A drain or inspection plug fitted with a permanent magnet that captures ferrous particles circulating in a gearbox, engine or hydraulic sump. At each oil change mechanics read the debris: a fine gray paste, called fuzz, is normal wear, while flakes, slivers or chips mean something is failing. Electrical chip detector versions in aircraft and helicopter gearboxes trigger a cockpit warning when debris bridges the gap.
Mag plug's got a little fuzz but no chunks, gearbox looks healthy, put it back in.
Photograph the plug every time; a fuzz pile that grows from one change to the next says more than any single look.
Also heard as
- magnetic plug
- chip detector
- magnetic drain plug
Related terms
Glitter
Visible sparkling metal flakes in drained oil or a cut-open filter, a red flag for serious wear.
PQ index
Magnetic measure of total ferrous debris in an oil sample, including large particles that spectrometers miss.
Analytical ferrography
Microscope study of wear particles pulled from oil by a magnet, used to identify how a machine is wearing.
Live zone
Spot in a circulating system where oil is moving and well mixed, the right place to pull a sample.