A drain tap threaded 7/8-14 UNF with a 1/8 outlet, made for the base of an oil catch tank. Catch tanks fill steadily and need emptying, and without a tap the routine is unscrewing the tank, spilling oil down the chassis and refitting it, which is why so many go unemptied until they overflow. Fitting this tap turns the job into opening a valve over a container. The 1/8 outlet is small enough to control the flow and takes a short hose if the tank sits somewhere awkward. Fit it at the lowest point of the tank so it actually drains, and check the seal after the first use.