How did this happen?!

I never used to like rigid bikes. I've never had one either, this is (or rather will be) my first.
Some of the guys from my club have hardtail bikes and I have ridden those a couple of times.

Slowly the appeal of a stripped-down, light, nimble bike with a strong vintage flair revealed itself to me and there was no going back.
I toyed with the idea of a 1200 twin-spark Sportster chop for a while (we had an engine in the workshop that eventually went into a tarmac-ripping beast another friend built) but that just didn't happen.

Then another friend from the club decided to sell his BSA café racer and everything that came with it and I was lucky enough to snatch that at the time.

I kept it for a few years, used it as a bar hopper, went on rallies, on long trips, had fun with it and then sold it on to another friend. What's that got to do with this you say?
Well, amongst the things I got when I bought that bike (a small box of spares, the aluminium tank, the shorter seat, etc.) was an A50 engine, which I kept.
It seemed almost natural to plan a hardtail bike with that engine, although at first I had imagined a Rickman-Métisse bike,

or possibly a special built around an ACME trackmaster frame.
That wasn't really going to happen though, for various reasons, so the A50 got put on the back burner.
Then the whole chopper thing resurfaced and with it came thoughts of wild summer rides, sleeping under the stars, going on a big trip with a small bike and with all your gear in a roll-up strapped to a sissybar.
I had the engine. I had the handlebar, the mufflers and a carburetter. This thing was practically building itself!
That was it, time to decide on one thing: it would be a rigid/hardtail framed BSA.
Then came the dozens of variations on every component and trying to decide the overall look I liked best. Since there is more than one type that I really like, that was always going to be difficult and it is possible that this bike's shape will evolve.
I realised that what I liked best in terms of proportions and stance was an original front loop from, say, a Triumph Daytona or a Bonneville, with a straight-to-axle style hardtail bolted on.
The reason for this preference is probably down to a single photograph in a book called "hot rods and pin-ups" by David Perry of an orange 1970 TR6 hardtail.

I can honestly say there was very little I didn't like about that bike and out of the multitude of looks and parts I had seen, this was one of the most coherent and well put together.
I thought I could get a Triumph front frame (and I had even sourced one locally, though it was pretty badly mangled) but there were never any guarantees it would fit.
Not being the most skilled guy out there I wasn't sure what would have gone into modifying a frame and making it so that everything could line up correctly (sprockets and chain for one).
After the local Triumph front loop turned out to be too dodgy I started looking at British-only as a possible source of frame + bolt-on hardtail. Having bought spares from them in the past I'm pretty sure they would have been able to deliver.
But then something else happened that I hadn't thought possible in the first place. There was a frame that was exactly what I was looking for. At just the right time. Uncanny.
Enter Lowbrow Customs. No introductions necessary (unless you've been living in another dimension where there are no cool rides, in which case please go check them out right now: www.lowbrowcustoms.com).
And sure enough in their stock they had a variety of complete frames as well as bolt-on/weld-on hardtails for Triumph and BSA. They've since expanded their range and if what you're after is a tidy little chop with a proper frame, they have what you need.
The best part was, there was a complete frame (all welded, no bolt-on parts) for the A50 engine (or "motor" as they say over there). Score! and you could have it in lots of flavours too: from standard wheelbase up to six inches longer at the back.
From standard height to 2.5in lower. With or without a battery base, with a looped or straight-to-axle rear end. That means something like four dozen frames to choose from just for your particular engine!
So you can imagine that just choosing the specs for the frame took some thinking. In the end the winner was:
4in stretch, with battery base, 1in drop and with straight rails. Phew.

Over to David Bird who actually makes the frames and is clearly very talented at what he does.
Both him and the guys at Lowbrow were incredibly quick in getting everything (it wasn't just the frame I got from them...) packed and shipped over to Rome.
I like the fact that this frame has already travelled more than 5,000 miles... I got some catching up to do!

Battery.

So, yes it's going to have a battery, because although you can run these bikes without one (assuming your alternator, capacitor and wiring in general are in *perfect* shape), I think, why not make life a little easier?
I've salvaged a pretty compact sealed battery off my dad's Yamaha TDM850 (destroyed in an accident thanks to some coked-up whore on her cell-phone), rather than a standard acid type, as they can be bulky and need a bit more maintenance.
I should be able to hook up one of them fancy modern chargers to it pretty easily without having to take it out of the frame or leave any connector leads behind.

These are a good idea, they work very well and I recommend you get one if you don't already have one.
I'll need to cover up the battery somehow though, because it is fugly. Perhaps some sort of battery box that could also house the Boyer Powerbox and any other avionics I want out of sight.
Note the greasy thumb-print in the first photo. Nice.

Score!

The valve cover on my engine is the late finned type found on most A50/A65 bikes. The first ones were smooth and I've always liked the look of those, so when I found this on e-Bay for a decent price I decided to get it:

It's in good shape overall, it just has a slight scratch but I'm sure that'll come out with polishing.

That's right, polishing.

I've already given it a good clean, scrubbed off the old gasket and it's looking good. It will look great when it's polished to a shiny, mirror finish.

A place to call home.

I think it's fair to say that anyone of us who's into the British motorcycle thing would love a place like this to just disappear into and get on with it, without the rest of the world to get in the way.
Get lost in the details, look at every little thing and be there.

Parts: engine.

Parts: clutch.

Parts: kickstart & gear lever.

Parts: handlebar mountings.

Parts: footrests.

Parts: front wheel.

Parts: rear wheel.

Parts: forks.

Parts: manifold.

Parts: cables.

Zdravo!

