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Gearbox selector shaft.


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Despite a fairly recent gearbox rebuild (In miles not years) the car will occasionally jump out of 3rd gear on over-run. Having stripped off the rear extension  there is clearly wear on the top of the indent on the selector shaft. If I put a spring balance on the shaft it requires around 30-35 lb to come out of 4th but only around 15 lb to come out of 3rd which i believe is the problem. I have bought new  plungers & springs for both the forward selector shafts but the shafts themselves are unobtainable.

I have not yet fitted them but if i screw the 3rd/4th plunger in to give the correct reading on 3rd whether 4th will be too high to be usable. Does anyone know how high a reading is still workable on 4th.

Does anyone have an alternative solution, know if the existing shaft could be machined to remove the wear or know of a source of new shafts?

tIMG_20220113_220027.thumb.jpg.6dd25d79f072b028f7def590bedc2abb.jpghanks,

Allan

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It could be that, but could also be worn dogs on the synchro hub / 3rd gear dogs, weak springs in the synchro hub, or even simpler is the rubber gear lever gaiter encouraging the gear lever towards neutral (I had this once on a Jeep Cherokee)

Bob

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I have just overhauled my gearbox for the same problem. For one thing, the synchro rings were worn, allowing what I thought too much end float of the hub assembly on the shaft. So did it ever quite go into 3rd as there was a lot of movement? Having said that it never came out of 4th. The angles of the cones had worn meaning they’d effectively be too large for the gear. When replaced they reduced free movement quite a bit. I also shimmed about 0.012” out of the bushings on the shaft as well, to bring it back into its 0.003”-0.009” spec. 

Then I rebuilt the selector housing after Roger kindly fitted modified seals, and used a spring balance on the shafts. The correct poundage for release of the 3rd/4th selector shaft is a lot more than 15lbs. The correct figure is in the workshop manual, and is more like 38lb or so. Even with the imbalance you have it’ll need more than 15lb, so it’s either another shaft, weld up and re-file the wear on the indent, or you could try a ball bearing instead. The screws ideally need to remain flush in the housing. I found and fitted a couple of thin BA washers under the screw to increase the pressure, which is the correct way to do it according to the W/m. 

Kevin

Edited by boxofbits
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My gearbox is a Stag unit converted to fit the TR.    It spits out 3 rd gear on overrun when cold.   It was a new factory unit ( not rebuilt) when fitted some 30 k miles ago and has had the issue since fitting.   

 Once warm no problems at all.

It also leaks from the selectors so I feel a bit of tampering may be in order eventually.

 

 

 

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Hi Allan,

 the wear on the shaft does not look excessive enough to allow the 3rd gear to pop out.

After 30,000 miles the GB is hardly 'Run in' but if the synchro etc is dodgy then that would cause the wear.

As Bob says above it could be a stiff rubber gaiter around the gear stick.

Roger

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4 hours ago, RogerH said:

Hi Allan,

 the wear on the shaft does not look excessive enough to allow the 3rd gear to pop out.

After 30,000 miles the GB is hardly 'Run in' but if the synchro etc is dodgy then that would cause the wear.

As Bob says above it could be a stiff rubber gaiter around the gear stick.

Roger

I’d agree worth checking superficial things first. Thinking again about the selector shaft, I believe they might be chromium plated so not a good idea to fill/ re-file with weld. I do have a used selector shaft you can have , but it has picked up a little bit of surface rust where the seal is positioned, but the shaft and detents are fine.

Kevin

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Thanks for this. Have been out today so no further forward. I agree the gearstick rubber may be a contributing factor as I have the dual rubber arrangement (I think from the 6?) rather than the original 4A one piece. However with only a 15lb pull off anything would probably do it.

Having had 3rd synchro problems before the rebuild this feels different (or is that wishful thinking?) in that a slight nudge back up on the gearstick keeps it in 3rd without having to apply any further pressure.

Will try the new detent plungers & springs & see how I get on. Will also call Pete Cox next week - good idea thanks.

failing all of that might like to try your spare Boxofbits if you didn't mind. 

cheers,

Allan

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20 hours ago, Allan Westbury said:

Thanks for this. Have been out today so no further forward. I agree the gearstick rubber may be a contributing factor as I have the dual rubber arrangement (I think from the 6?) rather than the original 4A one piece. However with only a 15lb pull off anything would probably do it.

 

later 4a has the two part gaiter, only TR4 and very early 4a have single part boots.

Stuart.

Edited by stuart
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  • 4 weeks later...

Just to close this off i bought new shorter springs and plungers as per the book. To get around 30lb ft on third I was over 40 lb ft on 4th. During a conversation with Cox & Son about something else they said if I took it down to them they would service it while I waited.

They replaced the short spring & plunger with the longer spring & ball on third/fourth, polished the shafts, replaced the O rings & copper greased the gear lever socket. There were a few broken pins involved and a bit of drilling so in the end I was pleased I had taken it down. The spring tension was measured by hand!

The car is now back together & while fourth is a bit notchy there is no way it jumps out in third so success!

I now have the two part boot fitted although I remember a one piece being fitted in the dim distant past.

thanks all.

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If you were to fettle gearboxes every day, as do Pete and Tom, you would be able to measure spring tension by hand.

Ian Cornish

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I used a spring hook gauge from Ebay to do mine, and it worked well. I hooked it on the selector rods and pulled on the scale to get the correct poundage.
 

Like Allan’s it has resolved jumping out of 3rd on the over-run. I did however shim the mainshaft bushings for 2nd and 3rd gears which were about 0.025” combined out of spec. The tension on 3rd was only about 20/21 lbs. I put two thin BA washers under the selector screw as per the WM and it’s all good.

https://www.ebay.co.uk/itm/174649938777?hash=item28a9f29359:g:nUIAAOSwT~dgNSQu

Kevin

 

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I use a fixed weight, in this case cans of water as the weight is easily adjusted. 
I find it much easier to manage than trying to pull on a spring gauge and read the force on a fluctuating scale. 

48B15C83-10FB-4139-AB3B-E3625ECCBEC6.jpeg

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Now I call that getting sophisticated. When I bought my first TR (late 60s) this kind of science just didn’t exist. The summum of engineering effort was to remove an engine using a fencing stob (post) and two young lads - outside in the street in Aberdeen. Snowdrifts added to the interest.

james

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