In a message dated 04/10/2005 8:02:13 PM Central Daylight Time, canarder@frontiernet.net writes:

<< What if one were to check a plug by installing it in an old rotor housing, and if it came within 1 mm or so from the chamber, add a gasket to that plug to shim it at least 1 mm clear of the apex seals?
Provided you don't have taper seat plugs ... Jim S.
 >>

Murphy never sleeps.

You must constantly strive to develop systems that are intrinsically safe, and don't rely on any activity or thought beyond that required for installation. You and I (of course) would never forget which of those plugs gets the extra spacer. But your helpful brother in law may not notice any difference. And so it is, that if a part can be installed incorrectly, it will eventually be installed incorrectly. Murphy's law. Naval Aviation News, 1962.

There is not enough room in a rotary for an extended tip plug. There is no need to install one, or even a plug that looks like one. There is a possibility that you can get the plug in and torqued, and the engine will run fine until that little metal flake machined off by the ground electrode slips into the chamber and gets run over by an apex seal. Note that the ground electrode is welded to the plug shell and that weld in some cases has some flash sticking out sideways. Look at every plug, every time. Cracked porcelain, weld flash, misaligned ground electrode, wrong resistance. Open.

Medium gap and retracted gap (nothing sticking out past the end of the shell) come in a wide range of heat ranges and most work well in a rotary. Many even made in America. I use NGKs because Mazda gives them to me, but many guys use AC's.
Like $2.50 each. Or Champion. That's American.

Lynn E. Hanover