The Rudge Gearbox

In last month’s Classic Bike I wrote a bit about Rudge gearboxes as part of the feature I’m doing on restoring a 1937 Special.

But there wasn’t really enough space to do the subject justice so I said I’d put a more in-depth look at it on here. Last week Roger Murray emailed to say he hadn’t managed to find anything about it, so I thought I’d better get on with it!

What actually happened was this, that particular issue was a bit tight for space so I had to keep the feature brief, hence the idea of putting the full story on here in the hopes it might be useful to someone. But at the last minute another feature in the magazine fell through so I had space again and was up till all hours rewriting the feature with loads more gearbox  pictures, taking some comfort from the fact that at least now I wouldn’t need to put it on the Blog. When the magazine came out a couple of weeks later I found that they hadn’t used the new material and had got around the extra space by just enlarging a couple of pictures… Doh! That’s just how it goes in magazines sometimes and so now I was back to putting it on here. But I admit, I did wonder whether anybody would actually come looking for it… Well, apparently so, so here we go!

Firstly, Rudge were very fond of taper/spline combinations, indeed I think I was told that it was their patented idea and presumably relates to the Rudge Whitworth backbone industry of automotive wheel manufacture. These appear in several places and in the gearbox, are to be found on the kick start and clutch centre. Having loosened the nut, the kick starter can be freed by giving it an outward tap on the pedal end with a soft mallet. The clutch centre has a thread for a puller which is available from the Rudge club. Since most British clutch pullers are 20 or 26 tpi and somewhere around an inch diameter; it dawned on me to have a look at what pullers I already had – how many variations can there possibly be? I had a look through my selection and right enough, I found a BSA six spring one is the same thread, saving me a job.

Before dismantling the gearbox it is worth assessing end float.

The engagement of Rudge gears is slight; excess end float is ‘lost motion’, backlash if you like, that reduces the engagement, further increasing the chance of gears jumping out of mesh. Using a depth gauge, push the main shaft fully inwards and take a measurement from the end of the shaft to a convenient fixed point, like the bearing retainer ring. Tighten the screw to lock the gauge at this setting and then pull the main shaft as far out as it will go and compare the measurement. You should now have a gap beneath the depth gauge tip which can be measured using feeler gauges; I find this an easier method than re-measuring and calculating the difference by subtraction.

Next check the layshaft float.

This you do by moving the kick starter shaft in and out in the same way. How much end float should there be? Well since the gearbox shell is aluminium it is likely to expand with heat increasing end float rather than reducing it, so many say ‘zero’, personally I prefer just enough that you can feel it but probably not enough to measure. Our gearbox measured up at 25 thou on both shafts. A fairly thick paper gasket, measuring 20 thou was fitted on the outer cover. On reassembly I replaced the gasket with ‘instant’ and was happy with the result, the end float being what I would call ‘just perceptible’. If you can’t get it right that way you would need to alter the thrust washers inside the box and it all gets complicated. I’m not sure what availability there is for alternative sizes.

The Rudge sprocket nut has a left/hand thread. Underneath the sprocket is a castellated ring, also left/hand, which compresses a pigskin washer onto the sleeve gear as a gland seal.

These washers are available from the Rudge Club. Our one wasn’t particularly snug so I ordered a replacement. On Rudges the main shaft, unusually, is at the bottom of the box with the lay shaft above. This is actually quite a good idea since the main shaft does most of the work but usually has to rely on splash lube from below; the downside is that the output shaft is thus below the oil level – asking for leaks. When I laid out the bits to build my Ulster’s gearbox, I couldn’t find any kind of seal at all – or even any space to fit one. The only part I seemed to be missing was a big coil spring. I ordered one and discovered that before the pigskin gland seal, they relied on putting a spring behind the sprocket, which gently pulled the sleeve gear so that the thrust washer between it and the bearing was gently pressed against the bearing outer race, making a seal. The same principle was used on the ceramic water pump seal Honda fitted on CX500s, there’ll be a name for it no doubt but whatever it’s called it seems to work very well on my bike.

