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Gearbox gremlins

Submitted by tony_wood on Sat, 01/08/2020
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Heavy Twins

Hi. I have stripped and rebuilt the gearbox of my Dommie 99 barn find project. (In fact I’ve now stripped and rebuilt it eleven times - such fun!).

The gearbox is mounted in the correct orientation in my workbench vice. I only have the shell and inner cover bolted together - the outer cover isn’t assembled yet.

I’m having problems consistently getting all 4 gears.

I can put it all together and get all 4 gears, then I spin the mainshaft a few times by using the gearbox sprocket and all of a sudden I can’t get 4th gear - the box is locked in 3rd gear and won’t go to 4th or 2nd. If I rotate the mainshaft a few more times and rock the camshaft up and down using a spanner on its fixing bolt, I can get 4th again and get back to 1st. On other occasions, I go from 1st through neutral to 2nd, then I can’t get 3rd or back to neutral. 

There’s no problem when the box is bare apart from the camplate, quadrant and plunger - they work smoothly and consistently so I know that bit is ok. I’ve also built the box with just the camplate, quadrant, plunger and pinions that have the selector forks to make sure that the forks aren’t jamming in their tracks- everything works fine.

I have ascertained that sometimes the dogs on the sliding gears don’t mesh with the dogs on the adjacent free-running gear but instead the dogs come together face to face (i.e. they don’t interlock). That prevents correct gear selection and seems to be the cause of the lockups.

Am I getting that because the gear pinions aren’t being spun by the running engine as they would be in ‘normal‘ operation?

I’m really confused by all of this I’m not sure if I’m chasing something that is a problem or something that isn’t a real problem in everyday riding. 
Any advice and help gratefully accepted.

Thanks

Regards

Tony

Submitted by peter_shand on Sat, 01/08/2020

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I have ascertained that sometimes the dogs on the sliding gears don’t mesh with the dogs on the adjacent free-running gear but instead the dogs come together face to face (i.e. they don’t interlock). That prevents correct gear selection and seems to be the cause of the lockups.

Am I getting that because the gear pinions aren’t being spun by the running engine as they would be in ‘normal‘ operation?

In short Yes, 

Submitted by jim_hill on Sun, 02/08/2020

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Instead of using the gearbox sprocket to test the selection, put the clutch on the mainshaft and spin that while testing selection, i am pretty sure will find a better selection, that is how it will work when it is installed in the bike.

Submitted by ian_soady on Sun, 02/08/2020

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.. with the above, it's always going to be the case that with a static test sometimes the dogs will meet end to end but when the engine is running / bike rolling they'll find their way home.

You don't even need to put the clutch on, just turning the layshaft by hand should be enough. At least you've had plenty of practice at rebuilding it!

Submitted by tony_wood on Sun, 02/08/2020

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Thanks everyone. This is my first Norton gearbox rebuild. I’m used to building unit Triumph gearboxes - they seem to just go together and work - that’s why I was a bit uncertain about my Dommie’s gearbox operation. I’m going to assume that all will be fine when it’s back in the bike so I’ll carry on with putting it all together.
Thanks again.

Regards

Tony

Submitted by robert_tuck on Sun, 02/08/2020

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Shafts can be very slightly bent by mishaps in the past . Worth rolling them on a flat surface to check . They can be trued up with skill and a press.

 



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