[burner31]

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
W18
W14
W10
W85N
W80N

Also the original 3X and W18 which Champion lists as norm. Also according to Champion, the W16Y is hotter than the W18 at low speed but cooler at high speeds over 5500 RPM....(over 5500 RPM? ... maybe Speedy ED J, Pete, George Miller, Dennis Pierce would be interested... maybe even Brent)

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
by Larry Brumfield