You look at how many threads are showing heat for heat range. For Champion’s I use 2 threads max. If more threads are showing heat I drop a range.
With NGK’s, 3-4 threads is where the heat should be. Autolite’s are 2-3 threads showing heat.
All these plug manufactured use different coatings on the plugs so they all “read” differently. You learn this by testing. And yes, it’s a royal PITA but that’s the only way I know of to learn this stuff.
To check for WOT you need to be able to see the very bottom of the plug where the porcelain meets the shell. You want to see about a .100 wide carbon ring down there. If you are really good at keeping up on your tune up, you can go down to .080 wide but if the weather makes a big move and you don’t correct for it you can start hurting parts. W you can cut the shell off the plug to make it easier to see the carbon ring.
On a street engine the fuel ring can be .120 or so wide and not hurt you power wise.
