Tuesday, 23 February 2021

The Clutch: Part 1

A complete clutch came with the project bike and can be seen attached to the gearbox in the “as received” picture below.


My experience of clutches fitted to early AJSs is that they are marginal in their ability to handle the power of the engines. On both the AJS K7 and AJcette bikes that I had built previously I had found that the clutch slipped under power at high engine revs. These clutches were new in every respect and identical copies of the originals but the number of plates fitted (one behind the chainwheel and one in front) is, in my view, insufficient for a powerful OHC engine. Not wishing to make new clutches for a second time for these bikes I solved the problem by increasing the spring pressure and although both clutches will now take full power without slipping the increased pressure results in a heavier clutch.

The original Sturmey Archer clutch fitted to this gearbox has another couple of plates compared to the lightweight singles however it is still a pretty puny clutch and I decided to design and make a new one that would take the power from this V-Twin engine.

As I have mentioned in previous blogs, I often design and make parts “on-the-fly” in the workshop but, here, I needed to at least sketch the bare bones of the complete clutch as there are a few design constraints that need to be accounted for. In particular, I planned to use an off-the-shelf bearing between the clutch centre and the chainwheel rather than the lose rollers of the original and the chainwheel sprocket needs to end up in the correct axial location on the gearbox mainshaft to align with the (not yet made) engine sprocket and the (also not yet made) crankshaft.

Although some details changed slightly during machining, this is essentially the design that was adopted.

I decided to proceed in the following way:

1)    Select a suitable bearing. I choose an NSK 6812ZZ 60mm x 78mm x 10mm which is good quality metal shielded deep groove ball bearing.

 2)    Use a triplex sprocket with an adequate OD between the rows of teeth that will accept later Norton clutch plates. Interestingly, the friction plates on a Norton Dominator clutch from the 1950s are the same as those on a 1930s Sturmey Archer clutch ….Norton didn’t change much after they bought the design rights to Sturmey Archer gearboxes. Machine off 2 rows of teeth, bore the centre and mill slots for the clutch plates. Machine the other end to accept the bearing and make a flat plate cover to protect the clutch innards from road dirt.

 3)    Machine a clutch centre to fit the existing gearbox mainshaft taper, spark erode the keyway, mill slots for the plain clutch plates, thread ¼” BSF for the 6 clutch spring studs and machine the diameter on the inner end to support the bearing.

 4)    Make new longer studs to support the springs and spring holders.

 5)    Bond friction material onto the chainwheel inner surface and on both sides of new friction plates.

The first step was to order the various materials for the main parts, shown below.

The material for the triplex sprocket and the large round piece of steel from which the clutch centre would be made are both EN8 which is a good medium strength steel and quite adequate for these components.

In the end, I didn’t use the upper plates shown in the picture but ordered a set of 3 Surflex plates which are of better quality.

The first step in making the chainwheel/clutch basket is to machine off the hub of the triplex sprocket. I have made a number of clutches using this method in the past and found that the hub is actually welded onto the 3-row sprocket, ie what appears to be a single, large piece of metal is in fact 2 pieces welded together. This is shown clearly in the picture below with the sprocket held in the lathe chuck on one set of teeth and sufficient material has been machined away such that the hub has detached from the sprocket – it literally fell off!

It’s useful to be aware of this to avoid some nasty machining accident!

I have found in the past that the centre hole/hub/sprocket are not usually concentric by quite a significant amount and the only foolproof way of setting this up for machining is to check by using a dial gauge, carefully, on the teeth. It is also much easier if the number of teeth is divisible by 3 as the sprocket can then be mounted in the 3-jaw chuck; the alternative is to use a 4-jaw check. If this is not done, then the sprocket will end up being mounted and machined eccentrically and it will subsequently be impossible to set up the chain tension properly.

After machining off 2 rows of teeth and boring the inside the main body of the clutch is finished

and, after boring the other end for the bearing


 it is set up on the rotary indexing table on the milling machine to machine the slots


The picture below show the finished clutch housing together with the original.


The completed chainwheel, together with the new friction plates, was then sent off to have friction disks bonded onto the surfaces, specifically, a 2mm thick disk on the chainwheel and 2x 1.5mm thick on each friction plate. I have used the company Saftek in Cleckheaton for many years for all friction material and always been very happy with their work. 


Finally, the friction plates and the plain plates were inserted into the new chainwheel to check that all the dimensions had been calculated (and made!) correctly …which they had.


The second part is to make the inner core of the clutch; this is a bit more complicated than the chainwheel.

