Drain plug and washer

The drain plug is the bolt or cap at the lowest point of the engine that lets old oil out. It seals with a soft washer (aluminium or copper) or an O-ring. On many commuter singles, including the K-Light, the large drain cap also holds the oil strainer and its spring.

What it does

The plug holds oil in; the washer crushes slightly to make the seal. Over-tightening stretches the threads in the soft aluminium crankcase, and a washer that has been crushed once does not seal well again. Some plugs carry a magnet that catches steel particles.

Signs it is worn or failing

• Oil leaking from the drain plug.
• A plug that keeps turning and will not get tight: the thread is stripped.
• Rounded hex from the wrong spanner.
• On a magnetic plug, a beard of metal: see Metal particles in the oil.

When to replace it

Replace the washer or O-ring at every oil change. Replace the plug if the hex is rounded or the thread is damaged. A stripped crankcase thread needs a workshop repair insert.

What to match when buying

• Match thread size and pitch (for example M12 x 1.5), length and head size. A plug that is too long can hit parts inside.
• Washers: match inside and outside diameter. Use the material the maker used.
• On a strainer cap, match the O-ring size and the spring.

Fitting notes

• Home job: Replace the drain plug sealing washer, usually during Change the engine oil.
• Hot oil burns: let the engine cool a little and keep your arm out of the stream.
• Start the plug by hand. Tighten to the value in your bike's manual with a torque wrench, not "as tight as possible". MAVCC warns: be sure you have the drain plug and not a gearbox detent plug.

Reference (K-Light 125)