I've used the RCJ7Y and the little RDJ8J both in the same engine (not at the same time). I didn't see any difference in the way the engine performed(same rpm). Actually there's not much difference in the actual size of the plugs. They are the same height and the ceramic insulator is about the same size. The difference being as the "C" plug has a gasket and the "D" plug is tapered seat with the smaller hex. The heat range is different but doesn't seem to matter that much?

Right now there is a Bosch WSR6F (Says R10 ### on the insulator) plug in my Homelite 30 which normally uses the Champion RCJ7Y but I have run the RDJ8J in it. What ever's in my flight box?

The non resistor plugs are trouble city and a invitation to disaster in a R/C model.

I bought a box of each specification Champion plugs from the local NAPA auto store. They didn't have the "R" type in stock but had them the next day for me.

I've never had any luck getting the right part from those teenage girls at the Pep Boys store. There's too many other auto parts stores to go to without bothering with Pep Boys.

Enjoy,

Jim