Gearbox boss locating peg

[gammakeith]

Dear all,

I’ve managed to make some serious progress with my 3 speed gearbox so I’m feeling chuffed. The new boss is now bushed and bored to give me satisfactory clearance. I’ve checked the end float of the layshaft and faced the bush so I have ended up with 11 thou end float which sounds good to me. This is rather better than the old boss which I measured as giving nearly 50 thou. That clearly explains why the layshaft gear was able to ride up enough to gouge the mainshaft bearing.

However, it appears I have one problem left to sort. The kickstarter boss is prevented by rotating in two ways. Firstly by a metal plate clamped by the gearbox end-plate bolts. Secondly by what appears to have been a peg screwed or pressed into the outer cover that engages in a hole in the boss. The latter has been sliced cleanly flush with the housing and the former somewhat mangled. I’ve cleaned up the metal plate so it engages nicely but I am assuming its best not to rely just on this so can someone advise if the peg is threaded or pressed in before I attack it with the drill?

Thanks,

Keith

[BRIAN MARSHALL]

Hello Keith,
Only later machines, Brums, had the boss located by the notched strap type fixture, but a lot of earlier gearboxes have been thus modified during repairs and overhauls, mainly because the earlier dowel method was VERY poor. Few old gearbox end covers are without the dowel hole showing signs of tearing ! The dowel is identical to a clutch roller, ie. a 1/8″ x 1/8″ peg, no threads, and just a push fit. The damage is caused by folk starting their bikes when it is still on the rear stand….
This allows the rider’s leg to give an excessively long ‘swing’ on the pedal, letting the pedal to reach the bottom limit of the pawl slot, and then all the rider’s weight puts the dowel into an attempted shear, usually just ripping the alloy around the dowel as it tries to stop more pedal movement. Drill out the damaged alloy, and install a steel bush with a 1/8″ diameter hole in it to accept a new dowel, that can be made a lot longer than 3/16″ long, and therefore having a much stronger location into the end cover.
The moral of this story is DON’T START THE BIKE WHEN IT IS ON THE REAR STAND ! Exactly the same bit of starting advice applies to two-speeders, as the kickstart goes beyond its design limit and breaks! TWO-SPEEDERS SHOULD NEVER BE STARTED WITH BIKE ON THE REAR STAND ! DO IT ASTRIDE THE BIKE SO THAT YOUR LEG SWINGS BACKWARDS BUT NOT DOWNWARDS. ( Think of it being like a horse kicking backwards ).
Brian

[gammakeith]

Many thanks Brian! As usual, I am now much better informed. It seems I am fortunate as there is no sign of any stress to the cover around the ‘dowel’ area. Thus, I’m hoping the drilling operation will proceed OK. Who knows I may even be able to make the next area meet on the Scott 😀

Judging by the state of the fixing strap I think the “start on the stand” technique has been used repeatedly by previous owner/s as the boss had forced itself beyond its correct position. It probably also accounts for the large crack in the old boss as well. Its an elegantly simple design but (as with all things) susceptible to abuse.

Keith

[BRIAN MARSHALL]

Hello again Keith,
The kickstart mounting boss is really the worst bit of design on the Scott gearbox, as apart from that silly little locating dowel, the boss itself is seriously weakened by the slot for the pawl to operate in, as it goes through more than half of the boss periphery. As the kickstart lever is a non-folding design, if the bike falls over and the pedal hits the ground it can create a crack in the boss, starting in the right-angled end of the pawl slot. Starting the bike when it is on the rear stand also encourages cracks in the boss. A slight redesign, with a bigger diameter boss that doesn’t need such a big slot to give operating clearance for the pawl, would transform the wretched thing, yet they continued with it from late 1926 to 1972 !
Brian

[gammakeith]

Well it certainly was not 1/8″ in my bike! I needed a 5.5 mm drill (sorry but I’m a mostly metric) to clear the contents which included the remains of a theaded bolt. Now all sorted with a new peg but the strap then didn’t fit as it had been built up with weld. A few minutes work with the Dremel and a file got the weld cleaned off and the notch in the strap lined up beautifully – most satisfying. Obviously, my old boss had just been relying on the strap for quite some time and it was certainly not in the correct position.

The boss and shafts are now fitted to the outer case with new bearing so tonights job will be a trial assembly to double check there is still correct layshaft float and see how bad the mainshaft is. There was clear movement on the output shaft before so its going to be waaaay more than 10 thou……..

Keith

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