After fully breaking in the engine and you have ran several qualifiers (real or simulated) it is possible that the clearance can be reduced for more performance! This is something that is done at your risk and only recommended for expert mechanics. We assume no responsibility of damage from improperly adjusted head clearance by the consumer. Ideal head clearance varies with conditions; Nitro, Altitude, Ambient temperature, RPM range, ETC. When checking head clearance it is critical that the measurement is taken at the very out side o.d. of the squish band were it is the closest (the bottom of the chamber is on an angle not parallel with the piston). The following are Tips on "reading" the glow plug.

How to "READ" your glow plug.
OS and Turbo glow plugs go gray sooner [easier] than McCoy plugs; this is not necessarily bad. Actually when a new plug wire just goes slightly gray after a 5 or 10 minute hard run it means your very close to an optimal horsepower tune, but be careful the next step is TOO LEAN! My rule of thumb.

Note: if the wire become distorted or broken but stays shiny and wet, it is a indication the head clearance is to close or the straightaway RPM is too great!

Note: You can only "Read" your plug in a nearly new state (Wire like new and shiny) A gray plug can still operate well. However, after it has totally gone gray performance can start to fall off. To test just put in a new plug and if there is no difference in performance save the gray one or put it back in. If your engine does not feel or run right tray a new plug before making major tune change's.