Gearbox = coffee grinder

[RichardL]

I'm having a bit of trouble understanding this because the picture on the left strikes me as the version WITHOUT a gasket, because the pieces are closer together. What gives?

Richard L.

[Greybeard]

I'm having a bit of trouble understanding this because the picture on the left strikes me as the version WITHOUT a gasket, because the pieces are closer together. What gives?

Richard L.

Yes, I've been trying to rationalise that. I still think the principle of the claw meeting the cam with a very close tolerance yet fully engaging when working properly has to be the explanation.

[RichardL]

I've never been inside a plunger gearbox and it's been 10 years since being inside my SA box, so I know next to nothing. With that disclaimer, I can say anything. Maybe the issue is that it was always intended for the claw to be a close fit with the cam, and the cam (or claw) has worn down, requiring the removal of the inner gasket to engage with any regularity at all. I doesn't make sense to me that BSA shows an inner gasket in the parts book, but using it fails the gearbox.

Richard L.

[Greybeard]

...Maybe the issue is that it was always intended for the claw to be a close fit with the cam, and the cam (or claw) has worn down, requiring the removal of the inner gasket to engage with any regularity at all. I doesn't make sense to me that BSA shows an inner gasket in the parts book, but using it fails the gearbox.

This is a rebuilt gearbox so should be good  . I searched the forum for similar problems and found a few posts referring to the gasket issue. I did some work on the box during the bike rebuild four years ago. I presume that there was no inner gasket when I stripped it so didn't fit one on assembly. Until now I only had a slight problem when the bike was really warmed up; changing gear sometimes required a second dab. Since the recent work all changes are really positive and easy.
Any road up, (as they say somewhere), I'm now a happy Plungy, (thanking duTch for that name) owner 

[RichardL]

... I doesn't make sense to me...

...about sums it up.

Richard

[JulianS]

The available gaskets come in different thicknesses, depending on source.

An original old stock BSA gasket in my gasket folder very thin at about 0.006 inch thick.

A modern paper gasket a bit thicker at about 0.009 inch thick.

A modern composite material gasket much thicker at about 0.018 inch thick.

[duTch]

...... I'm now a happy Plungy, (thanking duTch for that name) owner 

 Geez thanks Geebs- I don't remember that, thought it must be one of yours, so was giving you the credit. 

 I've pondered the relationship of 'the claw' some time ago, but put it on the back-burner. It may require fabricating a dummy carrier for the camplate to attach to a spare inner biscuit to fully research it....actually.. .. just remembered I did start to do it to check out the selector fork clearance, but been sidetracked with stuff like work...

 One other thing to keep in mind is the end-float of the layshaft with each different gasket thickness

[RichardL]

Is there an opening big enough to fit a flexible boroscope? I ask this recognizing that GB may not give-a-hoot anymore, as he rides smilingly along.

Richard L.

[Greybeard]

...GB may give-a-hoot anymore, as rides smilingly along.

I hope to not have to give-a-hoot during the time I have left to ride my BSA. 

[Greybeard]

After completing the reassembly of my gearbox and engine I was gutted to find that gears were not selecting properly. Oh boy I was not happy to think that I'd have to take the engine out again! I spoke to Richard Clamp who'd renovated the box. He said that he had fitted a gasket on the inner case because there wasn't one on the box as I'd taken it to him. I had a feeling that I'd read something on this forum about plunger gearbox gaskets so I searched the forum and from various posts I could identify that I probably had the same symptoms. With Richards agreement I stripped off the gearbox covers and quickly refitted them with no gaskets. Gear selection immediately felt positive. I cleaned off the old sealant, applied Blue Hylomar to both sides, waited for it to dry a bit, reassembled the covers and took the bike for a run. The gearbox is now GREAT!
Richard Clamp has been as upset about this as me. He even offered to drive down and take a look at the box on the machine.

A couple of things:

1) A theory about the notorious problem associated with inner gearbox case gaskets on Plunger models (I don't know if this applies to swinging arm models)

I was pondering why the presence of the gasket should make a difference to the selector arm claws. I reckon the problem is because the selector arm being pivoted the claws move in an arc toward the selector cam slots. As the claw approachs the edge of the selector cam slot they must be very close to the top edge of the slot. The gasket thickness must be enough to cause the claw to collide with the cam rather than going right into the slot. When the claw is properly positioned into the slot I'm sure there is really good contact. I've made a drawing to try and show my thoughts on this.

2) When adding oil to your gearbox do not stop when a little oil comes out of the level plug. The level plug is in the outer case which fills quicker than the main gearbox casing. Wait for the oil to stop dripping and then add some more, wait a while, add some more etc. until a real stream of oil issues forth from the level plug hole. I'm pretty sure that that is how I allowed my gearbox oil to become so low.