OTHER SPARK PLUG PROBLEMS
The cure for threads that are stripped, crossed, or badly nicked
is replacement of the spark plug with a new one. Dirty threads in the engine may
cause the spark plug to seize before it is seated. Dirty threads also cause poor
contact between the spark plug, spark plug gasket, and the engine seat. This
results in poor heat transfer and will cause excessive overheating of the spark
plug. This condition can be corrected by making sure that threads are clean, and
by observing the torque specifications when installing new plugs.
Connector well flashover is caused by an electrical path along
the surface of the insulator, from contact cap to shield. It occurs when the
voltage required to arc across the electrode gap exceeds the voltage required to
track over the surface of the insulator. This condition is caused by a too-wide
electrode gap, oil, moisture, salt track, or other conductive deposit on the
terminal well surface or lead-in assembly. When flashover occurs, combustion
chamber residues quickly coat the insulator tip and electrodes so that the
condition may be interpreted as oil or gas fouling. If the ceramic of the plug
is not broken, the plug may be cleaned and reused. Thorough cleaning of the lead
in assembly may solve the problem, or it may be necessary to replace the
assembly to affect a cure.
Electrode Gaps
The size of the electrode gap has a very definite effect on
spark plug service life and also on the performance of the engine. Insufficient
gap size will not only cause misfiring during idle, but will also misfire during
cruise power with lean fuel/air mixture. This intermittent misfiring during
cruise lowers the temperature of the insulator tip to such an extent that lead
deposits forming on the insulators may not vaporize sufficiently to keep the
tips clean.
HINTS FOR SPARK PLUG SERVICING
Spark plugs with cracked, broken or loose insulators or highly
worn electrodes should not be cleaned and reused. Replace with new aircraft
spark plugs.
To prevent damage to spark plugs during removal and
installation, use the right tools for the job.
It is helpful to use anti-seize
compound or plain engine oil on spark plug threads starting two full threads
from the electrode, but DO NOT USE a graphite based
compound.
If a spark plug is dropped, discard it and replace with a new
one.
A torque wrench, reading in either foot pounds or inch pounds,
is essential to proper installation of spark plugs.
This is a subset of info at:
http://www.lycoming.textron.com/main.jsp?bodyPage=support/engineOperationTips/index.html
Scott Krueger