Sunday 27 September 2020

Where to Start….?

Having decided to build an approximate replica of this vintage world speed record attempt beast the first question is …..where to start? The engine dominates the bike and so the obvious place on which to embark on this project would be the V-Twin engine itself. And so the top-down plan was ….build a V-Twin OHC engine that is similar in design and appearance to the original AJS engine, find a chassis to put it in plus an appropriate gearbox and then build the rest of the bike. Clearly, this wasn’t going to be a 2 month project…!

As with the AJcette, the intention was to use Mk1 cammy Velocette cylinder barrels, heads and camboxes, as I had plenty of them, and to graft those onto bespoke crankcases, using chain drive to the camshafts. There are variants of these early OHC engine cylinder barrels; the earliest barrels for the road bikes had the part number K22 and these evolved into barrels with the part number K22/4 which have a noticeably thicker section where the barrel joins the base flange, seen below.



I chose 2 of the latter, thicker barrels. These are distinct from the KTT barrels of the period which have a thicker base flange and part numbers K22/2, K22/3 and K22/5. 

As I mentioned in a previous blog, I do not make drawings and do the detailed design “up-front”. I do make scruffy drawings (more like dimensioned sketches) as I’m going along so that I know what dimensions to machine but I do not, as a rule, know those dimensions until the previous “bits” have been made. However, for the V-Twin, I made an exception and made some general layout drawings before I started. Why? Well because there is otherwise a high probability of getting halfway through the manufacture and build and finding some silliness, such as a screw thread needing to be in the same place as a bearing.

The 3 drawings, which were made at 1:1 scale, are shown below:

The yellow line down the middle of each drawing is where 2 pieces of graph paper are stuck together with tape. Design features of the single-cylinder AJS OHC counterparts are carried over including:

-       2x drive-side main bearings separated by an oil distribution ring

 -      The same gear and sprockets arrangement for the camshaft drive, the only difference being that there are now 3 sprockets in total – the 2 inner sprockets to drive the camshafts and the outer for the magneto

-       Although it can’t be seen in these drawings, the same Weller chain tensioner arrangement would be used.

I planned to use Alpha Bearings to make the entire crankshaft and conrods, as I had with the AJcette, and after a lot of discussion with Max Nightingale, I decided to use side-by-side connecting rods rather than a knife-and-fork arrangement and this can be seen in the drawing in that the cylinders are staggered. Why? Because it is easier to make con-rods for a side-by-side arrangement rather than the more complicated knife-and-fork design. As it turned out, this fairly major design decision was changed during the build and knife-and-fork con-rods and in-line cylinders were used ….but more of that later.

Apart from the design, the project kick-off also signalled the time to start collecting bits and pieces that would be needed. As this is a 500 V-Twin an appropriate magneto would be required and, by luck, a lovely 500 BTH magneto with anti-clockwise rotation turned up on ebay; it wasn’t cheap but was in excellent condition.


And so this was the starting point: some layout drawings, cylinder barrels, heads and camboxes and a magneto. The barrels would need appropriate pistons and the heads and camboxes would need refurbishing ….but it was a start. As not much could proceed without crankcases the first task would be to figure out where these would come from …which was the next part of the project.

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