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.