The ACDelco Spark Plug Identification Table
Prefix and suffix letters are used to identify a specific type plus the numbers relate to thread size and heat range. For example, the R45TS spark plug shown is:
R = Resistor
4 = 14 mm Thread
5 = Heat Range
T = Taper Seat
S = Extended Tip
Plug Identification Table Prefix Identification Code Description
B Series Gap
C Commercial
CS Chain Saw
F Fine Line 14 mm 5/8" Hex
M Marine
LM Lawn Mower type
R Resistor
S Shielded (5/8-24 thread)
V Surface Gap
Prefixes are sometimes combined e.g., VB, CR
Numbering
1st number denotes THREAD SIZE
2nd number denotes HEAT RANGE
In the early 1990s, an all-numeric, identification code was developed to ensure that the proper spark plug would be selected for replacement in specific engine applications. The numeric code does not in any way correspond to the heat range of the spark plug. Therefore, selection of a spark plug with a different code number is not recommended. The heat ranges may be drastically different between two different plugs, and engine damage or poor performance may result. See your local installer or retailer for details.
The prefix “41” stands for the manufacturer’s product line (e.g., 41 = spark plug)
Numbers after the prefix indicate the type of spark plug and also the specific application:
100 – 199 = Iridium
600 – 699 = Resistor
800 – 999 = Double Platinum
Plug Identification Table Suffix Identification Code Description
A Cap Gap
C Copper Core Center Electrode
C Colder Version of M44 Plub
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.
Here's the code list for AC spark plugs.
