049 Glow plug conversion perfected
The Norvel plug adaptor will work in Cox engines. What Norvel
does is they don't drill the hole all the way through the head. The
"small" original sized hole is in the combustion chamber and a few
thousands of head thickness remains before the hole diameter opens up for the standard plug. The standard plug then screws down into a blind hole and bottoms out. If you look up at the plug through the combustion chamber side of the head, all you see
is a hole the size of the coil diameter.
The loss of rpm by opening up the diameter all the way through for the 1/4-32 standard plug is primarily due to the loss of combustion chamber shape and to a lessor degree, the reduction in compression. The Norvel concept seems to be a great compromise.
I haven't seen the RJL adaptor, only the Norvel.