A standard glow plug uses a 1/4 32 thread. The Nelson Q500 uses only the larger diameter Nelson plug which comes in a standard or heavy duty variety. This plug uses no washers bet instead has a beveled surface which mates with a matching surface on the head. The plugs are not expensive about the same as most sport plugs but they are not interchangeable. In my post above I may have been a little incorrect concerning Mr. Jetts Q500 that I have not used. There may be avaliable an adapter for his engine allowing the use of a standard plug but not sure of this. Looking forward to meeting you at Speedworld in March.

John