I recently bought a 1963 BSA royal star a50 500cc and one of the teeth broke off the transmission. I bought it this way knowing there was an issue but was told it was a few hundred for a tranny and a few hundred to have it installed. I called a shop and they werent sure if they needed to disassemble the entire engine to install the tranny. Is that the case or should it be simpler than that. Any advice would be greatly appreciated, it is my first bike so I thought it would be a cool project, iand i done know too much about the. but I might be in way over my head. Also any advice on a place that would be a good deal on a refurb tranny would be help. Thanks in advance.
I see it's your 1st post!!! Wecome!! How did the seller know,...."and one of the teeth broke off the transmission.." ? You can't see inside so it's odd they knew what the problem is for sure. Does is run? How does it feel?
Anyhow, if you are going to become infected with the Brit Bike disease, you will end up getting a shop manual and a parts book. Better sooner than later, most here will agree. These aren't too expensive, as there are copies of the originals made for sale. The shop book would be really handy to explain the process to you and the shop you take it to. I gotta ask though, if they don't have BSA experience, maybe you can ask around and find someone that does. This bike has a few quirks !!(Hey! it's British!)You did a good thing coming here though, a bunch of good guys dying to help and offer advice. You know, spread the misery around....
SO, the job is kind of a messy deal, in that both sides of the motor need to come off, and there is oil and gaskets to deal with. Drain the trans as much as possible 1st. Motor oil need not be drained. The rear chain DOES NOT need to be removed, as the trans will come out the RH side once the nut holding the clutch on the shaft is removed in the LH side. LH is the Primary Side, the 1st of the 2 drive chains and clutch. RH is the Timing Side, the side with the timing gears and points.( assuming you did not know from your "1st bike comment" )
On the LH primary side, the foot peg(Nut is a LH, backwards thread!!!), cover, and chain slackened with the adjuster. IF,...IF, the shop has experience, they don't absolutely have to remove all the primary chain and clutch(special tool req'd), from the area to release the trans. BUT,......... they probably will, and there are loose roller bearings 'behind' the clutch. Kind of a pain.
On the RH timing side, the kick lever, shift lever, foot peg(NUT is RH thread), and outer cover come off. Next the points etc come off (released from a taper drive in the cam end), the kicker spring and gear, and DO NOT forget to pull the tach drive shaft out, the clutch cable from the lever, a bunch of screws, and the inner cover comes off, of course it doesn't just "come off" as it's stuck with gasket gook, ....both motor and trans oil dripping everywhere. (hopefully not too much more stinky Trans oil) Now you see the timing gears, and hopfully will leave them as is. You can see the gasket separates the 2 oils down the center.
Now, you remove the nuts from the round trans plate, and figure out a way to pry the plate loose from the case, moving the shifter plate a bit will help get the trans out of the hole, and the whole thing comes out as an assembly. (agian, assuming the LH stuff is off) Make sure not to lose a thick shim washer that will be on the end of the lay shaft. It usually stays on the shaft, stuck with gear lube.
KC's summary of the work needed to replace the gear cluster has nailed it pretty well.
The manual is definitely needed, and the parts book will come in handy as well. You can get copies from most of the bigger dealers who handle BSA parts (Raber's, etc), or Kim's BSA CD set is quite a good alternative (listed up the top of the BSA board)
Good used A50 / A65 gear clusters turn up on eBay all the time for around $150.
The work isn't hard to do yourself, just a bit fiddly. The other main things to watch out for when pulling it apart are: - it's good to have the right tool for undoing the sleeve nuts on the clutch pressure plate, but you can make one yourself by notching a big screwdriver or using a bit of steel strap the right thickness. - a rattle gun makes it much easier to undo the mainshaft nut. I don't like the workshop manual approach of putting it in gear and holding the back brake on because that can lead to breaking teeth off gears - I think KC missed the gear selector detent plunger. That needs to be undone before removing the gear cluster.
Things to watch out for when you're putting it back together: - early A50 / A65 timing is very fiddly to set. It's probably best to throw the whole points assembly away and fit electronic ignition. Boyer Bransden, Pazon and Tri-Spark are all good. Tri-Spark is the best ime, but not cheap. All work best with 6V coils. You might already have those with an early A50. - the clutch pressure plate adjustment can make a world of difference. Take the time to make sure it lifts evenly. I think you have the 4 spring clutch. They're a little more work to set up than the later 3 spring version, but work quite well once they're done. The clutch springs are probably worth replacing while you have it apart. It will probably take a few tries to get the clutch set up fully to get enough pressure to avoid slipping and not so much that the lever action is too heavy. Adjust the nuts to be flush with the end of the studs to start with.
If you don't already know, the earlier model twins used a mixture of BSF, Whitworth and BSC nuts and bolts, so AF spanners will be the wrong size. You'll need "Whitworth" spanners (probably 3/16" through to 3/8"). Combination (open end / ring) spanners and a set of 3/8" drive sockets should do the job. I think these are available in the US. I know British Spares in NZ have them, but the exchange rate may not be very kind to you at the moment.
Good luck with the project. Do you have any pictures you can show us? Don't be shy to ask more questions.
Thanks for the help, I am sure I'm gonna have heaps of questions. I will go out and by the parts book and manual right away. I have the original manual which came with the bike but it might not be overly helpful. I saw that the tooth was broken because the gearbox was already removed and the guy I bought it from showed me the broken teeth. There are 2 teeth broken on the first actual gear. I had planned on just learning to do the little stuff on the bike and gradually increase that too larger jobs. But maybe I should go after it. When I bought the bike he gave me a few boxes of parts being the transmission, chain, pedal, I think clutch, nuts/bolts, etc. it seemed really daunting. But I would really like to learn. Thanks again for the advice. I spoke to a few places that said I could send them the tranny and they will fix what needs to be fixed and send it back to me. Would any of you recommend this? Or should I try to myself?
You'll probably only need to replace the sleeve gear with the broken teeth and the corresponding layshaft gear. As a rule, if one of a pair of gears is replaced the other should be as well.
It might even be worth looking for new gears instead of chasing a used cluster since you only need 1 pair of gears.
It's a nice looking bike - should be good when you get it sorted.
The disassembly has already been done, so you only need to put Humpty Dumpty together again. That's not hard when you've done if a couple of times, but can be a bit daunting if you're faced with boxes of bits and you don't know if they're all there and where they go.
Having the factory parts book and manual will be essential if you're going to tackle this without local help. The early parts books didn't have very good diagrams. The later books do, and the A50 / A65 gearbox didn't change much over the years. All things considered, Kim's BSA CD will be money well spent. It has the factory workshop manuals and parts books for most years. The later books provide a lot more detail, which will give you a better feel for how it all goes together. The details changed over the years, so the correct year books will give you the specific info for your bike.
Lay out all the parts and post a picture of them. We should be able to help you with what sort of shape they're in and how they all go together. When you get the manuals, try to assemble the parts again in the right position relative to each other outside the bike, and post that as well if you're at all unclear.
Come to think of it, if you could do all of that on the Members Projects forum, it will help the next newcomer who is faced with a gearbox rebuild.