The gearbox on my '51 Tiger 100 was overdue for a rebuild. It jumped out of 1st gear when revved, the usual whine in 3rd gear had got very loud. And oil leaks. These issues have all been there for years because they didn't stop you riding. However, the rest of the bike is pretty much sorted now so it seemed like time to do this.  Plus, I've done a dozen gearbox rebuilds, so, no problem right?

Unfortunately gearbox removal involves tearing down the whole primary side which is a pain. New gaskets and lock-washers and special tools (clutch locking/clutch hub extractor) not to mention the usual risk of coming across damaged or wrong parts.

On stripping the box there wasn't a lot wrong. Main issue looked like the layshaft bushes, both badly worn. Both mainshaft bearings were good and all the springs measured up OK. Lots of other minor wear but nothing dramatic except a camplate with gouges out around the track, and a worn high gear bush. Layshaft bushes often get neglected because the drive side one is blind and so harder to extract. Incidentally,  google AI had already prophesied most of what I found when I asked it about the problems I was getting. Amazing what it knows now

I ordered the bushes from Monty's Classics (recommended, he knows his stuff, has a good stock and refunds without any fuss). The first high gear bush was a new one from LF Harris and didn't fit, but the second was a genuine nos and pushed on nicely. The blind layshaft bush is more fun. After trying a couple of ideas found on web forums (including using expanding masonry bolts) that didn't work, I went for the factory method. A coarse 3/4" tap (I used UNC) plus a bolt to match. My pal Allan welded a long scrap bolt to the short UNC bolt (bottom pic) which allowed it to be levered up and out after the case was warmed up. The timing side bush is in the inner cover and can be drifted out.

The bad camplate was replaced with a good used one from my stock. A couple of less worn plungers on the gear quadrant, plus two more new bushes on the same quadrant, using a similar tool on the inner blind one. The only other issue was a thrust washer worn thin on the kickstart ratchet which accounted for the kicker not returning properly.

With all the bushes removed it's a good opportunity to dress all the joint faces to stem the oil leaks. I use sheets of fine emery on surface plates. It's important after doing that to check the end float on the layshaft when fitting the inner cover but I've never found this to be even near the limits. I don't use any gaskets on these boxes, just a good sealant.

After assembly, gear selection was checked on the bench. Never very nice but seemed OK. Refitted box, inner primary on and build the transmission. It's a good plan to set up a wedge under the gearbox pivot to aid installation. You can set the right height before removing the gearbox. I had also found a clutch chainwheel with the teeth rounded off, wrong clutch springs and a primary chain made of 3 sections (the culprit must be me, I've had this bike for 25 years) but all now replaced. Final assembly, exhaust pipes on, fill with oil. Out of gate and up the road. Only two gears! It won't go up from 2nd. VERY disappointing, and I couldn't face doing it all again immediately so put it in the corner. My nice BMW f800 works fine thank you.

Eventually, I tore it all down again. Although there is always the fear that mixing new parts in with a 70 year-old gearbox is asking for trouble, I'm pretty sure it must be one of the components I've changed, and after checking right through, the camplate is the main suspect. This part is a bit mysterious. There are at least two different versions, possibly three. Despite this it always retains the same part number (typical Triumph) T500. In addition, on the exploded drawings in the parts book it is shown side-on which hides the different shaped outer forms so no clues there. Triumph pre unit gearboxes often changed parts but overall there are two main types, let's say early and late. Early is up to 1949, then nearly all new in 1950, after which many internals are interchangeable. The only significant differences post'50 are the case change in 1954 for the swing arm type mounting, and slickshift mods in 1958. All camplates have the same track but the outer shape changes. I do know the later is bigger and won't fit the early boxes without modifying. Also the plunger tip shape changed and doesn't like mixing up. There is a newly manufactured camplate generally available and it is the last version. Advertised as fitting all pre-unit gearboxes but I don't think so. I believe the later (1950-1962) used two different shapes as my 1951 is not the same as this last one. Later (new) camplate is shown on the left in pic.

Then, new problem: when I dismantled I found the primary chain tight. It was correctly tensioned I know because that particular job is nice and easy with the cover off. Only a few miles and it's tight so the gearbox must be moving around. Looking at the main case more carefully I saw the mounting hole across the base is ovalled. Not enough for much movement but any will stress the clamp beyond its designed function. The clamp on the rigid boxes is nasty to tighten anyway and clearly it can't hold a gearbox still that isn't held firmly at the pivot. So that needed bushing. I decided on a pair of phosphor bronze bushes 3/8" ID and 1/2" OD about 3/4" inch long. No problem, a few quid on ebay which was a break. Back to Allan and his milling machine. Actually quite a tricky job to drill it exactly as the bushes must be a tight fit and perfectly in line. We ended up using a long line reamer to finish. The 1/2" reamer had 'Austin Motors' engraved on it and apparently was for reaming Austin 7 kingpin bushes. From back when looking after your car involved a bit more than checking tyres and oil!

I bought a new camplate (£50 quid, ouch) and rebuilt the box very carefully this time. Gear selection was checked exhaustively before and after installing box. It felt better although it required a very deliberate action. Off to Draganfly for a couple more primary gaskets. The transmission was built up, and final assembly completed again. Most sources say 2/3 pint of oil in the gearbox, but my trusty original workshop manual says 1 pint. It seems too much as it runs past the level plug, and leaks past the mainshaft oil seal when the bike is on the prop stand. So settle for something in between? Use engine oil, not gear oil.

Out of the gate and up the road. 1st gear, 2nd gear. Out of village, try 3rd gear. Not great but found it. Turns out that is just nerves making me do it hesitantly because actually it's fine. A ten mile road test and all is well although it feels so different. Instead of the light touch changes before, it now needs a firm, 'measured' change but all gears are happening again! Phew.