Noisy gearboxes

[Chris KC]

The constant mesh gears are the ones that get replaced in a CR box; and the most likely candidate for generation of noise, all other things being equal.
By definition they are constantly in mesh with each other, the difference between 1/2/3 and 4th is that in 4th the 1st and 3rd motion (input and output) shafts are locked together, so no load is carried through the constant mesh gears and the intershaft bearing/ bush sees no (relative) rotation.
Thus if you have a noise in 1/2/3 only these must be your prime suspects - constant mesh gears and intershaft bearing.
I'd fully expect the gears to be noisy under load, the thing that surprises me is this level of noise with no gear selected.
Clutching at straws perhaps, but do the 1st & 3rd motion shafts move freely relative to each other in neutral? If the intershaft bearing is binding once installed it could perhaps be 'loading' the C/M gears in neutral. (Actually upon deeper thought I'm not sure that it could...!)
Have you tried to check the backlash between the C/M gears? Original gear sets are all etched with matching numbers, they were selected for size much like timing gears. This subtlety is lost when machining CR gears so it's possible they might be meshing a bit tight, or with excessive clearance.
The above makes sense (I think) for a CR conversion - but not obviously applicable to an original Nippy box. (I take it you are sure the gears inside it are originals?)

[Dave Mann]

    I think you need a rig like this then you can listen to the gearbox all on it's own, the brake drum can be used to provide a bit of load.
The Andrew Bird close ratio gear set in our 4 speed RN are helical and whilst not totally silent in the intermediate gears they don't emit the banshee howl I had with a Super Accessories straight cur set I had in a special 50 years ago.

[bob46320]

What size bolts are the "Cadburys GIANT SIZE" ones Dave ?

[Dave Mann]

I think those tins date from the 1960s not chocolates but Cadburys Smash and if you don't know what that is it was dehydrated potato.

[Reckless Rat]

(in martian accent): "They peel them with their metal knives, and then they smash them all to bits".
Classic.

[Ian Williams]

Another thought, I have come across gearboxes where the synchro cone has come off the 1st motion shaft, this makes one hell of a howl. In a converted box this may be the issue, the cone is shrunk onto the gear so if the person who builds the box removes and refits the cone cold rather than hot there is and extremely good chance it will come loose.

[daveg]

Thanks for all the pointers. I haven't had a chance to look at the gearboxes today. Chris your explanation of what happens in 4th gear and Ian's experience of distortion on the 1st motion shaft as a result of the close ratio conversion point strongly to an inter-shaft bearing or meshing problem on the converted gearbox. As this box is in a car an usable I am going to concentrate on the "original" box  first.

I don't know the history of the Nippy gearbox it came with the car and has the "S" stamped on the casing but it is always possible the internals have been swapped. When i get time I will see if it has matching numbers. The 1st and 3rd motion shafts rotate freely when in neutral as I remember checking this when setting up the selector fork positions.

PS Dave that setup is just what I need - I tried driving the box with a drill bu the drill was noisier than the gearbox.

[Ian Williams]

In fairness to our gear manufacturers Austin used to run in gear pairs on special jigs then mark them up in sets as Chris mentioned, of course our low volume modern manufactured sets are not afforded this luxury and consequently make some noise. In my experience provided the gearbox is set up correctly and layshaft not worn to the extent the meshing pattern is loose they are not excessively so, certainly a dam sign quieter than straight cut gears, they also seem to improve as they run in. I have one set which has done in excess of 80,000 miles and is virtually silent.

[daveg]

I got a chance to dismantle the original Nippy gearbox today. I found these numbers - Lay shaft gear cluster G760, 1st motion shaft and 3rd gear cog 351S. The gears and synchros all look to be in good condition

The lay shaft is grooved.

   

I don't know what the ring between the gear and the synchro ring is called but it is loose and can be moved about 2mm. It is hard to see in the picture but the synchro is not flush with the back of the gear. Can it be heated and pressed further?

   

There are marks at a funny angle on the lay shaft bearing face as if it has been filed.

   

The lay shaft cluster front bearing is worn but fairly smooth. Is the welded on gear normal?

   

I am going to fit new roller bearings, clean up the lay shaft bearing faces and reduce the end float by fitting a shim behind the front bush and see what difference it makes

[Ian Williams]

The synchro cone is loose on the ist morion shaft, probably as a result of being fitted cold, if you put is back together without dealing that you will have further problems. The difficulty that you now have is that the toothed bore of the cone is now stretched, some may suggest staking it onto the gear but in my experience this is often unsuccessful, I usually replace them with one from another box. The lay shaft appears to be not badly scored, provided there is not excessive play in the bronze bearings all should be well there.
I have just looked back through your pictures before posting, the cone is not loose on the ist motion shaft but on the main shaft 3rd gear. I also suggest that the box has had an alternative ratio fitted, the layshaft has been altered with a larger constant mesh gear, quite the opposite to what I would expect???