I have a reader from Croatia, hi!

The best laid plans of mice.

It's not a bad idea to have a build plan and at the same time make peace with the fact that you'll probably have to adapt and change several times along the way.
So far, this is my plan:
first, the rear wheel hub and brake plate get sandblasted, I can then assemble the brake, replace the bearings and spacer in the hub and then hand it all over to Peppe (he already has rim, spokes and nipples at the workshop) so he can actually build the wheel. I'll need to start thinking about getting tire and tube pretty frakking soon.
Next we'll focus on the front end. The fork has to be cleaned and overhauled, the yokes will be marginally cleaned but not overly polished.
At that point I can open up the front brake and check everything is in good condition (and actually, you know, there). We might decide to change the front tire if by this point I've gotten around to getting the new ones.
After the front and rear cycle parts are assembled and put aside we'll finish the engine rebuild.
Once the engine is complete we'll fit the frame over it sideways, then lift it up straight, insert the steering stem with the entire front end (the top yoke will lock it all in place and will already have the handlebar fully assembled) and finally the rear wheel. It's possible that it will be easier to leave the front wheel aside until the fork is fully bolted on the frame.
This will give us a free-rolling bike that can easily be moved in and out of the workshop depending on what needs to be done. One critical component to be added at this point is the side stand!
I'll bolt it down nice and tight for now, but when the frame is eventually painted and the bike is assembled again (after the mock-up phase) I might decide to weld it or tack it in place to make it more secure. We'll see about that.
Anyway, now that the bike moves on its own wheels we can tackle other, trickier components. The area to focus on is where the oil tank will go and then the mudguard. The oil tank is fairly straightforward in that all it needs is an extra mounting brace to be welded to the frame.

The mudguard needs to be positioned correctly in relation to several different points: the rear tip's position, the radius of the tire, the distance from the tire, etc.
It will need a pair of struts to support it at the back (these will be easy to take off and replace with a sissy bar when I need to carry some luggage) and some sort of attachment points at the front, underneath the oil tank area.
After all of that is taken care of - and believe me this will be one of the toughest jobs on the whole build - we can figure out how to mount the footrests and the rear brake pedal, the main caveat being the exhaust pipes. I reckon the footrests are going to be a total pain: I not only need to figure out which ones will fit, but they will also need a lot of welding to find their place on the bike.
Then comes the battery box, which will also house the Powerbox. This will be the start of the electrics/avionics chapter. We need to figure out where and how to mount the coil (ideally a single Dyna-type coil with twin leads) as well as the ignition module for the Boyer (this will probably fit under the fuel tank).
The final layer to be assembled is the top one, namely the fuel tank and the seat. The latter is a relatively simple matter, which involves welding a hinge to the rear of the frame's backbone and, crucially, two bungs (get these from Lowbrow)on the short crossmember between the rear top rails.

These provide an attachment point for the seat springs and must be properly positioned so the springs aren't subject to any lateral stress.
Fitting jobs such as the oil tank and mudguard, fuel tank and seat hinge all need to happen side by side, to make sure there is enough room for everything.
Hmm... let's seeeee..... anything else left to do? bit of petrol, kick it and off you go!

More AMAL wonderfulness.

Highly recommended reading right here folks.
This will answer pretty much any questions you might have regarding the Mk1 Concentric (A.K.A. the best carburetter in the world) as well as provide information that's otherwise impossible to come by - reliably - to do with jet stampings, changes by year, etc.
Detailed and very clear photos, step by step instructions, everything you need.
GO! read it now!

Cold over there...

Man, this winter is really kicking my ass...

I'll bounce back, just you wait.

More A50 literature.

There's something about period brochures that just makes me happy.

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.

Features for 1962.

You really do get the best of everything with a BSA!

the old '626'

I took apart the A50's original carburetter to see what was inside and get an idea of jetting and needle position.
When BSA originally produced the first 500 unit twin, they fitted a 1 inch Monobloc, which was then superseded by the Mk1 Concentric when the Monobloc line was discontinued. Switching from inches to millimeters, the 26mm was the closest to the 1 inch measure.
I'll fit another carburetter altogether but I kept this one for the original air filter and as a source of float-bowl screws (you can never have too many of those, trust me.)

Look at the petrol pipe feed and the throttle cable sticking out at the top: my guess is this was chopped off the bike with a pair of wirecutters when the engine was removed. Now, why the engine was removed from the original bike, we will never know. 



All the bits are there, further proof that this thing was just ripped off the original bike together with the engine - to which it was still attached, air filter and all, when I got it.

But what's this? The float needle is of the modern viton-tipped variety as opposed to the original all plastic kind. This would suggest that at some point, someone opened this up and changed it.

The stampings at the bottom of the slide are "622" and "3 1/2"

The needle is marked with two thin grooves on the head (these are not the ones where the clip goes, Amal needles come in several different types, identified by the markings on the blunt end of the needle) - the clip is in the 1st groove (or 3rd, depending which way you look at it) meaning that with the throttle off, the needle sits high in the jets assembly, allowing more fuel through.
This is one of several things you can tune on the Mk1 and they each make a difference. Do not be fooled by the apparent simplicity of the Concentric, for therein lies the brilliance of this design. You don't need anything more (pointlessly) complex than this, and you can tune them to suit your engine perfectly.

The main jet is a 200 and I don't even want to guess what that freaky residue is inside it. Probably something organic left by insects. I had just ran out of WD-40 so couldn't get the jet holder off to get to the needle jet, which I'm guessing is probably a 104 or a 105.

Speaking of insects, what the frak is that?

Definitely not in the Amal parts list.