[rotorwrench]

I've seen the X plugs referred to as 425 and 525 depending on the year of manufacture but the 525 is likely the single piece type but same heat range. There is a lot of unknown history on the Champion Model T plugs. The only way to know for sure is to compare plugs with other known part numbers from back in the day. One thing is for certain, Champion only makes the one OEM style plug design and there is no mention of its properties as far as heat range. This leaves a lot of unanswered questions that I have about them. On a recently overhauled engine that is broken in well with minimal oil consumption, the plugs will likely work just as intended. It's when there is more oil consumption or blow by and the plugs have a tendency to foul that a person would even consider a hotter plug. This can also apply to folks that modify the engine to get more compression or just for more performance overall. They just have to take what's available since there are not too many choices in this day and age.

I've heard folks making statements about the insulators getting loose in the non disassemblable type plugs like the Motorcraft F11 or Autolite 3095 when they are hot. That is brought about by the size of the plug and unequal expansion rates of materials. Watching them wiggle about would be sort of unnerving. The original style Champion design seems to have a check on that.

We pay over a 100 bucks each for fine wire iridium electrode plugs for the helicopters so that makes 36 bucks each seem cheap. We have two plugs per cylinder so 8 sparkplugs for a 4-cylinder can get expensive. Even worse for the 6-cylinder engines that go well over a 1000 bucks for a set.