Page 3 - Combined_141_OCR

Champion Spark Plug Identification System

Reach-Prefix Letters
Example: L cqJ

IMPORTANT! As stated previously, plug reach is the dis­
57 20 tance from the gasket seat to the base of the shell (see A
above). It does not include the distance the ceramic nose or
electrodes extend beyond the shell (see B above). This is an
important point to consider, as some racing engines may
have very high compression ratios and cannot physically
accommodate plugs having exposed electrodes or projected
The prefix letter to the left of the number signifies reach ceramic noses. (Some racing engines can easily fit exposed
and diameter. Reach and diameter are a matter of engine electrodes and even projected nose plugs.)
evolution and cylinder head design. Plug reach is the dis­ Engines that have milled heads, employ pop-up pistons,
tance from the gasket seat to the base of the shell. Not or are re-stroked may limit the plug space. Projected nose
inclusive of this measurement is the distance the electrodes plugs and, in some cases, outside electrode types may have
protruce ... or the firing projects. io be abandoned as room no longer exists for the valves or
Prefix letters are she a Cl < l > Each better defines nom- pistons to operate without striking the plugs.
■= e s Zt - z a~e:er. If doubt exists on plug clearance (especially with pro­
jected nose plugs), it is wise practice to rotate the engine
by hand to check for interference. To insure that “rod
LETTER HEX SIZE DIAMETER stretch” at high RPM does not tap the electrodes, it is
recommended that the plug gaps be opened wider than
D 1/2 7/8 18mm normal when rotating the engine manually,.
In some cases, spark plug clearance can be increased by
F .460 13 16 18 Taper using solid copper gaskets. (Gasket numbers and thickness
are listed on page 25.) If the plug is torqued properly, the
G .700 5 8 10mm heat range of the plug will not be affected.
H 7 16 13/16 14mm
J 3 = 13/16 14mm
K 1 18mm
L 12-.492 13/16 14mm
■' i 3 4 -.680 13 16 14mm
p 11 16 12mm
R 3 4 3 4 12mm In some special racing engines, the spark plug is
“masked.” A masked plug does not enter the combustion
Z 5 8-11 16 10mm chamber but fires thru an orifice smaller than the spark
1 2 plug boss. A retracted gap must be used.
(Masked plug design enables the engine to use larger
NOTE! In some instances, the letter “U” will precede the valves. More coolant can be circulated around the plug.
tetter signifying reach (e^. UJ-6). This denotes an auxiliary The position of the spark plug “cartridge-firing” into the
(booster gap) built into the top stud of the plug. The “U” combustion chamber frequently dictates different engine
gap is used in plug applications to combat severe fouling. timing.)
It is not extensively used in racing. (A booster gap plug can Ignition timing requirement in a racing engine is influ­
be e _ = . :er:”‘ez 2 ' re s:.: of the enced by compression ratio, RPM, plug location, gap style,
plug.) This he’e ventilates the interior booster gap. combustion chamber shape, induction system design, fuel,
and camshaft characteristics.
CUSTOM CYLINDER HEADS POPULAR SPARK PLUG FRACTIONAL & DECIMAL
SIZES CONVERTED TO MILLIMETERS
There are many brands of aluminum heads on the market.
These conversion types usually have larger water jackets INCH MM INCH MM
and spark plug boss depths may vary. They may not conform 3/8 .952 .680 17.27
to plug reach standards. It is recommended that plugs be in­ .392 10.00 11/16 17.46
stalled and checked for fit before heads are installed. A plug 7/16 11.11 .700 17.87
that does not extend the full threaded length of the plug
boss can invite hot spots and decrease compression ratio .445 11.30 .709 18.00
... a plug that extends beyond the length of the plug boss .460 11.68 3/4 19.05
can also invite hot spots and possibly interfere with the .472 12.00 13/16 20.63
valves or pistons. 1/2 12.70 7/8 22.22
In some cases, a change to another plug reach may be .551 14.00 1 25.40
in order ... or, again, solid gaskets can be used to insure .625-5/8 15.87
proper depth fit.
4 5