Originally Posted by rbabos
Yes. It will actually be an air out/in effect following what the pistons direction is.
Correct. If you lightly put your hand, finger or maybe a piece of paper or paper towel over the hole you should see and/or feel a light blowing and suction effect. This effect is more pronounced and noticeable on v-twin motors, the bigger the motor, the more noticeable the effect. Crankcase breathing and oil scavenging being managed correctly is very important especially at high RPM's.