View Full Version : Spark Plug dimensions

06-07-2007, 02:26 PM
I am in the process of changing Champion plugs in my newly-rebuilt 259 from H-12 to RH-18Y; I'm experiencing fouling and radio interference. However, I notice that the 18's will extend into the firing chamber about 1/4" further than the 12's did, due to the projected nose core. I have visions of destroying my piston heads--any cause for concern here?

51 Commander State Sedan
Butler PA

John Kirchhoff
06-07-2007, 04:18 PM
For sure you don't want the threaded portion extending any further into the combustion chamber. The plug will screw in just fine and even though nothing may get in the way of anything important, carbon will build up in the exposed threads and you'll never be able to unscrew it.

As far as the porcelian part sticking out further, that's because the 18 is a hotter plug than the 12.

1956 Hawk
06-07-2007, 04:22 PM
Yes and no. The spark plug is a long way from the piston, so you don't have to worry about that. I am not familiar with the plugs you are using, but you need to get the thread length correct. If the threads extend into the combustion chamber they will get carbon buildup on them. This will make them very hard to remove. Likewise if the threads are too short carbon will buildup in the head. If it is just the ground strap that is longer or shorter I wouldn't worry too much.



06-07-2007, 04:36 PM
H18Y was a factory specified plug for some years of 259s. You are right to be concerned in general, but in this instance you don't have a problem, use them and be happy.


55 Commander Starlight

Dick Steinkamp
06-07-2007, 05:33 PM
In addition, you may want to "read" them after a few miles to make sure they are the correct heat range.


06-07-2007, 10:49 PM
Thanks to all of you for the explanations and sage advice. I will install the RH-18Ys and check them 50 or so miles down the road for glazing.

51 Commander State Sedan
Butler PA