I found this quote while surfing distributors. It comes from the Accuspark retailer on eBay. I suppose there's a lot of marketing involved in getting a no-name rotor bought over eBay, but it offers some background I suppose.
Accuspark wrote:
Recently manufactured rotor arms have been failing because the typical "mix" used in the injection moulding nowadays contains more carbon blacking and is therefore more conductive. Still more importantly, the rivet which holds the brass inlay into the moulding is slightly longer than the original, bringing it too close to the spring clip on the underside. The high tension current, averaging 30,000 volts, is always looking for the easiest route to earth and shorts out from the tip of the overlength rivet, through the reduced thickness of more conductive plastic and the spring clip on the underside of the rotor arm, to earth out down the distributor shaft. Result - no sparks at the plugs. The situation sometimes rectifies itself on cooling, but then reoccurs with increasing frequency until the rotor permanently short circuits.
Modelled on the original, dependable 1960’s design, but made from modern materials on the latest machinery to very close tolerances.
These LUCAS STYLE RED ROTOR ARMS will not let you down.
The integrally moulded brass inlay dispenses with the troublesome rivet used on most versions which is known to short circuit.
The increased insulation properties of the body makes them suitable for use with high voltage coils & electronic ignition kits.
The spring steel clip on the underside, in conjunction with a revised internal tolerance assures a really snug fit, even on old & slightly reduced diameter mountings.
The moke was so bad I took the whole engine out, head off, clutch off and there it has sat. That'll learn it!