quarter incher

Get yourselves one of these 1/4 inch Whitworth spanners and keep it in your "don't-leave-home-without" tool kit. For one, you need it to undo the banjo bolt on the Amal carburetter.

Rear wheel: quick update.

The gorgeous conical hub and its brake plate have gone off to the machine shop to be sandblasted clean.

Should look pretty good when done.
I took the rim, spokes and nipples to the workshop. The rim cleaned up real nice with a brillo pad and I was pleasantly surprised to find that the chrome plating is in much better shape than I had thought at first. There are only a couple of spots where iron oxide has bitten hard but not enough to warrant having the whole thing re-chromed. There's something else to be said regarding re-chroming rims: very often, if you see a lot of oxidation on the rim, chances are it will have built up inside the rolled edges (where the tire sits) and there is no way to really get in there and get rid of the rust. So you could have a perfectly shiny rim and a bunch of rust on the inside.... would you like that on your bike? Thought not.
The rear wheel will be built up pretty soon. Things have started to happen, it's exciting at this stage because although it's small things that go slowly, you know they'll lead to progress (and it's exciting because we still haven't run into any problems so we're not jaded yet - but give that time). From now on, parts will fit together to make a machine that moves. How cool is that, really?

Score!

So, in addition to the single-carburetter A50, BSA also made a twin-carb version of the 500cc for competition. It was called the Wasp (but there was also the Cyclone - little difference I think) both in scrambler and in road going trim; not many were made compared with the standard version so it is somewhat of a rarity compared to the twin-carb 650s.

Really, the most important component of this version is the cylinder head, which has the twin port for two carburetters. Visually the biggest difference is in the space between the two combustion chambers, about two inches on the 500, whereas there's hardly any meat in between the 650's head.
A few months ago I was browsing around for spares when I came across a spares dealer (NOS and new) that had one up for sale at a very reasonable price and in good nick. I jumped at the chance and now I have the option to transform the engine into a fire-breathing twin-carburetter beast if I should find that the standard head doesn't have enough go.
What the heck, I just like having it sit on my shelf for now!

A bit of background.

Here's what I know about the engine's provenance: it was bought and sold at least a couple of times, the last of which was at one of the regular classic bike autojumbles where blokes go rummaging in big grimy piles of dirty, rusty, sticky, oily metal sitting in the open air, in the baking sun or under the pouring rain.
As I mentioned before, I have no idea why the engine became separated from the rest of the motorcycle in the first place, but judging by the way it seems to have been severed, I'd say it was probably done at a breaker's yard? Or perhaps the bike was in an accident and they salvaged the engine? Maybe they had a 650 engine and swapped it out?
I doubt I'll ever find out, though I might get in touch with the owners club and see if they can trace any history based on the engine numbers. Then again there are only slim chances of that yielding any results seeing as, from what I gather, BSA engine numbers are more of a rough guide than a precise indication.
What I know for sure is that many people have had this engine, but few of them have actually used it. It just sort of got passed around until it reached me - and even then, it's been sitting in a workshop half dismanteled for a good while now.

'sploded view

Behold! And learn all the secrets of the Amal Mk1 Concentric!
How glorious are vintage drawings?
There is only one small piece not shown (or possibly shown within the air slide spring) which is a small brass tube in the shape of an elongated top hat: this fits into the spring, into the air slide.