Brass type core plug for engine. Originally, engines had brass oil way plugs (2K1345) 1/4" hole fitted.
These cup type plugs replaced them about 1985. 12.7mm cup type plug about 9.6mm deep. Fits 1/2" hole.
All good engine tuners would remove them for cleaning sludge, but these later cup type are much easier to remove and replace.
4 required on Large Bore.
2 required on Small Bore.