C15S SCRAMBLER GEAR SELECTION

[Spaceman]

Just to summarise what we found when examining the gearbox today. Everything looked ok at first sight after disassembling the gearbox but, when put back into the engine, the gear change mechanism was found to jam and it was impossible to select all the gears. There is a question mark over whether the gear quadrant plungers align with the middle of the camplate cut-out slots and therefore the key dimensions for both components needs to be checked but they are probably ok.

However, after a more detailed look at what was happening when you tried to change gear, the problem appears to be that one of the quadrant plungers was projecting too far, possibly 1 mm more than the other, and was jamming the mechanism. In practice, the mechanism uses the upper plunger when changing up and the lower plunger for changing down. When the gear lever is moved to change gear, the appropriate plunger is pushed against the inside edge of one of the camplate slots thereby rotating the camplate to the next gear position. However, when the gear lever is released, the quadrant spring should rotate the quadrant back to its central position but, to do this, the bevel on the outside of the active plunger must press against the other side of the camplate slot, depressing the plunger and allowing it to slide out of the slot.

In this case, the straight side of the plunger rather than the bevelled part appeared to be hitting the side of the slot stopping it from being pushed out of the way and therefore stopping the quadrant from returning. It's possible, the problem is due to a slightly mangled plunger retaining plate although attempts to bend it to give equal plunger projections proved to be difficult. Short of changing the quadrant, and if it proves difficult to sort out the problem with the retaining plate, it might be possible to carefully grind down the plunger at fault to put the bevel back into the correct position.

Overall, a bit of a strange problem that must have occurred some time ago - perhaps, the previous owner who did most of the restoration work changed the quadrant because the splines had gone and didn't realise the plungers did not sit level?

[Spaceman]

This appears to be a very subtle problem with this C15 gearbox and the attached picture tries to show the problem in a bit more detail.

The slot marked in red is where the camplate pokes through which is 1/8" in width. The quadrant is in the position it would be in with the gear lever fully pressed down - the flat side of the central plunger will have just pushed against the inside edge of a camplate slot to move the camplate to the next lower gear.

However, under spring pressure, the bevelled edge of the plunger marked in blue will move until it hits the opposite side of the camplate slot. At this point, the plunger should be forced downwards out of the way so that the quadrant can then move back to its central position.

The problem here appeared to be that one of the plungers was projecting too far and, rather than the bevelled edge of the plunger contacting the camplate when trying to return to its central position, the lower straight sided part of the plunger was making contact meaning that it jammed in this position.

The picture shows an easy way to check whether this problem is present because, when the quadrant is rotated by +/- 20 degs, only the bevelled part of the plunges should move across the camplate slot. From the picture, it looks as though it would take a plunger to project about 1 mm more than it should to cause a problem which seems to be case here.

[DAVE BRADY]

Hi,

If one of the plungers is causing the problem changing gear on one direction then swapping the plungers should also swap the problem.  Perhaps worth doing to confirm or eliminate the plunger as the problem.  I suppose after that some different plungers??

Dave.

[Spaceman]

When compared side-by-side, the two plungers appeared to be identical. At the time, I assumed it was the somewhat battered retaining plate that was causing the problem but, in the limited time available, I wasn't able to change the shape of the plate enough to solve the problem. Although its unlikely to be case, its possible the quadrant may have been machined wrongly where the plate screws down and the only solution for this quadrant may be to grind down one of the plungers to make it project the same distance as the other?

[bobby12]

Hi,
Update on my gearbox woes!!
I have now fitted a new plate that holds down the quadrant plungers, I have also checked their position from the photo enclosed in the last post regarding the arc they move on the cam plate, one of the plungers was as expected high, I have ground down this plunger and it now fits within the required arc so when trying to engage gears the chamfer pushes the plunger down instead of a dead stop.
Feeling very optimistic as I can now engage all 4 gears whilst applying pressure with my thumb on the cam plate while the cluster is out of the engine.
However, when I fit the gear cluster back into the engine I can engage first, neutral and then a combination of the other 3 gears, I can feel some gears then sometimes a jump from second to forth and so on. Think I just need to keep trying.
I would like to take this opportunity to say what a fantastic site the BSA OWNERS CLUB is, without it and the responsive help from its members especially SPACEMAN, I would not have been able to sort out my problem, fingers crossed.
Regards, MW.

[Spaceman]

Mike.

It sounds as though your quadrant was machined wrongly because the plungers should project the same amount with a good retaining plate fitted. As discussed, I am happy to send you my spare quadrant to try - if this solves the gear changing problems then it will at least confirm the problems definitely lie with your quadrant which may need a bit more fettling.

However, I am not entirely sure which unit single my quadrant came off although I think it should be ok on a C15. The distance between the shoulders on my quadrant is 1.658" and the length of the splined shaft from the outer shoulder is about 2.863". If you can confirm these dimensions match those on your quadrant, I will stick mine in the post for you tomorrow.

[bobby12]

Hi Mike
The shaft length is 2.876 but the quadrant width where the plungers sit is only 1.240 is that where you measured your 1.658 ? If so, not the same quadrant.

[bobby12]

But I am not sure if my quadrant is correct for my bike, I've been trying to obtain some dimensions for the correct one but proving to be difficult, the part numbers are all different without any sizes given.

[Spaceman]

I believe the plunger quadrants are interchangeable on all the side point unit singles (C15-B50). It was the same part number for the B44 through to the B50. Although the C15 has a different part number, I am sure there is only a minor design difference between it and the later quadrants. Unfortunately, Rupert Ratio does not provide any information on the differences.

As said, I am not sure which bike my spare quadrant came off but the key dimensions are shown in the attached picture.

[bobby12]

Hi
Checking my quadrant dimensions it is within a few thou of yours so it looks to be same.
If you could send me your spare one to try, it would be most appreciated, might well prove what the problem is
Thanks,. M

[Spaceman]

I will post it off later today if I get back from my trip in time!

[Spaceman]

Unfortunately, sorting office closed at 2 pm before I got back and so will post it off this morning!

[Spaceman]

Apparently Royal Mail is on strike today and tomorrow!!

[bobby12]

Hi
Gearbox woes update.
I have now tried another quadrant loaned to me and at last I can engage all 4 gears, sometimes going up or down I do seem to miss one. But it seems to prove that my problem was a faulty quadrant for some reason. Now all I need to do is find another quadrant to purchase , all so far on eBay are all too short. If anyone has a Quadrant for sale please let me know as I cannot proceed with my rebuild until I find one.
Thanks, M

[Spaceman]

The problem found with jumbo12's gearbox was that one of the gear change quadrant plungers projected too far and this jammed it against the camplate when trying to change gear. This was confirmed when my spare quadrant was tested and worked correctly.

Unfortunately, replacement quadrants appear to be a bit rare on the ground at the moment and so I offered to swap the quadrants. However, the spline on my spare quadrant was not in good condition and so I have undertaken to repair it with a replacement splined shaft. The pictures show the process adopted.

The main issue was to keep the replacement shaft concentric with the remaining quadrant shaft and the way I did that was by boring a hole in the latter on my lathe and then turning a tight fitting spigot on the replacement shaft. The join between the two shaft was positioned so that it would be in the middle of the gap between the inner and outer timing covers.

To perform a strong weld, it was then necessary to machine a groove at the join as shown although this had to be enlarged a bit further to make the welding a bit easier. Even so, trying to weld a small 1/2" shaft was a bit fiddly. After the welding was complete, the shaft was then machined on the lathe to reduce its diameter back to 1/2".

Hopefully the repaired quadrant will solve all of Jumbo12's gear box problems?