Since the entire distributor housing moves every time the vacuum advance changes timing, it's possible for a poor ground to develop between the housing and the engine block. This can affect how well the points ground the coil primary winding, and cause a loss or reduction of the high-voltage spark. If a wire is added between the distributor housing and the block, the ground isn't subject to resistance changes as the advance moves. The wire must be extremely flexible, as it will be in constant motion. I like to use a coil-to-distributor wire for a mid-60's Ford distributor for a ground wire, which was made of extremely fine strands of high-flex wire. They used to be stocked by every auto parts store, since they were still prone to break up inside the insulation, just not as fast as regular primary wire.
Jerry