Having extracted the cover screws, the cover is still retained by dowels which may be a bit stiff. I deemed tapping the main shaft end with a nylon hammer safer than thumping the clutch cable stop on the cover. Either way, do it gently and once clear of the dowels STOP because you are about to spill rollers all over the bench.

Rudge ran their gearboxes on loose rollers in preference to journal bearings. They are expensive to replace and easy to lose, So ideally sit the box in an old oven tray or something before going further.

The rear cover can come off now. This is the next bit of Rudge mystique. The bush that holds the selector camplate is threaded to allow adjustment of engagement. Nobody else seemed to think this was necessary but Rudge did. Unfortunately it offers an opportunity to get the engagement very wrong so care needs to be taken. Sam Lovegrove (off the telly) is an ex Rudge owner and told me that he’d heard it was possible to adjust it so that you could get two gears at once which is not good news.  The short screw in the cover is a grub that locates in a notch in the bush to secure it in the correct position. Before removing the cover, take out the screw and make sure it is actually located in a notch. Then remove the cover and make sure there is only one notch, it may have been adjusted previously or even taken from another box and have several, so mark the correct one.

Now you can lift the selector camplate clear and slide the gears out of the box as a set, again watching out for runaway rollers.

There are rollers inside the kickstart shaft, the high gear bearing and inside the sleeve gear on early models (the ’37-on  boxes have plain bushes here) there is also a hardened spacer between the two sets of rollers in the sleeve gear. I had a puzzle with my bike because with the rollers and this spacer fitted, there was still room at the end, as though the spacer was too narrow – although a witness mark indicated it was the right size. A query on the highly recommended ‘Real Rudge Enthusiasts’ website (don’t ask; schisms occur in all clubs and the reasons are no concern of mine but suffice to say the Real Rudge site is free to join and offers free downloads of parts books etc whereas the Rudge Enthusiasts Club is an official owners club which requires club membership to access information/parts clb spares scheme etc) revealed that the rollers supplied by the parts scheme are slightly shorter than standard Rudge ones. I got around it by making a new spacer from titanium and hardening it, fingers crossed it seems to be okay so far.

Now; my bike jumped out of gear from the moment it went on the road. Mostly second and sometimes top. Because I don’t have the footchange ‘peardrop’ positive stop assembly I rigged mine up as hand change which has the advantage that I can see which way it jumps; it turned out that the lever jumped upward out of second and down out of top. While I had Ben Moor’s Classic Bike Project ’37 box on the bench, I acquired a set of internals for an earlier box which are more or less the same but with finer tooth pitch. This gave me the opportunity to inspect the movement through the gears in a complete box and duplicate it with the loose gears on the bench. From this I could see that although the lever was jumping out of the two gears in different directions the movement inside the gearbox is actually the same. A common problem on Rudge boxes is that in second gear the sliding pinion engages with splines on the layshaft – but only just. This means that if the spline end is at all rounded, jumping is very likely. My problem was concurrent with this issue but it seemed to me that if I just screwed the adjustable selector bush in by half a turn then the gear would sit a bit further on the spline and maybe all would be well. Like a lot of long-established owners clubs the Rudge club has an archive of technical notes collated from club magazines which are available on the website. Generally speaking, stuff written in club magazines isn’t the mixed bag you get on today’s web forums although a certain amount of reading between the lines is still needed because technology and the value of the bikes has changed and some old repairs would be considered horrible bodges now but nonetheless it’s well worth reading. In Rudge lore, biasing the position of the selector to accommodate wear is frowned upon and I can see why; it’s a dodge rather than a repair and having had a chance to analyse its workings I find that you can indeed accidentally adjust it so that one gear is still engaged when you select another and since there are only two full turns of the adjuster ‘lock to lock’ (ie before the gears bottom out in either direction) even my half turn is an appreciable – and so potentially risky – movement. But I’m getting a bit ahead, here’s how it works.

