Hello there, photo No1. This type, in the aviation world is know as
"Massive electode" The 3 electrode type are found on radial aero engines such as Pratt & Whitney that power the likes of Grumman Wildcat and Hellcat, Chance Vought Corsair and P47 Thunderbolt. They are better at coping with oil fouling as, if left for any length ot time not being run, the lower pots can fill up with oil and, if not drained can cause hydraulicing. I used to have to do ths job on a Corsair aircraft at Duxford every so often.

email (option): petethejeep@hotmail.co.uk

Those three electrode type plugs were very common as vehicle and motorcycle plugs as well. The centre electrode fires a spark at only one of the three points at any time but the spark has three options of which one to use taking the least line of resistance. Ron

email (option): ronpier@talk21.com

I was going to get a few if they fitted my M20.
I know the KLG 777 was the one mentioned for the M20.
Not sure the other makes around at that time.
They are on ebay for around 35 pound each.
from the green spark plug company.

http://www.gsparkplug.com/1x-klg-spark-plug-l777.html

The list I found on the new zealand website is interesting reading on types fitted to BSA's around 1930

http://www.barnstormers.co.nz/barnstormers/wp-content/uploads/2010/11/Model-Spark-Plug-Guide2.pdf



Paul Crisp

I think Henk has some genuine NOS KLG L777 plugs in their tins. Best email him for a price.

Ron

email (option): ronpier@talk21.com

This shows the evolution of KLGs from three electrodes in the early 1930s, two from the late thirties into the 1950s and then single. These are all the same grade KS5 / M80 18mm plugs specified for the 16H.



I've seen it suggested that the two non-firing electrodes at any one time tend not to burn off deposits and can then foul until they short out on the centre electrode.