4-Speed Gearbox for the Velocette Cammy Special

I have so far rebuilt 4 early Velo 3-speed gearboxes – those for KTT 55, KTT 305 and 2 flat-tankers. I had one last gearbox to prepare for all 5 Velo projects and this was my one-and-only 4-speed ‘box. It’s marked as #2 in the picture below.

I bought this ‘box many years ago and it was advertised as being rebuilt and in good condition. Superficially it looks fine and all the gears selected positively, however, I have learned never to take these assurances at face value. So, off with the cover to see how it looked inside.

Pretty good. Bearings had been renewed, teeth and dogs on the gears were good, there was a small repair to one of the lugs on the front but I would consider this gearbox to be in excellent condition and did not require any attention inside.

However, there is always something to do! When the positive stop was fitted it was immediately apparent that the “Outside Striking Lever” (Velocette parts book terminology), part number BK-65/3, that was fitted was wrong.

There are 2 fundamental differences In comparison to the B-65 component on the 3 speed gearboxes: BK-65/3 has a spline fitting rather than a square and it is 1” shorter. The lever that should be fitted is an earlier BK-65 that has the same spline but is longer.

I didn’t have a correct BK-65 lever but, luckily, there is an easy solution: add a 1” extension piece

made from a tough steel, EN24T, threaded internally and that positions the linkage to the positive stop in the correct place.

The only other part that I had to make was the cable adjuster for the clutch, CK-21, that screws into the top

by copying one from one of the other gearboxes. This, I found, had another weird thread of 19 TPI (the other one was the oil filler that I had to make, see here, with 13 TPI) which I cannot find on any thread chart for this diameter. Luckily, my 76 year old Harrison lathe can cut these obscure threads.

On the clutch side of the gearbox, a new thrust bearing and the wire clip for the thrust cup, BK-52, were replaced.

I have found on 2 of the gearboxes that I have rebuilt that the back of the clutch has contacted the wire clip and this had ground a flat on the wire – in one case it had nearly severed it into 2 pieces.

I have not yet decided what chaincase to fit but in the meantime, I have lock-wired the 4 screws that would fix the chaincase to the gearbox.

One of the 7-plate clutches that I rebuilt was fitted with new C6/2 Thrust Pins

and 16 new clutch springs

before putting on the sleeve gear nut with a new C8 washer.

All 5 gearboxes that I need - 3 for the immediate Velo projects and the 2 that I plan to work on subsequently, have now been rebuilt.

There are a couple of minor things to finish off but, essentially, this part of the project is complete. This particular gearbox is now in the bike.

This is the pile of bits that has been replaced (and there are a few more parts already in the bin) from the gearbox rebuilds.

Before moving on to the next part of the “transmissions sub- project” (which is to get 3 working positive stops) I should mention that it is essential to have a few special Velo tools to assemble and adjust the clutches, shown below.

Without these, it is pretty well impossible to set up a Velo clutch. I acquired these many years ago for my Venom and Thruxton ….not much changed from the 1920s to 1971.

I also have a lot of clutch parts left over.

If you want any of these bits then let me know (email address at top right of page) as I have no further use for them. There are some 3-plate clutches in this pile of bits. Ideally collect from my home close to Worthing, West Sussex or I can bring to a Kempton autojumble, Founders Day or the Banbury Run in 2025.

There is also a complete set of standard ratio gears in good condition available that I replaced with a CR set in KTT 305s gearbox.

 

And finally…..

For those of you that are members of the Velocette Owners Club and that read the clubs magazine, Fishtail, you will be aware that there has recently been a series of concise and well-written articles by Brian Agnew about flow and combustion in engines.

I spent 40+ years of my working life engaged in exactly this topic, namely developing, validating by way of experiment and applying Computational Fluid Dynamics software for in-cylinder flow, fuel injection and combustion in the design and development of IC engines. In his most recent article (Fishtail 504) Brian refers to “Barrel Swirl”, more often referred to in the industry as “Tumble” – flow rotation about an axis parallel to the crankshaft. To illustrate this, I poked around in some old directories on my PC and found an animation of an in-cylinder flow calculation for a modern 4-valve engine (well, reasonably modern – I retired 9 years ago) which I have shown below. The surfaces of the intake and exhaust ports with the piston at TDC in the cylinder look like this:

and the flow generated during the intake and compression strokes are shown in the animation below.

The flow structure in earlier designs of 4-valve engines, such as the Rudge, would be completely different. Intake ports in modern engines are designed to achieve separation as the flow enters the cylinder to generate the rotation, seen in the above animation. The rotational speed increases as the air is compressed during the compression stroke (conservation of angular momentum - spinning ice skaters decreasing their moment of inertia by moving their arms inwards) and dissipates into small-scale turbulence as the piston approaches TDC. As Brian mentioned in his article, it is the small-scale turbulence that determines the turbulent burning velocity - the speed that the flame propagates; without these processes engines simply wouldn't work.

Such calculations were carried out routinely for competing designs, more often with fuel injection to assess fuel-air mixing, in the R&D departments of major manufacturers. Whether that is still the case I doubt very much ...the engineers that I used to work with are now either working on electric vehicles or pensioned off.