Thomas Head Spark Plugs

[Vicky]

What would be the recommended spark plug for a Model B Engine with an original Thomas Head. Stock Bore & Stroke, Counterbalanced, Cam is a Winfield 3/4 grind, 1 1/2 inch Stainless Steel Valves, Two 94 Carbs and a FS Electronic Ignition.

Thanks,

Bob

[mrtexas]

r44xls

[Benson]

A word of caution:

The original Thomas head that I had back in the early 60's used a 3/8 reach plug.

The R44XLS is a 3/4 reach plug.

Sorry I do not remember what the plug was that I used.

I have been told that the Reproduction THOMAS heads use a 3/4 inch reach plug.

I suggest that you remove a plug and "look see" which reach your head has.

[Vicky]

Thanks, I check the head for the correct reach, and start with a medium heat range plug.

Bob

[Jim Brierley]

With Champion plugs, find the correct reach and go for around a 14 heat range. D-14 would be 18mm, etc.

[Cajun]

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If you have the Thomas with the 3/8 reach-check plugs for mid 60's Pontiac and Olds. I don't remember the number.

It is not good to have the threads exposed in the combustion chamber.

[mrtexas]

Originally Posted by Benson

A word of caution:

The original Thomas head that I had back in the early 60's used a 3/8 reach plug.

The R44XLS is a 3/4 reach plug.

Sorry I do not remember what the plug was that I used.

I have been told that the Reproduction THOMAS heads use a 3/4 inch reach plug.

I suggest that you remove a plug and "look see" which reach your head has.

Thanks for the caution. Today I measured how much room I have. My plug hole in the Thomas head is between the valves and has 1 3/8 inch of space for a plug.

[Benson]

Originally Posted by mrtexas

Thanks for the caution. Today I measured how much room I have. My plug hole in the Thomas head is between the valves and has 1 3/8 inch of space for a plug.

1. It is not only the valve to plug clearance we are concerned about.

It is important to use the correct reach plug so that the threads are not exposed inside the combustion chamber of the head, as Cajun said above. Reach can be thought of as the number of threads on the plug. The exposed threads glow red hot and will cause detentation. Bad news in any engine!


2. Also if using oversized valves (standard size valve is 1 1/2 inch as I remember) it is possible on some heads that the valves will hit the side of the head as the valves come up and bend the valves. I never tried the Original Thomas head on an engine with oversized valves.

I have seen 1.7 inch valves hit the side of the chamber on an un-modified Lion Head III on an A engine.

3. NOTE: My original Thomas used 14mm plugs.