Gearbox problem

[watts]

I am looking for a little guidance.
I can no longer change gear on my 1970 A65 Thunderbolt.
Looking at it the shaft rotates up and down.
It failed while I was out but by keeping my foot on the change lever it stayed in gear and got me home about a mile so I hope I did no real damage.

I was wondering if the selector is a bit sloppy, but I don't think it is cause

The problem can be seen here it goes into 1st (probably maybe 2nd) but does not stay.  I think 3 and 4 word ok nut there are lots of false neutrals. In the video it is in gear until I take my finger off, if keep it under pressure with the rear wheel it stays in but realise back wheel and take finger off it come out of gear.  I assume this will be the cam plate?

Does anyone have any suggested where the fault maybe, before I embark on any unnecessary work.
Thank you

[Allan G]

you've got at least two problems going on there.

1. YES the slop you have there will cause an engagement issue, when you are pressing on the lever to change gear, instead of rotating the quadrant, it will try and lift up in the cam plate trapdoor as there is less effort to do that, than reposition the cam plate.

2. you should be able to rotate the gearbox and the cam plate position not change. Its possible you have a broken spring but just as possible that the indent plunger has worn a groove in the camplate tack, then if thats the case, you need a new cam plate..


Ideally your looking at a replacement trapdoor and possible cam plate minimum. Time to remove the clutch and pull the box.

[watts]

Hello Allan

Thank you for that, in my mind that is what I thought but it is reassuring to have confirmation given the amount of work.

Last time I contemplated  taking clutch off I struggled to get the centre out and gave up, time to do it.

Thank you

Matthew 

[watts]

Hello Allan

I have been offered a NOS gearbox end cover, which has bearings and bushes in.  This would help with the sloppy selector hole, but does this create a problem with misalignment between layshaft bushes?  Or will it just 'bed' in?

Thank you
Matthew

[Allan G]

Providing the cover has the same part number as yours ( there weee at least 3 types) measure the gap between the cam plate.

It'll be spot on. I've swapped covers before for better ones with no adverse effects. I did start making a jig so I could bush the worn hole. But I've collected a few since so should be fine for my projects for now.

[watts]

Hello Allan

I have started to take it apart a the problem can be seen here in this video.  If I select 1st and hold the tension with rear wheel it stays in but when I release the selector plate raises as in video.  I assume when it is working properly when 1st is selected the plate turns as in the video but it remains in gear.

Thanks

Matthew

[DAVE BRADY]

Hi Matthew,

What happens if you engage 2nd and can you move the plate to select 3rd and fourth?
As Allan suggests, initially I suspect the sprung plunger that holds the gear change cam plate in place.  If you have a parts book it is item 48 on page 27.  It could be the spring has broken.  It shows this in the workshop manual in fig. B58a.  This plunger exerts quite a bit of pressure so changing gear just by moving the selector plate should not be easy.
Now I hope I get this right but I think that on earlier models it is necessary to remove the gear cluster/cassette to get to it but later models it can be accessed once the primary has been remove along with the gear box sprocket.  This would have to be done either way.  With the plug removed the plunger and spring can be replaced. 
There is an after market plunger available that instead of just a shaped end to the plunger there is a semi-captive ball bearing that makes gear changing a bit smoother. 

Dave.

[Allan G]

I'll second that dave. It'll be spring or the track in the cam plate has worn from the sharp point on the plunger. This will take a while to wear through but when it goes... it goes and it will jump out of gear with little to no effort. If it's more resilient in higher gears then I would strongly suspect this. Since you spend more time between 1 N 2 than in 4. 

I round the end of these plungers to get rid of the sharp point. Then polish them on my polishing wheel.

Either way, your wasting time and fixing nothing until that cassette comes out.

[watts]

Thank you everyone for your help.
I hope to get it out today or tomorrow.

Who supplies the aftermarket plungers?

Thanks

Matthew

[DAVE BRADY]

Hi,

Try this link.

https://images.app.goo.gl/kvLHkewTv9uQ3KyC9

Dave.

[watts]

Hello

Thank you

Matthew