Well while everyone is at home with lots of time to read and ponder, let me introduce the topic of sparkplugs in radial engines. Because most radials were designed as aircraft engines it is not surprising that in the early days tanks radials used aircraft sparkplugs. Aircraft plugs were shielded, had 18mm diameter
short reach (.5 inch) threads and had multiple electrodes ( usually 4). The premium manufacturer of aviation sparkplugs in the USA was the BG Corporation of New York.
These were not run of the mill disposable spark plugs. They were manufactured in two pieces and a well equipped shop had proper fixtures to adjust both the electrodes and the core. There are a number of BG plugs that were used in the 975 engine( and the 5 & 7 cylinder aircraft versions), 4B2S, 314-GS and 417-S. While the BG manual does not say they were suitable for the W670 Continental, it seems that in practical terms they were. (the finned 314-GS was intended for aircraft, not to build the economy version of a C4 !!)
By wars end the Champion 63-S plug was standard in the 975. It was a single electrode conventional plug but retained the shielded connection. See the photos below.
The 975-46 also used an aircraft type plug (AC) and it also used multiple electrodes. Perhaps this style was retained because they were used in helicopters. Note the longer threads on the -46 plug have been cut back to .5 inches so the threads don't foul with carbon (check out the Champion HO-14S beside it).
So anyone tell me why the change? Are multiple electrodes more prone to fouling and failure? Could it be about the cost of the part? All comments appreciated. There must have been vast quantities of the BG plugs made as they are much easier to find today than the Champion 63S.
