Monday 6 July 2020

Making the Timing Case Castings: The Patterns – Part 1

The camshaft and magneto are both chain-driven and the entire drive system is enclosed in L-shaped inner and outer aluminium chaincases. The inner chaincase is bolted to the timing-side crankcase and provides part of the structure to support the Weller chain tensioner and the outer is screwed to the inner to retain the oil and to provide the surface onto which the external oil pump is affixed. These castings are not particularly complicated; nevertheless there are many small bosses for screws and there needs to be sufficient material in certain places to allow for machining.

So, how to make the patterns? I digress briefly ….. Back in 1975 I lived in a flat in Muswell Hill, London, while I was studying and, as luck would have it, I met a guy that owned an antique shop. I hasten to point out that the antique shop was not in Muswell Hill, for reasons that will become apparent, but rather a less salubrious part of North London. Chatting one day, he showed me a bronze statue of Venus de Milo that he had for sale. This famous ancient Greek statue must have been reproduced thousands of times over the centuries, nevertheless a quality, signed bronze will make a good price.

For some time, I had been playing around making resin castings of various objects – not easy working on the floor of a small London flat, and was having reasonable success. I borrowed the original signed bronze from my buddy and made a 2-part split mould using RTV silicone. The 2 parts of the mould were held together by a multitude of screws that were cast into the silicone and, although it was quite tedious assembling and dissembling the mould for casting resin copies, it worked and worked well.

I then perfected the art of making fake bronze statues. Bronze powder filler was added to the resin and the mould was then part filled and the resin “swilled around” to give the surface a good thick coating. When that had set, more resin was added, this time with a lead powder filler to give the “bronze” added weight and the base of the bronze was then filled with a final dose of bronze powder resin. The final cosmetic touch, after gently eliminating and then polishing any imperfections at the mating line, was to artificially age the bronze with a home-brewed chemical concoction and stick a piece of green felt on the base. 

The result is pretty convincing and quite a few ended up on the shelf of his antique shop! The picture below is the last one that I have.

So, how does that relate to making aluminium castings for a motorcycle? At the time of starting the AJcette project I was restoring the AJS K7 and the various bits and pieces of the K7 engine were scattered around the workshop. What better way to make patterns for the AJcette timing cases than to copy the K7 castings in resin and “cut and shut” them to fit the new engine?

The first step in making the mould is to make a plywood box that surrounds the original part and to insert screws a couple of turns into the wood.

The original part, with various openings blocked off with cardboard and tape, is then suspended with stainless steel locking wire approximately ½” above the base.

RTV silicone is then poured into the mould to the top of the casing.

Finally, 3 pieces of round aluminium bar are lightly glued to the casing that will form sprues for feeding resin to the cavity and vent holes for letting air out. It is also necessary to coat the already-cast silicone on the bottom part of the mould with a release agent (I use Vaseline) because the new liquid silicone will otherwise adhere to the surface and make it impossible to separate the 2 halves of the mould.

After the second pouring of silicone, the upper and lower parts of the silicone mould can be separated by unscrewing the multitude of screws (which have threads cast into the silicone), disassembling the box and removing the original casting. 

The box and 2-part silicone mould can now be reassembled and used for casting resin copies of the original. Obviously this process must be repeated for both the inner and outer timing case components.

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