Spark Plug Service

Get the spark plugs cleaned and tested. Normally part of a 50/100 hour check but can be done at anytime.

The bottom plugs will usually be heavily fouled with lead - especially if the engine is being operated with a full Rich mixture and not leaned. The lead is removed manually with a pick. It is difficult to clean the ceramic insulator nose without an air/abrasive blast. 

We have the correct machine (known as a "bomb tester") (ATS - see top photo) for abrasive blast and air cleaning of massive electrode spark plugs followed by testing of the spark at high voltage (24kV) and under pressure to simulate engine conditions.  The strength and colour of the spark can clearly be seen. Gaps checked and adjusted (3rd and 4th photos).

Resistance Check

We check the resistance between the contact well and the centre electrode. New plugs will have resistances of about 1 or 2 kilohms. Plugs with resistances of 5 kilohms or more should be scrapped. A plug may be producing a spark when tested in the bomb tester but excessively high resistance is undesirable because (a) it affects the strength of the spark and can make starting difficult and (b) it can force the electrical energy to take a lower resistance path which could be by arcing within the magneto casing.

Note also that any spark plug which has been dropped on a hard surface like a concrete hangar floor should be scrapped as there is a definite risk that the insulator ceramic has been fractured. This is another reason why we inspect the plugs under 10X magnification ( necessary to properly see cracks in the central insulator ceramic nose).

Wear/Erosion Check

We also check the electrode diameter for excessive wear i.a.w..spark plug manufacturers recommendations. If either the centre or ground electrodes are worn to less than half their original dimensions, then the plug is at the end of its life. E.g. Champion specify that plugs which pass through a 0.261" diameter hole are worn out. 

Manual cleaning only cannot verify that you actually have a functioning plug!  Plug post cleaning - bottom photo.

We usually fit new gaskets if we are re-installing plugs in an engine post cleaning & testing. Copper anti-seize grease is applied to the threads (except the first 2 or 3 threads). Plugs are tightened with a torque wrench to the value specified in the AMM.

Plug Rotation

We rotate the plugs to equalise differing erosion rates due to different capacitances associated with the difference in length of the ignition leads supplying front versus rear cylinders. The capacitance difference results in difference in spark energy at the plug electrode which means the plugs nearest the magneto are eroded more quickly than those furthest from it. Also the magneto produces positive and negative impulses - half the plugs receiving positive, half negative. If plugs aren't rotated, half of them will be eroded at a much greater rate.

Dessicator/de-hydrator plugs

If your aircraft is sitting in the hangar and not being flown - especially during the winter - we can remove one spark plug from each cylinder and fit dessicator/de-hydrator plugs  (see middle photo) while the engine is not in use. These plugs absorb moisture protecting the cylinders (where most of the corrosion damage is done). The dessicant quite visibly changes colour from blue to pink when it has absorbed water and can be dried out very easily for re-use.

Dehydrator Plugs