Atom Minor Mk III
by Jan Huning
6. Rotary Disc Valve
The disc valve was turned from a slice of free-cutting mild steel. The OD, end face and bore were turned, and then the disc parted off a few thou overlength. The parted face was cleaned up by holding the disc on a faced stub of bar with double sided sellotape. This face was the non-contact face. The cutout was cleaned up by milling, clamping the disc onto a piece of bar with a spigot to centre the valve, the piece of bar being held in a small 3-jaw on the rotary table. The hole for the drive pin was drilled at the same setting, making it easy to put it in the correct position relative to the centre hole and cutout. The hole for the drive pin was countersunk on the contact face, and the pin rivitted in place.
Rather than case-hardening the pivot pin for the disc-valve, I turned up a little bronze disc, which is clamped to the backplate by the 6BA pin. The bronze disc is a few thou thicker than the disc-valve to provide axial clearance. (I cannot remember how much thicker, but there is no need for a very small clearance as the oil film will keep the disc against the backplate for starting, and when running, the mean crankcase pressure is above atmospheric, so this will ensure the disc stays in contact with the backplate.) I think the diameter of the stationary bronze bearing was 3/16", with a diametral clearance in the disc valve of around 1 thou.
On final assembly of disc-valve, pin and backplate, a little thread retainer (Locktite 222) was put in the threaded hole before screwing the pin in place. Putting the locktite in the hole before screwing in the pin should prevent the locktite getting anywhere near the disc-valve. Locktite 222 was also used for the locknut on the outside. Screwing the pin in tightly, use of locknut and Locktite thread retainer should ensure the pin stays put. I have had a disc retaining pin unscrew during operation on another engine, a 10cc diesel, hence the belt and braces approach. The head of the pivot pin was broken off by the conrod, and then jammed solid between crankweb and crankcase, stopping the engine very quickly indeed, followed by the 14" propeller spinning gracefully through the air, landing a few feet in front of the engine.