The following may be "old hat" to a lot of folk, I suspect?  To me it represents a new piece of trivia, in that in 1955 I was all of 9 yrs. young!  All I knew, until I stumbled upon this, was that the "-5" in the spark plug designation had to have some meaning.  However, I had no idea as to what.  That mystery is here revealed —

Spark Plug Cross-Reference Chart from the 1950's

14mm Thread x 3/8" Reach

Heat Range Champion 2-Stroke Suppressor AC Autolite Bosch KLG Lodge
Hottest J-14 N/A N/A 49, 48 ---- W95T1 ---- BB14S
Warmer J-12 J12J N/A 47, 47Com, 48X A11, AR10 W125T1 FT20 BAN,BS14
Warmer J-11 J11J N/A 46-5, 46, 46X
A9 W145T3 ---- BB14S
Medium J-8 J8J XJ8 45Com, 45, VF9,
AR8 AT8 A7 AN7 4S-140 4S-165 W175T4 TFS50 CAN
Cooler J-7 J7J N/A 44, 44Com A5 AN5 AR5 W175T4 ---- ----
Cooler J-6 J6J N/A 43 43Com AT4 AH4 W225T3 FS70 HAN
Colder J-5 N/A N/A ---- ---- ---- ---- ----
Colder ---- J3 N/A 42-5Com ---- ---- ---- ----
Coldest J-2 N/A N/A 42Com A3 AR4 W240T3 ---- ----



Engine                              Normal Service                              Hotter Plug                                       Colder Plug
                                          (Original Equipment)                     (For City-Type Operation)              (For Continuous Heavy-DutyOperation)
1955 Passenger Car—    AC 44-5*                                          AC 46-5                                            AC 43-5 Commercial

*Note that spark plugs recommended for use in 1955 engines are suffixed by a "-5".  This indicates that the insulators on these plugs are of the smooth straight shank type , as contrasted to the ribbed insulator plugs used for service on later engines.

Under no circumstances should 1955 style plugs be used in later year cars, as the spark plug boots would fit loosely and thus increase the possibility of spark plug flash-over.

AC Spark Plug Numbering (Example:  AC 44)

· 1st number (4) denotes THREAD SIZE      · 2nd number (4) denotes HEAT RANGE

AC Spark Plug Identification Table - PREFIX

Identification Code



Series Gap




Chain Saw


Fine Line 14 mm 5/8" Hex




Lawn Mower type




Shielded (5/8-24 thread)


Surface Gap

Prefixes are sometimes combined e.g., VB, CR


Plug Identification Table SUFFIX

ID Code Description                                                                                         ID Code Description
A- Cap Gap                                                                                                      C- Copper Core Center Electrode
C- Colder Version of M44 Plug                                                                         E- Special Design Electrode Extended Tip
F- 1/2" (12.7 mm) reach FF 1/2" (12.7 mm) reach, fully threaded                     G- Pin Gap (cold-running) I Iridium Center Electrode
J- Boot Release Agent, Anti-Seize Compound                                                 K Special Design
L- Long Reach
    7/16" (11.1 mm) reach, (14 mm) thread

    3/4" (19 mm) reach, (18 mm) thread
    3/4" (19 mm) reach, (14 mm) thread (fineline)
LT- Long reach, .715" (18.16 mm), Tapered Seat                                             M- Special Design Electrode
N 3/4" 19 mm reach, 3/8" 9.5 mm thread length                                               R- Resistor (Sport Vehicle Plugs)
S- Extended Tip                                                                                               S 7/8"- (22.3 mm) Moderate Long Reach 23/32" (18.25 mm)
T- Tapered Seat Shell Design                                                                          TS- Tapered Seat with extended tip
XL- Extra Long Reach, 3/4" (19 mm) fully threaded                                         Y- 3-Prong Cloverleaf Electrode
Z- Special Gap (usually denotes wide gap)                                                      5- .050" (1.3 mm) gap
6- .060" (1.5 mm) gap                                                                                       8- .080" (2 mm) gap 100 Iridium
600- Resistor Type 800 Platinum Type                                                              900- Double Platinum Type
Suffixes are combined to form such AC suffix designations as FG, XLS, TS, FFM, TSX, SZ, etc.


Heat Range Reference

AC DELCO - The higher the number of the plug, the hotter the plug.  Example:  AC 46 is hotter than an AC 44, while an AC 43 is cooler.
1955 Cadillac use AC 44-5

AUTOLITE - The larger the last digit in the part number, the hotter the plug is.  Example:  AR3935 is hotter than an AR3932 plug.
1955 Cadillac use 86 or AP 86 (Platinum)

BOSCH - The lower the number, the hotter the plug.
1955 Cadillac use WR10FC+

CHAMPION - Typically in the same series the larger the number the hotter the plug.
Example:  Champion RJ12YC.  A RJ18YC would be a hotter Champion plug.  A RJ8YC would be colder.
R = Size
J = 14 mm
12 = Heat range
Y = Standard Projection
C = Copper
1955 Cadillac use RJ18YC

DENSO - Like Bosch, the lower the number, the hotter the plug.
1955 Cadillac use W14PR-U

NGK - The larger the number after the dash the colder the plug is. Example: R5671A-7 is a colder plug than a R5671A-2.
1955 Cadillac use XRG, gapped at .045

AC 44 Spark Plug Modern Day Replacements

AC R43
Autolite 75 or 85 = AC44S or AC45S (projected tip versions of AC44/45)
Accel 137 or 138 projected tip (equivalent to AC45S/46S)
Champion J14YC, J8C = AC44/45, J11C = AC46 (RJ12YC or RJ14YC for projected tip)
NGK XR4 or XR5 or B4
Denso W14P-U or W9P-U

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Created 4 Jun 2009 - 06:02:41 Hrs. 

© 2009, G. David Yaros.  All rights reserved.