The Rudge gearbox is symmetrical through a diagonal line. In first, the two pinions top right engage and the two bottom left have to separate far enough that the dogs on the sliding pinion are well clear of the mating holes in the outside gear. In top, it is these two that are mated while the top right pair separate. The aim is to position the screwed bush (and therefore the camplate) so that in both gears, the mesh and separation is the same, in other words the selector is completely centralised. If you adjust too far off-centre one way, the separated gears will not be far enough apart and the dogs may catch, engaging both first and top at once and locking the gearbox…nasty. If you adjust it too far the other way, the dogs bottom in the holes before the detent plunger has clicked into its notch and it will jump out of gear. So it is pretty important to get it right.

To adjust it, you need to unscrew the detent plunger a bit so that the camplate is readily movable by hand – don’t make it too slack, the spring loading of the plunger holds the camplate in position. You also need to secure the bush and camplate without the cover fitted so you can see what’s going on; people used to cut a spare cover in half to use just the clamping section but I wouldn’t want to do that today. I actually found that it was possible to invert the cover and fit it as in the picture but alternatively you could probably make up something out of a bit of wood, it doesn’t need to be too fancy.

I’ve screwed up here, the idea was a set of photos of the box in specific gears with a cluster below showing what’s actually happening inside but I now see I’ve got the gear cluster the wrong way up, so the selectors are in the opposite positions to where they should be, dammit!

So ignoring the selector being in top and imagining it’s in first, like the gear cluster, here’s how it works. Drive comes in via the mainshaft (top) from the clutch on the splined right end of the shaft. The sleeve gear that carries the gearbox sprocket (top right) is free spinning on the shaft. The small pinion at the other end of the shaft is fixed to it and so drives the large layshaft pinion (bottom left) which is free spinning on the shaft but in first, the adjacent sliding gear – which is splined to the shaft – is attached to it by dogs so the drive is passed to the shaft itself. The next layshaft pinion (3rd from left) is free spinning in this position and so does nothing but the small one on the right end is fixed to the shaft, so it transmits the drive to the sleeve gear and thus the rear wheel.

This is neutral (both gears and selector position correct – at least I got this one right)


Here the drive, again coming in on the mainshaft, passes via the small pinion on the left end to the large gear below but this time because the dogs are not engaged, the large pinion is free spinning and no drive is transmitted there. Look along the shaft; of the next pair, the layshaft gear is splined to the shaft but the mainshaft one is free spinning, next set it’s the other way round and the layshaft gear is free spinning. We know the sleeve gear is free also so although the two shafts are connected by four pairs of gears, in all cases one of the pair is free to spin, so nowhere can a driven gear on the mainshaft transmit drive to the layshaft and thus back to the sleeve gear and sprocket.

In the next picture the gears are in second (but the selector is in third).

The smaller sliding pinion on the mainshaft (2nd from left) has moved right onto the shaft spline. The layshaft gear with which it meshes is permanently splined to the shaft, so this time the mainshaft drive is sent via these two gears to the layshaft, driving the fixed pinion on the right and thus sleeve gear and sprocket.

In third gear the reverse is the case (selector now shown in second)

The pinion on the mainshaft has backed off the spline to spin freely again but the one on the layshaft (third from right) has now moved left, onto the layshaft spline, so this time the layshaft is being driven by these two and transmitted to the sleeve gear and sprocket.

Finally in top (bearing in mind the selector shows first in error)

This is the easy one, the mainshaft drives its splined pinion (3rd from right) which is moved right so that its dogs engage with the sleeve gear. At this point the sprocket is thus turning at the same speed as the mainshaft, so top gear is always referred to as 1:1 gearbox ratio. Being the gear you do most miles in, this is the least stressful gear for the box as it’s virtually direct drive – the layshaft being completely out of play.

If you have managed to stay awake through this analysis, you can see that if my bike was jumping out of top gear and second gear, in both cases that involves moving cogs to the right in the picture, which is why I adjusted the selector position.

