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Navigator first gear jumping out


Hi, took my newly refurbished Navigator Delux 1962 around the block for its first run this morning and it jumped out of first gear at every opportunity, (this was not unexpected) although I had tried to ensure against it during gear box re-assembly by fitting a shim, although to tell the truth it wasn't that clear in the manual what size shim I should be placing exactly where.

I've looked at the Lightweight twins compendium but can't see anything to help, Can anyone kindly give me chapter and verse on this please?

Thanks in advance, John.



Hi John,

If you are logged in you can see the lightweight compendium under 'technical'.

I'm going to assume you have read page 26. 

IMHO the first gear can jump out if the dogs on the sliding 2nd layshaft gear don't fully engage with the 1st layshaft gear. As the dogs of both gears get rounded, there is a tendency for them to try and push apart and overcome the restraint of the selector fork holding that 2nd layshaft gear in engagement..

Looking at the illustration on page 26. 

Is the correct hardened steel spacer present as illustrated in position 60? If not this is your problem.

Does the kickstart shaft have free axial play? How much? Measure with a dial gauge preferably.

Aim for between 0.05mm and 0.15mm by shimming between 1st gear pinion item 48 and starter ratchet item 59 as described in the text of page 26. A side benefit of shimming for no axial movement in the kickstart shaft is that the o-ring oil seal on the shaft will seal much better.

After checking the shims as above, the 1st gear is held as near the sliding pinion as the designer intended.

The position of sliding pinion when 1st gear is engaged is determined by the fork engaging with it. If the selector fork ears are worn, the sliding gear may not be fully engaged.

If the round pegs on the selector forks are worn the sliding gear may not be fully engaged.

The position of the selector cam relative to 1st gear pinion is affected by the shims (not shown) either side of the cam, (item 16) on its shaft (item 17). And the shims Items 18 and 19 either side of the bush item 65.

This is a lot to take in. Sixty years after assembly there is a good chance your gearbox has been assembled with shims missing, or in the wrong place. It does make a difference.

I suggest you go for shimming the 1st gear pinion to start with. Andy Sochanik suggests this will fix the problem. I think it probably will.

Cheers - bike looks great.




Wow! Thanks Peter. Must admit that I skimmed the index of the tech compendium rather than looking at every page. Hopefully this will solve my problem. Will let you know.

Cheers, John.


Looks great with those "shiny new exhaust pipes" John. Trust you got them to fit OK after your initial concerns?


Hi Michael, I returned the first set of p[pes I received as the dimensions of the hot end were a mile out and the cold end  was not a nice taper to the silencer but an ugly great butt weld. I got a refund easily enough and another pair from a fresh batch the guy had just had made. These were perfect from the looks point of view and only needed just a little fettling to fit. One happy bunny.

 How's your project going?

Cheers, John


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