Spark Plugs - Originals Differ From Replacements
86mitsubishi01SUBJECT:
REPLACEMENT SPARK PLUGS
NO. STB-86-08-008
DATE December, 1986
MODEL All Models
PURPOSE
Technicians have noticed that the replacement spark plugs specified for Mitsubishi engines are not always the same as the original-equipment plugs they replace. Some have questioned the suitability of the replacement plugs because of these differences. This bulletin explains the differences and the reason for them.
FIGURE 1:
DESCRIPTION
Original-equipment spark plugs can differ from replacement plugs in two ways:
1) The gap between the insulator and the barrel is much smaller (Fig. 1)
FIGURE 2:
2) The number on the insulator is different (Fig. 2)
The original-equipment plug is an anti-fouling design. The first few trips a new car makes are short ones
- from the assembly line to the staging area, then on and off the ship, around the preparation area, on and off the haulaway, etc. The conditions are conductive to spark plug fouling. If a conductive film of oil and carbon should foul the plug, the spark can easily travel across the conductive deposit and jump the narrow gap between the insulator and shell, allowing the cylinder to fire normally. Inside the narrow gap, the space between the insulator and shell widens so that under normal operating conditions the spark plug functions like any other in its heat range.
To identify these special plugs, a special nomenclature is printed on the insulator. (Additional information about the codes on the insulator is given in the chart on page 2.) These factory-installed plugs can be left in the engine for the recommended interval, or replaced whenever misfiring begins to occur. Replacement plugs are selected using the chart in the Owners Manual or Service Manual.