After a bit of research I think that the UNF is a squared off thread and the BSCy is rounded. If that's the case with everything else being equal, a 1/2"x20 BSCy bolt would be tight going into a UNF nut and if it's the other way around, it would probably be an easy fit and possibly weaker. Some of the taps advertised are simply marked 1/2"x20. I phoned one supplier to ask about this and they told me that they supply the UNF tap for both threads.
So, I agree with the above comments, always use the correct tools, and make sure you insist on it being correct.

I've a 1/2" x 20tpi BSCy thread that needs cleaning up. I've a 1/2" x 20tpi UNF tap but somewhere at the back of my mind I think there's a small difference in the thread which affects the strength if the two are mixed. Does anyone have an opinion on this?

I've had a few fins repaired over the years. The last time the welder used 3mm sheet steel to fabricate the missing piece of fin and I think the key is to use a high chrome welding rod. By welding it, the fin will then act as a cooling surface as originally designed. Its obviously a lot easier if the broken fin is at the end of the row.

Hi Guru
There's plenty of choice around but I've just fitted a Yuasa 6N6-3B-1 to mine.

Have a look at Andy Tiernan's bikes. There are photos of a 1939 B21 deluxe with sidestand.

I took the photo from the web.
It's certainly possible to fit one, but it's a bit awkward. There're plenty of after market ones available. You need to know the diameter of the frame tube that it'll clamp onto and the length needed to allow the bike to lean a few degrees. The brake anchor strap is a bit in the way. If the stand angles forward during use it will help prevent the bike rotating around the stand.
If you have a go, let us know how you get on.

Yes he spent a fair bit of time getting things lined up and clamping it absolutely solid. He said that the drills were designed for removing broken taps. Impressive. I didn't know such things existed. It's good to keep learning.

Update: had a go with cobalt drills and they just blunted. Had a go at annealing by heating it up until it was red then cooled over 1 1/2 hours, but still just as hard. My engineer friend said that martensite had probably formed during welding and the way to sort it was to heat it to 800' for three days then cool it very slowly. That wasn't going to happen, so I took it round to his workshop and over an hour he gradually drilled it with some very special carbide drills. So job sorted.

I thought I'd seen one somewhere. Its a bit tricky and has to be quite far back, so don't know how stable it'll be, but it's possible. You might need to braze/weld the clamp to the frame to stop it turning and dropping the bike on the floor.

Yes the 2mm drill was a pilot. The tapping drill is 7mm for a 5/16" BSCy. I'm guessing that the combination of welding material and heat hardening has caused the problem.

I had a stripped thread in a cast iron cylinder head so I've had the hole welded up so that I can drill and tap a new thread. Unfortunately I can't make much of an impression on the weld. I'm using a 2mm cobalt drill bit in a pillar drill. What do I need to drill this really tough steel and then my next problem will be can I cut a thread?
Hoping someone is knowledgeable to point me in the right direction.

I finally got round to making one. I started with a round brass box, piece of brass sheet and some very fine brass gauze. Using the diagram in the parts book plus measurements of my oil tank, this is what I put together.

Many thanks
When I got a new bike/car in the past the first thing I would do was to change the oils, plugs, points and filters etc before I drove them. Unfortunately in recent years I have found that replacement plugs have sometimes failed after a year of little mileage and have resorted to putting the old plug back in. You wouldn't think that there was much to wear out in a spark plug apart from the electrodes after thousands of miles.
I'm gradually working my way through the B21 and of course it's fitted with the wrong plug. It will be renewed.

Has anyone got any suggestions for a modern equivalent spark plug for the old KLG L777 fitted to many BSAs in the 30s?
Would a Champion L87 do the job?
Cheers

Have a look down the gap top centre and see if the gear selector thrust washer is at an angle and jammed on the shaft. If so release it with a screwdriver. Try putting a little pressure on the end of the mainshaft to leave it behind. There should be nothing holding it.
Once it starts to move be careful that the other thrust washers don't end up on the floor; they're probably different thicknesses and need to be fitted on the correct shafts. Also watch out for the little plunger and spring behind the ratchet.