Tuesday 28 September 2021

The Final Build of the AJS V-Twin: Part 3

There were a few more little jobs to do and checks to be made to complete the engine build.

Although this had already been done at the time of the rebores, the piston-to-cylinder clearance was re-checked – mistakes can happen. The clearance was exactly as the recommendation that came with the pistons at 0.0035”:

the above information courtesy of Lacey Ducati

and the rings gapped to the recommended values with the grinding wheel in the ring-gapping jig.

The engine and gearbox could then be assembled into the frame and the barrels, cylinder heads, camboxes etc added. A few other cycle bits and pieces were also added.

The next step was add the magneto and set up the ignition and cam timing. I always use a timing disc for determining the crank angle as it is by far the most accurate method of doing this with the piston close to TDC. I have a number of timing discs with different hole sizes in the middle that I use for different diameter crankshafts. Rather than buy a timing disc, it is straightforward to generate a simple pie-chart with markings every 2 degrees using a spreadsheet (I use Windows on my PC and therefore have Excel), print it and glue it to a piece of cardboard. With an appropriately sized hole cut with a scalpel in the centre and some masking tape it can then be made a snug fit on the crankshaft. For a reference marker, a piece of locking wire wrapped around a nearby bolt suffices.

The other item in the above picture is an indicator that screws into the spark plug thread and moves up and down with piston position. It is not difficult to make one of these from an old spark plug but I bought one (they are cheap enough!) because it has a graduated scale. I always set TDC position by adjusting the timing disc until I get the same displacement at + and – 45 degrees either side of TDC. This is much more accurate than trying to locate TDC directly.

The first check was to make sure that the front and rear TDC positions were separated by 50 degrees. This might seem like an unnecessary check but, as the cam timing is set up independently for each cylinder it is wise to ensure that the rear cylinder is leading by the expected 50 degrees. It was …and would have been quite a surprise if it wasn’t!.

The magneto points had already been gapped and the timing of the front cylinder was then set up using a thin strip of cigarette paper to determine points-just-opening and making sure it was the correct cylinder on the magneto cam ring. This is done with the ignition lever in the fully advanced position. The Vernier adjuster was set on the magneto sprocket and the rear was checked. The timing on the rear should be 310 degrees later; it was exactly this and I give credit to whoever ground the cam ring on this BTH TT magneto.

So far, so good. And now to set up the cam timing. The way in which Velocette recommend setting up the cam timing (remember, this is a K17/2 cam used on early KTTs) is to set the tappets to 0.012” and then detect when all slack is taken up; that is then defined as the opening (or closing) timing for that particular valve. In this way, the effect of long opening and closing ramps is eliminated. I have set my clearances to 0.015” (the original OHC AJSs had this level of clearance) and used a 0.004” feeler gauge to detect the opening/closing. The cam timing on the front and rear cylinders is set up independently by positioning the crankshaft at the desired crank angle for IVC and then rotating the camshaft backwards until the feeler gauge is gripped. The pin on the Vernier adjuster washer is then dropped into the holes that align on the sprocket and camshaft and the job's done.

Before leaving the task of setting up the cam and ignition timing, I always turn the engine over for a few revolutions and then re-check as the taking-up-of-clearances can give a different result from those originally measured.

The valve and ignition timing were set up as follows:


Front Cylinder

Rear Cylinder

“Book” Value

















Ignition (0 BTDC)




The last job on the engine assembly, for the present, was to make gaskets for the end and top covers on the camboxes


The timing case front cover, oil pumps and oil pipes could now be attached. The oil tank was also sprayed 2k black and added to the bike.


Apart from a few minor jobs, there is not much else that can be done on the final build until the petrol tank, exhaust pipes and handlebars come back from being chrome plated and the cover for the gearchange positive stop has been engraved.


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