plunger gearbox

[RDfella]

Getting frustrated & short on available time. hence the query instead on diving straight into the problem.
Bike is a '62 GF with my latest vee twin motor & bolted on plunger gearbox. Box seemed fine on bench and in bike (currently on lift).
But now it appears to have got to 4th gear and the gear change feels as if it is changing up (when it isn't) whilst trying to change down produces nothing, just the lever moves. The selector has new springs and everything appeared OK during rebuild (turned down layshaft & made new bush).
Would happily investigate but am seriously running short on time. Currently waiting out recovery time after last week's hernia op. Before that I was flat out working on a boat restoration and have another that'll need a month's work starting next month, so seriously don't need more delays by having to go backwards dismantling. Bike is getting close to rollers / start, so it is frustrating. Assemble carbs, make cables & fit tank would make it ready for a trial start. Front mudguard (ready to fit), front brake pipes, headlamp, primary case (half made), seat and number-plate would complete.

[Greybeard]

I had a problem shifting gears after using a gasket on the inner case. Removing the gasket the box worked fine. Might be worth checking that.

[Swarfcut]

 Just a suggestion....Try it with lubricant in the box, rather than dry. Also my feeling is that the spring and cam plunger play a large part in completing the change of gear. So spring adjustment may be the answer, can't remember if this box has a roller plunger mod, but with this or the original plain cone tip plunger the cam plate needs to move smoothly by itself for a clean change.

 Swarfy.

[chaterlea25]

Hi RD,
If the selector fork is not hooked over both gears it will give similar symptoms?
You can check through the inspection plate
Other than that GB' s reply is a good suggestion

John

[muskrat]

G'day RD.
Years ago my plunger g/box wasn't changing properly. The claw 67-3132 was the problem, worn & a chipped tooth. Prior to finding that a thinner or no gasket helped for a while .
Cheers

[RDfella]

Today my convalescence consisted of sitting on a chair achieving very little with the bike. Apart from silver-soldering a couple of banjos, I started on the gearbox.
Tried with OE cam plunger instead of roller. No change, so started dismantling.
Outer cover off - all looks OK. Tried changing gear with mole grips on the ball - no joy.
Inner cover off and all seems OK. Can change gears fine by rotating the cam, so all I've achieved is destroy two gaskets.
Cam teeth look fine, as does the rocking selector, so no idea what's going on.

Addendum - some suggested leaving out the rear gasket - but what about endfloat for the laygear and mainshaft?

[chaterlea25]

Hi RD
Is the ball lever moving at all?
The rocking bit has to grip onto the cam plate when the lever is lined up with the footchange , it needs to rock each way a touch to do this
There is a little spring loaded plunger acting on the rocker (judging from the diagram, but I cannot remember this???)
Is the footchange shaft/ lever moving to the ends of its travel
This diagram is from a C11G as the A10 diagram I found is missing the rocking ball shaft but they are the same as the A7/10 in design
As already said the distance from the arm pivot to the cam plate is critical for correct operation, adding or removing gaskets seems to be.
I put a plunger A10 gearbox together a few years ago and did not have problems or cannot remember problems with a C11G many years ago.

John

[Swarfcut]

     Correct CJ, unlike the later S/A box, the selector claw changes its position relative to the camplate at every gearchange.

   So the claw on the ball ended rocking lever must be free to pivot, as must the little plunger be free and the spring strong enough to return the claw back to its starting position to pick up on the camplate ready for the next  change. There is a small indent in the back of the claw for the tip of the little plunger to positively centralise the claw. Stiffness in the movement of the claw means it may not locate reliably on the camplate rear tangs as required. Gasket between case and inner cover determines the depth of mesh between the camplate and claw, which can be critical.

 Yes, the gasket also affects the layshaft endfloat, but this can be adjusted by the thickness of the thrust washer under the big first gear.....  washer inner chamfer goes towards clutch side. Mainshaft also moves slightly, but this is academic (up to now!!)

 Swarfy.

