Casting the distributor cover from Urethane

Please be patient... lots of photos!

While the distributor base can be made from aluminum, the distributor cover must be made from a non-conductive material. The plans caled for a machined distributor cap from a linen phenolic material, which has excelent insulating qualitites, but unfortunately is messy to work with, expensive, and difficult to machine into a shape which is both pleasing and effective.

I was introduced to a casting resin called Alumilite from a friend and fellow machinist, Paul Stelly. Paul is a master moldmaker and does incredible work with fibreglass and composites. Alumilite is a two-part plastic with a viscosity roughly similar to dish soap... pretty thin stuff, and it pours wonderfully. I used alumilite earlier with some foundry work, to replicate a master pattern allowing a pattern board which casts more than one part. This was far more effective (and easier) than machining or carving multiple patterns.

Alumilite also machines cleanly and without abrasive dust. An added benefit was that I could easily replicate many distributor caps from one silicone RTV mold.

The first step in making the mold was to produce a master pattern. For me, the easiest way was to machine one from aluminum. Here, the pattern begins life as a 2" dia. al bar. I have produced the cone which will become the high-voltage distributor connection point. After parting, the pattern is placed on the mill, and 9 distributor points were drilled into the perimeter of the body. Into these...
...I glued 9 turned aluminum stubs which will be drilled to accept the individual spark plug cables and fittings. The base of the cover is a teardrop shape which must match the distributor base to be secured. I produced this from a piece of sheet acrylic, which was then glued to the base of the aluminum pattern. The overall effect looks silly (acrylic glued to aluminum) but this is just a pattern... looks don't count! Accuracy does.
The finished pattern was glued to another piece of sheet acrylic to act as the bottom of the mold. A plastic drinking cup was cut and hot glued to the sheet base to form the walls of the mold. Next to the cup (and pattern) is a 1 lb container of alumilite RTV rubber, and catalyst.
After thoroughly mixing the RTV rubber and catalyst, the rubber is gently poured into the cup containing the pattern. Initially, the flow is directed towards the base away from the pattern. This allows the rubber to gently flow around the pattern and fill crevices with as little air as possible being trapped as bubbles within the mold. Alumilite RTV sets up rather quickly, having a 30 minute working time and full cure in 24 hours.
Success! Unlike sand casting, draft is not required when working with RTV and alumilite... as long as the rubber will flex enough to free the pattern (and the casting), you will be all set.

The mold cavity will replicate PERFECTLY every detail of the master pattern, down to 600 grit polishing marks on the aluminum. It is amazing. If the pattern is wood, you will have a wood-grained pattern. Every pore will be replicated. Therefore, be sure your pattern is as perfect as you can make it.

The alumilite resin is a 2-part product, available in white and brown. Alumilite does sell dyes, though. A small bottle of black dye will last a long time and really does the trick.

The resins are mixed 50-50 by weight. I use a gram postal scale to weigh the resins. It sets FAST, perhaps 2 minutes before the resin is no longer pourable, so after the resins are combined, you can only mix with a popsicle stick or similar for perhaps 20 seconds. The alumilite is then poured into the RTV mold, with the mold filled slightly above level to form a convex surface, which then rapidly hardens.

The finished product! With the RTV mold complete, I can crank out caps at the rate of one every 4 minutes. The cap will still require insertion of machined terminal posts, and final machining of the interior portion of the cap.

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