[stinkwheel]

Wow.

Mr H used to make an oversize bush with a larger OD, I was wondering if you could ream the gearcase and fit one of those but they are out of stock.

Priceparts does oversizes in iron up to 0.040" oversize but I wonder if that's enough? Measuring would tell you.
https://www.pricepartmotorcycles.co.uk/ourshop/prod_6902096-111051-osize.html

I still think that would be the solution but you'd may have to have one made. Not a huge job for anyone with basic lathe skills. I think you'd probably want the case off the bike (not much more stripping from where you are now) and then drill/bore/ream the hole until you're back to good metal and measure up for what oversize you need.

Or a new inner gear case. £150+VAT and delivery from Mr H or take a chance on £60-100 from India.

Or go the whole hog and go 5-speed?

There's a constellation inner gearcase for really cheap on ebay just now. It looks VERY similar but I have no idea how similar or if it's similar enough...I do know a meteor gearbox bolts into an indian bullet (there are other issues but the fit of the casing isn't one of them). Are you a betting man? Or maybe someone knows? https://www.ebay.co.uk/itm/125975729847

[richard211]

I was working on a 4 speed gearbox that had worn bearings last week. The counter shaft cast iron bush was worn out as well. When I had a closer look at the sleeve fitted on the end of the countershaft, I noticed the groove had sharp edges that was slowly over time taking material out of the cast iron bush.

 If you know someone or have access to a lathe machine, the repair is fairly straight forward. To sort out the damage on the gearbox casing, I would use a 3 leg brake caliper hone mounted in a drill and resurface the damaged area. To get the cast iron bush to fit into the gearbox casing, I would then machine a sleeve that would press fit onto the bush and then install it on the gearbox casing, it would also be a good idea to use a sleeve retaining compound.
  I would strongly suggest replacing the sleeve on the end of the layshaft with one made / machined out of bronze so that it prevents the same failure from happening in the future.
  Another upgrade that can be done is converting the gearbox to run using gear oil. It involves replacing the stock bearing with bearings that have seals on one side. I used SKF 6206 2RS1 and 6303 2RS1. These bearings have seals on both sides, we just have to remove the seal from one side and during installation of the bearing keep the side that has the oil seal towards the outside of the gearbox casing, this will allow gear oil to lube the bearing but not leak past the bearing.
 The reason I suggested using a brake hone to resurface the damage on the gearbox casing is that this work can be done without having to take the engine out of the frame and remove the gearbox casing.

[stinkwheel]

You can remove the gearcase without pulling the whole engine out. You need to pull the four studs out of the back of the gearbox. You may be able to use the existing nuts if there's enough thread showing or you might need a couple of 3/8" cycle thread halfnuts to jam onto them. Then you need to dismount the triangular plates at the top of the gearbox. You sometimes get lucky and can undo the bottom two, pull the bolts and pivot the plates up, you sometimes need to take out the top bolt too, clearance varies. The the whole case can be lifted up and out leaving the short-engine where it is.