Which of the following is the correct method for checking piston ring end gap?

1. Place the ring into the cylinder bore using a piston to square it, then measure the gap with a feeler gauge
2. Measure the gap of the ring while it is installed on the piston ring grooves
3. Hold the ring ends together and measure the outer diameter with a micrometer
4. Place the ring flat on a surface plate and measure the thickness with a vernier caliper