought I'd get stuck in to something a bit smaller and more manageable. So I started on the gearbox. I used to fret about taking gearboxes apart - you know the sort of thing - as you split the case a couple of springs and gears burst out and you don't know how to get it back together again. Then I learned to look at gearboxes in detail, and when you know what each part does and the relation to the other parts they are actually elegant and simple. None more so than the GB30 fitted to the Colt which is a lovely, compact 4 speed box. I gave the box a good scrub before taking apart and had a camera ready to catch the action. Even before taking it apart I knew there were problems, as the kick start shaft splines were stripped (still haven't found a replacement, but more of that later) and 2nd gear was difficult to find.
The picture here is after I split the cases and shows all the gears, on both shafts in their working positions. Fourth gear is selected, where the mainshaft drives the 2nd mainshaft gear (top row, second from left with the selector fork on it) which is engaged with the output gear. The mainshaft effectively drives the output gear directly, so the ratio is 1:1.
Here is the view from the back, where it is clear to see that the 2nd mainshaft gear is pushed in to the sleeve gear so that the dogs from the former engage with the holes in the latter. Remember that the sleeve gear is free to rotate on the mainshaft, and it is the 2nd mainshaft gear that is driven by splines on the mainshaft. The picture below also shows well the poor state of the kickstart shaft.
For future reference I photographed all the other gear selections, and then just for the fun of it worked out the reduction ratios from the number of teeth on the gears. I salute the designer of this gearbox, it would be difficult to improve - maybe just a stronger kick start shaft! So here we are getting down to basics - the gear cluster out of the box with the forks taken off.
Next pic below is the gears taken off the layshaft (bottom) and mainshaft (top) with their offside faces uppermost. I don't know if it is clear to see on the photo but the bronze bush on the 2nd layshaft gear has collapsed (bottom row of gears, second from left). An interesting note on how the gearbox is designed: on both shafts the gears are alternately splined or bushed (free to spin on the shaft) and pairs of gears on the adjacent shafts are paired as splined and bushed.
Now assembling the kickstart ratchet - this is a good time to point out how the problem of the stripped kickstart shaft was fixed. The stripped KS shaft has an outside diameter of 5/8", or about 18mm. I bought a kickstart to fit a 16mm splined shaft (its from a pit bike) and dremelled the shaft until the new kickstart fitted. Time will be the test of this adaptation.
Square 4 mk1, Square 4 mk2, BSA C12.
my attention moved to the front wheel and forks. Here is a picture of the ring of rust that was the front wheel. As with the back wheel the spokes were completely shot and the rim was possibly technically repairable, but economically probably not. Would be like trying to repair a light bulb - it can be done but it's cheaper to buy a new one. The inside of the hub was in a sorry state too, it must have been that when the bike was left standing the wheel leaned to the left and water collected inside the brake drum.
I like that this is just like the set up on a push bike, and wasn't tempted to change it to roller bearings. Maybe I'll do that at the next rebuild in 20 years.
