This listing is for an unmachined cast aluminium 3-screw type points cover, to fit the manual advance/retard housing BTH KD1 TT model magneto. The casting is a nice quality (lost wax) casting, with (lathe) machining boss attached, and we supply it with an engineering drawing giving machining dimensions taken from an original cover.
As you can see in the accompanyning photographs - this nice quality alloy casting replicates the outer contours of an original BTH KD1 TT magneto cover almost identically - including the unusual little vertical boss coming out from the centre, which was originally intended to allow fitment of an ignition cutout switch if required. This could still be machined and fitted if required - but is also the main reason why a casting like this is really required to make an accurate reproduction, otherwise it could be machined from billet.
You will also see that the casting includes a machining boss - that allows the outer circumference and rear face of the castingn to be relatively easily machined in the home workshop using a small lathe (and pillar drill or similar for the 3 fixing screw holes).
And finally, to assist - you will see that this listing includes a detailed technical drawing of all the machining dimensions, them being taken from the original BTH KD1 cover shown in the photographs. However - it is the customers responsibility before machining to ensure that the magneto that this cover will be fitted to corresponds with the dimensions on the drawing - this magneto was made for many years, and like all old motorcycle manufacturers - there can be slight variations in dimensions and tolerances over the years. It is worth noting that the protruding ring on the inside of the original cover is intended both to locate the cover on the original advance/retard housing - but also to help stop the points ring sliding out, so you should check that the protuberence is only enough to to stop the ring coming out - but should not be touching the points ring once gasket fitted, so as not to make that difficult to move.
It is also worth noting that original covers had a condensation filter fitted. As can be seen from closeup photos of the original BTH inner cover, this was simply a brass or steel disc, with 4 small holes drilled that were positioned at the top of the cover, so to allow air into the points area, but to capture condensation or damp. On the original cover, the recess this disc was fitted into was recessed around the surrounding area, but I believe this was because BTH manufacturing then allowed a tool to crimp around that disc. In our drawing the recess is not included - as if fitting the disc now, we would retain the disc with industrial Loctite retainer, then just centre punch the alloy around the circumference of the disc.
Finally, you will see the original cover has a hole cast in the small boss area on the inside of the cover. This was to allow that hole to be fully drilled by original customers and fit a cutout mechanism. We have not included that on the drawing, but there is no reason that could not be fitted. It would originally have been for a 2BA or similar screw that is retaining a strip of spring steel approx 1.5" long which is bent so when the cover is fitted to magneto the spring steel touches the centre points plate retaining screw on the magneto. Then where the 2 BA screw protrudes through the cover, a nut would be used to retain the spring, but additional thread - so that a wire with connector could be fitted - running to a cutout switch on the handlebar. The most important thing about fitting this mechanism (if required), was that the 2BA screw and spring must be entirely insulated from the cover itself, otherwise it will just permanently earth the magneto. Normally therefore, that hole was drilled larger than the screw so a fibre or non-conducting material could be fitted to isolate the screw. Email if you wish to fit a cutout mechanism and require more details - some mags used them, others did not.
Final Note - Jan 2021: Depending on popularity - we may be offering this cover fully CNC machined in the future and with condensation disc fitted. Obviously the machined price will be considerably more than shown here, but email on sales@racingvincent.co.uk if interested
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