Spark plug reach

[John Illinois]

Does the plug sit flush with the hole or does it protrude into the chamber? I am making thread repair Bushings and my old threads are worn so so it is hard to tell.


John

[DanTreace]

The plug will seat into the combustion chamber of the head, but will sit up as not all of the pipe threads on the plug are used, so the plug will stand up a ways above the pipe thread hole. This is a high head and Ford script Champion X. Other plugs may have different reach or heat range.. This view of cutaway the plug is just placed for reference, it won't actually sit this low.

Don't tap the pipe threads too deep if you are going to rethread those in the head.

[DanTreace]

Better views of plug reach, plug is Champion 25.

[John Illinois]

Thanks Dan. I figured some threads should show on top. I am tapping existing threads in a brass bushing to get the right height.It is working out good on my test head. I will mark the tap and get each one the same on my good head.

John

[Steve Jelf]

Even if you restrict the question to Champion X, they come in two different lengths.

Other plugs you can use on a Model T also vary in length.

[George House]

some people are known to carefully wrap aluminum foil around the plug threads a few times. I haven’t.

[Norman Kling]

I only use aluminum foil if the oil or combustion comes through the threaded area. If the threads are not too worn, it will seal it combustion tight.
Norm

[DanTreace]

George

I have used that technique to stop 'weeping' around the plug threads on older cylinders heads with worn down threads. Works fine. Normally use anti-seize on the plug too, when fitting into the cylinder head, saves the plug from sticking or threads from rusting when water sets in the plug hole depression from rain or washing the T.

Cut a piece of alum. foil and wrap around the plug threads, provides a good seal to a compromised plug hole.

[Mark Gregush]

I use ether antiseize or white teflon tape. Both seem to work fine on weepers.