Twin-Stack; This one has me beat, so any help would be appreciated. The glow plug on the left is obviously quite old, but just look at the reach of the thing ! That's a FOX Long plug alongside for comparison. From the copper washer to the end of the plug, it measures 3/8 inch, and it also has a small & large hole on either side. The deepest plug hole that I've been able to find on an engine, is the Miniature Motors Torp Special, but when I fitted this plug, the piston hit it ! Anybody know what it was designed for ? BOB