Magnetic Sump Plugs

A magnetic sump plug replaces the oil drain plug with one carrying a magnet, which catches steel particles and holds them at the lowest point of the sump. The plugs are listed by thread, in M12 × 1.25, M14 × 1.5, M16 × 1.5 and M20 × 1.5, all in black.

Every engine sheds fine steel and iron particles from gears, chains, cams and followers, most of all while new parts bed in. The filter traps much of this, but the finest particles pass through and some settle in the sump. A magnet holds that debris out of circulation, where it would otherwise wear the bearings and the oil pump.

The plug also tells you how the engine is doing. A light grey paste on the magnet is normal wear, but flakes, slivers or chips are not, and they call for a closer look. A magnet only catches ferrous metal, so aluminium, bronze and bearing material will not show on it: check the filter and the drained oil as well.

The choice comes down to the thread and its seal:

• Diameter and pitch: both must match the sump. A plug with the right diameter but the wrong pitch will start, then bind and strip the thread, which is easily done in an aluminium sump. Measure the old plug with a thread gauge if in doubt.
• Sealing washer: the plug seals on a washer under its head, so use a new one of the right size and material for your sump.
• Other drains: where a gearbox or differential drain uses one of these threads, a magnetic plug works there too, provided its length suits the casing.

Clean the threads and the seating face, start the plug by hand for several turns, and tighten it to the figure for your sump. At each oil change, wipe the magnet clean and note what it collected, so a change in the pattern stands out. Check for drips after the first run; a plug that keeps loosening points to a damaged thread or the wrong washer.