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#1
13-04-20, 23:21
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R975 sparkplugs
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. sparkplugs1.jpg sparkplugs2.jpg sparkplugs5.jpg sparkplugs6.jpg Last edited by Bob Phillips; 13-04-20 at 23:31. 
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#2
14-04-20, 10:05
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Good stuff, Bob. I've always been interested in plug design and behaviour because it causes me so much grief in our fleet of vehicles!
From what I've read, including some good stuff on the NGK website, one reason for the multiple ground electrodes is to reduce electrode erosion because the spark is shared between 2, 3, or 4 electrodes. (Only one electrode sparks per firing stroke.) I see this design is still used in aviation plugs. I suspect aviation engines have to deal with accelerated erosion because they normally run in the medium to high load range, especially under wartime conditions. These loads require spark voltages in the upper range, maybe 10kv to 20kv, hence more wear. Maybe that's why the switch to single electrode plugs on tank engines as the load is much more variable so average spark voltage would be less, so less erosion. I wonder if they were hotter plugs too, as i believe fouling was a problem due to longer periods idling than these engines were designed for. Malcolm
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#3
15-04-20, 02:34
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Just from a practical standpoint, how do you adjust the gap on a four electrode plug if you don't have the special tooling. I suspect that this cannot be done in the field therefore necessitating plug replacement.
Single electrode would have to be a better proposition for earthbound application and work just fine on millions of engines. David
__________________
Hell no! I'm not that old! Last edited by motto; 15-04-20 at 02:40.
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#4
16-04-20, 04:09
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R975 sparkplugs
Thanks Malcolm and David for your thoughts.
I think maybe you have it- aircraft plugs are too difficult to service in the field? seems to make sense. Much of what I have read suggests radials have a bad habit of fouling plugs and need frequent changes of plugs. So this could be due to a too rich mixture at low speeds or too much oil working its way up to the plugs? I have pulled apart engines where the top rings are all seized- probably due to oil leakage toward the combustion chamber. Too much oil will certainly foul the plugs, an ongoing problem in radial type engines.
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#5
17-04-20, 21:06
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Just a footnote to this discussions on sparkplugs. I was out rooting through one of my trailer, looking in boxes I put away 25 years ago and I found some more, radial/aviation style engine sparkplugs. The first another Champion C63, but with a twist. check out the twin electrodes. It the first one I remember seeing.
Second I found a couple of cartons of plugs for the Light Tank (Locust with Lycoming air cooled aviation engine) which are also Champion plugs. Note they are single electrode and unshielded. Here are a few pictures. plugsa.jpg plugc.jpg plugd.jpg
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#6
17-04-20, 21:26
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If anyone finds spark plugs with electrodes like the above, single or double, the thin piece welded to the threaded portion is platinum. Do not try to change the gap as they will fall off. However if the plugs are scrap it makes it easy to separate the platinum which is well worth doing - check the price !
David
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#7
18-04-20, 13:45
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Quote:
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Charles Fitton Maryhill On., Canada too many carriers too many rovers not enough time. (and now a BSA...) (and now a Triumph TRW...)
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