Here’s the issue. This is roughly the amount of spline available for the sliding pinion (middle in photo) to grip when it slides left for second and third, not a lot really and if you add a bit of wear on the leading edge you can see how tempting it is for it to slip off. The dogs (top gear) are a more positive engagement, I would say, which may explain why top was less prone to jump than second.

Having made my half turn adjustment on my box, I have had no further trouble and I think it may have been the right thing to do, despite Rudge lore. Obviously it needs to be done carefully and of course it will not restore a knackered gearbox but I did notice when setting up Ben Moor’s ’37 box that having centralised the selector carefully, if you changed up and down a few times it never seemed to come out exactly the same twice so maybe setting up on the bench and then adjusting it slightly ‘by feel’ is a better approach; given that second is a common weakness and I hadn’t ridden Ben’s bike to see if the gears were okay as they were, I set his box up with a  slight bias like mine as a precausitoin and it seems to be fine. Just remember that, with only two turns of that adjuster from one extreme to the other, any of centre adjustments you make must be very small.

That’s it from me. Apologies to anyone with no interest in Rudge gearboxes, I’ll try and put some more general stuff on very soon, and also for the lack of posts this year. It seems to have been an extra busy year but I’m hoping to get back on track from now on!

Cheers Rick






Comments (8)

  1. Raymond Albeson

    Luckily the gearbox on my 29 Special was rebuild by Tom Gaynor. There was, however a handwritten list of do’s and Dont’s in the file, especially relating to the damage the kickstart could do to the cases.That’s Rudges for you!

    1. Rick (Post author)

      Interesting, not sure why the kick start should come in for particular comment, although the ‘stop’ on the cover is adjustable and it s important to get that right (or not mess around with it in the first place) but I think it was pretty straightforward when I built the box on my bike.
      Probably the most important thing is oil level, because the shafts run direct on rollers, which are concealed inside them to a degree splash lubrication is not ideal and running low on oil will not just clap out a few easily replaced bearings but will take out the shafts as well. The ’36 or so onward bikes that that the tin cover (sorry ‘mechanism shield’ – don’t want it to sounds like it’s a bit of pressed tin to cover a cobbled up foot-change conversion) also have an extended neck for the gearbox filler/level which I have heard applauded for slightly increasing capacity but of course it doesn’t actually raise the level. My gearbox being remarkably oil tight, I lean the bike over to the left a bit when I check the level so it gets a little bit extra.
      Cheers R

  2. Ben

    Crikey – I’m ever so pleased I didn’t attempt it after reading this Rick!

    1. Rick (Post author)

      In the words of Gladys Knight, ‘take a walk in my shoes’ ! But we are getting there….

  3. Bob Tuck

    Very helpfull Rick, I am working on my own 36 Ulster,similar story,got to ride a 35 Ulster Grand Prix barn find when a boy(sold for £10),wanted one all my life,now 71 and up to my elbows in the weird world of Rudge. Looking forward to the ManxGP.

  4. john regan

    hi rick, the ceramic seal like on the hondx cx motors water pump is called a mechanical seal. theyr commonly used on larger commercial and industrial pumps where the motor and pump are seperate. as you say, theey rely on 2 ceramic disks pressed together by a spring to seal them.
    and im glad i found your site. i often posted on the old unnoficial classic bike mag forum but dont get in there much of late. anyhow, i spotted a link to here, and well, here i am, poised to post inane questions and mindless comments, as you probably realise now…..

  5. John Crispin

    Thanks Rick an excellent exposition of the workings of the late Rudge box and will be a help to the enquirers i have had .
    thanks John Crispin
    REC 37-9 500cc support

    1. Rick (Post author)

      Thanks John, praise indeed! It’s always a bit worrying bouncing into a topic that has been minutely explored by Club members over many years but at least I can say that right or wrong, what I write is based on my own experience and has worked for me.
      I have just edited it slightly to try and tidy up a few confusing bits.
      Cheers Rick


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