socket gap

[sunrise111]

1. socket gap - ?
2. metal rule - металлическая линейка?
3. The rule must penetrate to the same depth around the whole circumference - Линейка должна проникнуть к той же самой глубине вокруг целой окружности круга?

Check that the gasket is correctly seated by inserting the end of a metal rule through the annular spigot and socket gap until it touches the gasket. The rule must penetrate to the same depth around the whole circumference.