Here’s a typical Honda CX500 or GL500 piston;
As you might guess from the “L” on the crown, this is a left piston. There’s also a mark to indicate which side is the intake:
The exhaust side has no mark:
There’s a little round spot at the top – a leftover from the casting process:
On the sides we have the ring grooves – three in total – and some oil passageways (the holes):
The three large holes are also easily visible on the inside of the piston:
These receive splashing oil from the underside of the piston and allow it to pass through to the other side. The holes behind the lowest ring – the oil control ring – help blow it out and maintain contact with the cylinder bore. There are also similar holes for the wrist pin or gudgeon pin, one on the lower side:
…and one on the upper side:
To measure the piston you need a micrometer that covers the ~78mm range; a 75-100mm micrometer is a common choice.