Friday, 12 February 2021

Carburettors and Inlet Manifolds

I had already acquired a couple of carburettors for this project some time previously and had decided to use early AMAC instruments. These bronze carburettors were in production in the mid 1920s – a few years before the original AJS 1000cc V-Twin was designed but it is not clear from the pictures which carburettors were actually used on the original bike. Whatever they are they have massive float bowls

 The carburettors that I have are an AMAC 15 MDX and an AMAC 15 MDY

According to the period AMAC catalogue and spares list, these were produced in 1928

The only difference between them is that the MDX would have been supplied with a top-feed and the MDY with a bottom-feed float chamber – the carburettor bodies are the same. Here, I have fitted them both with bottom feed float chambers and they have been refurbished and converted to use a needle (by Martyn Bratby) rather than the needleless carburettors that left the factory. They are both 1” choke and 1 1/8” stub fitting.

Inlet manifolds are required for the front and rear cylinders. For the front cylinder, the carburettor needs to be located between the “V” of the timing cases and is a stub fitting onto the cylinder head. The fitting on the rear cylinder head is a 2-stud flange (the KTP cylinder head has a different carburettor fixing to the other Mk 1 OHC engines) and the carburettor positioning needs to avoid a frame bracket. This bracket could be removed if necessary but it would be advantageous to keep it if possible ...it might come in use for something later.

Although I have a decent pipe bender for small pipes and tubes up to 1/2" diameter, I do not have a bender that can accommodate a pipe of 1" ID. Luckily, pre-formed bends in stainless steel are available on ebay at a reasonable price and in a variety of sizes and angles.

 They are sold under the banner


Two 900 bends were ordered and these turned out to be high quality and very well made (in Italy). Stub inlets for the carburettor ends and flanges for the connections to the cylinder heads were machined and, after reducing the angle of the bend a few degrees in both cases, the pieces were silver soldered together to make 2x one-piece manifolds.


A flange was silver soldered onto the front cylinder head stub to provide attachment and, by putting the float chamber on the right side of the carburettor, the front carburettor could be attached.

and it “sits” snuggly between the timing case castings.

By carefully positioning the bend on the rear cylinder it was possible to fit the carburettor without removing the adjacent bracket.

Overall, this turned out to be a fairly elegant solution ….although it does give the bike a bit of a “Mad Max” look… like the original....


Tuesday, 2 February 2021

Fixing the Gearbox in Place

Whilst the engine was now securely fixed in the frame and there is large “hole” in which to insert the gearbox, the gearbox requires securing to the engine plates. Although the gearbox fitted in the “hole” the amount of space available severely limited the movement for adjustment and the curved edge on the front of the plates was therefore flattened to allow longitudinal movement.

The flats on the engine plates have allowed movement to give approximately 2 links of adjustment of a 5/8” pitch chain.

At this stage, the gearbox is resting in position on a couple of blocks of wood and held with G-clamps. The next stage is to machine an aluminium block that would be sandwiched between the engine plates and with through-holes (or, to be more precise, slots) through which would pass one set of gearbox mounting studs.

This was made of 6082 Aluminium alloy – a medium strength aluminium and the highest strength of the 6000 series alloys.

Longer studs (EN24T) were made for the gearbox

and the alloy support block together with the engine plates were drilled and tapped to accept 3x 5/16” high tensile BSW bolts on one side


and a steel clamp with 2x slots for the gearbox studs to pass through and 3x  5/16” BSW  cap screws on the other side.

This entire collection of bits

now provided the means to secure the gearbox to the engine plates and in the correct transverse position for chain alignment and vertical position for the chain to avoid the intermediate rear frame tube.

However there was one final job to make this work. When the gearbox was slid into the fully-forward position it was found that the gear lever (referred to in the Sturmey Archer spares book as the “Rocking Shaft Lever”), located at the bottom of the gearbox, would foul the engine plates when selecting top gear (the fully up position) and a new curved lever would be needed to avoid this problem.

The lever was first sketched and cut out in cardboard


and then a new rocking shaft lever was made in 2 pieces; the curved lever was made using 4mm thick mild steel (leftover scrap from the engine plates) and the cylindrical portion from silver steel. The 2 pieces were fixed together with a square section and then silver soldered. The tip of the rocking shaft lever that engages with the Sliding Gear Fork (also SA terminology) was hardened and tempered.

There is one small outstanding job remaining, namely to make 4 dome nuts rather than the off-the-shelf nuts used to hold the gearbox in place but that can wait for another time.