Incomplete review of all the information in this document can cause errors. Read the entire Service Instruction to make sure you have a complete understanding of the requirements.

This Service Instruction identifies the various short reach (Figure 1) and long reach (Figure 2) spark plugs (Tables 2 through 5) approved for installation on Lycoming engine models as well as guidelines for correct spark plug installation.

Cylinders that use short reach spark plugs have gray paint or no other special identifying color on the cylinder fins between the spark plug hole and the rocker covers (Figure 1). Engines with long reach spark plugs are identified by yellow paint on the cylinder fins between the spark plug hole and the rocker covers where shown in Figure 2.

Figure 3 shows spark plug electrode end types, structure, and part number designations. The term UNEF indicates that the diameter-pitch combination is from the extra fine series of Unified Thread Standard (UTS) thread. The standard designation for a UTS thread is a number indicating the diameter of the thread in inches (e.g. 5/8) followed by the pitch measured in threads per inch (e.g. 24).

Since the same engine models can use either short or long reach spark, do not use the engine model as the only basis to identify the spark plugs to be installed on the engine per Tables 2 through 5.

If an approved spark plug is not available, Lycoming can supply an equivalent substitute spark plug.

Tempest has acquired the Unison AutoLite spark plug line. This Service Instruction identifies the new Tempest spark plug part numbers in Tables 2 through 5.

Spark Plug Installation Guidelines

  1. Spark plug gap must be set at 0.016 to 0.022 in. (0.40 to 0.60 mm).
  2. Always install a spark plug with a new gasket (P/N STD-295).
  3. Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound.
  4. Use installation torque values shown in Table 1.