[RDfella]

Thanks for that, folks.
I seem to be 'twixt the devil and the deep blue sea. ref Chaterlea & Swarfy, all seems in remarkably good order, meaning there's nothing indicating change needed. On the other hand, I agree with Einsten's statement, that 'madness is doing the same thing and expecting a different result', meaning re-assembly as is will surely bring me back to the problem. The other vee I made has the same type of gearbox, and it's fine, so dunno what's up here. Very frustrating.
Clearly the issue seems to be with the selector picking up the cam but, as both are fixed in position, I can't see why leaving out the standard fitment gasket should make a difference. Maybe, as some have said, today's offerings are thicker? Perhaps I'll try (with little hope of success) cutting one from my stock of .012" gasket paper. 
Getting close (getting on a bit, pressure of work etc) to deciding to put it in one gear and (hopefully) start the engine, then leave it at the back of the shed for someone after me to sort out. 

Update Sat:
Spent the afternoon playing with the box. Checked shaft endfloats had sufficient space before doing a dry re-assembly sans gaskets etc. No joy. Tried changing back to the original detent plunger, which seemed a little better. Dunno why - I fitted roller plungers to my other rigid box and the GF S/A box and they were great straight off.
To cut a long story .... it now seems fine (on the lift turning rear wheel) after finding the plunger force was critical - a couple of turns either way and gears were essentially non-functional. I've re-assembled with no rear gasket - but loads more hylomar than I'm happy with - and hope that with oil inside and engine running it'll be OK. Certainly don't need any more hassle - a pile of jobs waiting for me means I have little time to spare on a project which is hogging the lift that two bikes my son uses are waiting for.
At least the sun is shining ........

[muskrat]

G'day RD.

[RDfella]

Before we consign this thread to history, thought I'd give my observations regarding the selector / claw issue on plunger gearboxes.
The mesh between these two elements has been described as critical, but the 'agricultural' design would indicate a mesh tolerance of maybe an eighth of an inch, meaning that if they fail to mesh because they're not close enough, the claw should be moved far closer than a gasket's thickness - at least a 1/16th. I measured the gearbox gasket thickness from two suppliers - both were .018".
Maybe the answer is a hole in the casing so an endoscope can see what's going on, followed by plugging the hole? And no, I'm not an anorak and I don't have the time ......

[Greybeard]

When I wrote up my gearbox problem and how I discovered that leaving the inner gasket out made all the difference I was gobsmacked. I concluded that the end of the claw must have only just been missing the tooth. Without the gasket the claw was just managing to engage the tooth. A new gearbox would probably not be so sensitive but an old slack box, who knows.

[muskrat]

G'day Gb.
That's exactly what I (and others) found with the plunger box. Even after putting in a NOS claw I could only manage a newspaper gasket. As you say a new box will work for 50 years but with wear on bushings & other bits the problem arises.
Cheers

[Swarfcut]

  I'll agree with that, either it works or it doesn't and by default the relationship between the pivot axis of the camplate and the ball lever would be the deal breaker here, in that as all new  the bits worked fine, old and worn, not so.

 Moving the axis of the camplate closer to the ball ended lever pivot looks to be the answer, involving some refacing of the gearbox main case or the inner cover. Certainly worth an experiment on some damaged or scrap castings.

 Swarfy.

[RDfella]

Following Swarfy's input, was thinking about how to cut out the guesswork and time spent fiddling around. Even if ultimately the gears work (gasket or not) one still doesn't know whether the pawl engagement is satisfactory or whether it's all operating on luck.
I appreciate these gearboxes are getting rare and they're not being rebuilt every day, but after an afternoon's frustration and still not sure if all is as it should be, I thought about how to ensure proper engagement. Given the engagement relies on so many machining operations being simultaneously accurate to a couple of thou using elderly machines and practices, I reckon that to avoid getting a bad reputation BSA must have assembled these gearboxes from bins of parts selected for fit. We have neither the jigs nor measurements to do that, so if ever I face that issue again, here's what I'd do:
1. assemble box case and cover using increasingly thick shims (say 3 x feeler gauges) until gear selection is compromised.
2. If shims are in the region of 2mm (.018" gasket + 1/16" engagement) then all is fine. Go ahead and assemble.
If not,
3. measure the depth of gearcase to cam lobe (say in 1st gear).
4. measure selector / rocker protrusion.
Then whichever of 1 or 2 is incorrect needs machining, but we need a few measured examples to determine which one.
There might instead be the option of making an eccentric selector pin, but I don't have the parts in front of me to tell.

Small addition - just measured distance to cam lug using a spare gearcase shell and a new cam I have - it's 1.510" at its highest between gears.