Model A Spark Plug Heat Range Table

[JayJay]

I remember seeing in the past a table or list showing the relative heat ranges of Model A spark plugs, from coolest to warmest (or the reverse). I thought it was in the Les Andrews Red Book but I couldn't find it when I looked the other day. I'm currently running Motorcraft TT10 in what I think is a 5.2 head and overheating, and I suspect at least some of that may be from a too-hot plug. At least I want to try plug change.

Anyone know the table I'm talking about? Or are the ghosts talking to me again?

Thanks, folks.

 

[Y-Blockhead]

I seem to remember Vince Falter having a heat range chart on Ford Garage. I have been unable to access Ford Barn or the Archives so I can not check.

 

[Fullraceflathead]

Running too hot of a spark plug can cause the spark plug to overheat but it will not cause the engine to overheat.

 

[Gary WA]

I remember seeing in the past a table or list showing the relative heat ranges of Model A spark plugs, from coolest to warmest (or the reverse). I thought it was in the Les Andrews Red Book but I couldn't find it when I looked the other day. I'm currently running Motorcraft TT10 in what I think is a 5.2 head and overheating, and I suspect at least some of that may be from a too-hot plug. At least I want to try plug change.

Anyone know the table I'm talking about? Or are the ghosts talking to me again?

Thanks, folks.

Here's one of many out there The late Larry Brumfield had many.

 

[BButturff]

This is from a post Larry Brumfield made a long time ago, all credit to him.


For those who may be interested, the following 7/8"-18 spark plugs will fit the Model A and were available as of a few years back. They are also not all the ones that I know of.

Champion (hotter down to colder):

W95D
W89D
W16Y
W20 (520)
W18 (518)
W14 (569)
W10
W85N
W80N

Also the original 3X and W18 which Champion lists as standard.

Motorcraft (hotter down to colder):

TT15 (the same as Champion W89D)
TT10 (the same as Champion W14)
TT8 (the same as Champion W14)
TT4 (the same as Champion W10)

Autolite:

3076 (the same as Champion W14 and Motorcraft TT10)

Mopar (hotter down to colder):

P-9-7S
P-9-6S
P-9-5S

Prestolite (hotter down to colder)

78-10
78-8

Stitt (hotter down to colder)

137 BSP
197
147 B
13/10

Range is a key word when one speaks of heat range.

Reading these Champion numbers is not real difficult. For example, W20 or W18, etc.:

The W is the shell design and it always means 7/8-18.

I should also add that Champion has made changes over time. For example a C-16C, used in many a Model A, starts with a C, BUT it is a 7/8-18 plug. It was discontinued about 1979 and replaced by the W16Y. There are many like this.

The number is the heat range and the lower the cooler (unless other letters immediately follow, like Y for example, which depending on the letter or letters can change or affect the heat range):

1 - 25 = Automotive and small engines

25 - 50 = Aviation

51 - 75 = High Performance

75 - 99 = Industrial and Special Applications

 

[Fullraceflathead]

Lots of good info thank you!!!

 

[Mike1291]

I appreciate that the chart has a more correct heat range for the Autolite 3076. Most sources say that the heat range is 4 vs the 18 of a 3X but the two spark plug manufacturers used different ways to measure the advertised heat range.

 

[Diastole]

Here is a chart comparing all the plugs except the Champion 3X

https://drive.google.com/file/d/1Q_nb2Hb7hotndLYPZ-zMAy4b9WnriIUu/view?usp=drivesdk

 

[katy]

Running too hot of a spark plug can cause the spark plug to overheat but it will not cause the engine to overheat.

Ditto

 

[drgoetz]

The spark plug thread on a Model A is really a 22mm metric thread, HOWEVER, a 7/8" x 18 tap works as well. I use a 22mm tap; the 7/8" tap is a little tight.
I clean up the threads with a 22mm and I use 3076 plugs for driving.

 

[Y-Blockhead]

The spark plug thread on a Model A is really a 22mm metric thread, HOWEVER, a 7/8" x 18 tap works as well. I use a 22mm tap; the 7/8" tap is a little tight.
I clean up the threads with a 22mm and I use 3076 plugs for driving.

I don’t use a tap at all, I use a thread chaser to clean the threads if needed.

 

[Brad in Germany]

.
Here is an interesting visual guide to the heat ranges of the various Model A era Champion spark plugs (from a Champion spark plug flier):

Brad in Maryland



.

 

[TomInCologne]

Question: Is Detonation because the spark plugs got too hot just an‘ old wives tale?

 

[Big hammer]

Not , only one of many things that can cause detonation/spark knock, but you already knew this I think ?!

 

[TomInCologne]

i was surprised that people said that a hot spark plug cannot cause the engine to overheat...


Yes, combustion s part of my everyday professional life...