That's a "vacuum break" piston bore, and the hole is supposed to have a flat steel plug pressed into the stepped end of the bore. Vacuum against the piston pulls against the automatic choke spring and helps open the choke slightly as the engine starts. If it won't run with the hole plugged, turn the idle speed screw clockwise to raise the airflow past the throttle plates.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!