Drain plug

1. A drain plug for an enclosure comprising a female-threaded opening, comprising:

a male-threaded portion;

a head portion comprising a drive structure and an annular groove formed on an under-head surface of the head portion and adjacent to the male-threaded portion wherein the groove has an inner boundary with a diameter less than an outer diameter of the male-threaded portion, the annular groove comprising a square cross section having a dimension of about 0.3 mm or more and 2.0 mm or less by about 1.0 mm or more and 10.0 mm or less, and an outer wall enclosing the annular groove;

a washer of a square cross section in the groove, wherein the washer does not comprise an O-ring washer and comprises an elastic material selected from a group consisting of diene rubber, olefin rubber, epichlorohydrin rubber, polysulfide rubber, silicone rubber, urethane rubber, and styrene rubber.

2. The drain plug of claim 1 , wherein the washer protrudes partially from the annular groove from about 0.01 mm or more and about 5 mm or less.

3. The drain plug of claim 1 further comprising a magnet at an end of the male-threaded portion.

4. The drain plug of claim 3 , the magnet comprises a rubber magnet or a rubber coating.

5. The drain plug of claim 3 , the magnet does not protrude from the end of the male-threaded portion.

6. The drain plug of claim 3 , the magnet is in a recess at the end of the male-threaded portion and the outer surface of the magnet is substantially coplanar with the end of the male-threaded portion.

7. The drain plug of claim 1 , the drive structure is selected from a group consisting of a square head, flat head, hexagonal head, polygonal head, Phillips head, torque head, Allen type head, slot, square recess, hexagonal recess, and polygonal recess.

8. The drain plug of claim 1 , wherein a surface of the outer wall is positioned to contact the enclosure when the drain plug is secured to the female-threaded opening.

9. The drain plug of claim 1 , wherein the annular groove and the washer have square cross sections.

10. The drain plug of claim 1 , wherein the square cross-section of the washer is maintained within the annular groove when the washer is compressed.

11. A drain plug for an enclosure comprising a female-threaded opening, comprising:

an annular groove of a square cross section having a dimension of about 0.3 mm or more and 2.0 mm or less by about 1.0 mm or more and 10.0 mm or less in an under-head surface portion of the drain plug and adjacent to a male-threaded portion wherein the groove has an inner boundary with a diameter less than an outer diameter of the male-threaded portion;

an outer wall enclosing the annular groove;

a washer of a square cross section in the annular groove, wherein the washer does not comprise an O-ring washer and comprises an elastic material selected from a group consisting of diene rubber, olefin rubber, epichlorohydrin rubber, polysulfide rubber, silicone rubber, urethane rubber, and styrene rubber; and

an orbital forming an end of a male-threaded portion to hold a magnet at the end of the male-threaded portion.

12. The drain plug of claim 11 , wherein the washer protrudes partially from the annular groove from about 0.01 mm or more and about 5 mm or less.

13. The drain plug of claim 11 , wherein a surface of the outer wall is positioned to contact the enclosure when the drain plug is secured to the female-threaded opening.

14. The drain plug of claim 11 , the drive structure is selected from a group consisting of a square head, flat head, hexagonal head, polygonal head, Phillips head, torque head, Allen type head, slot, square recess, hexagonal recess, and polygonal recess.

15. The drain plug of claim 11 further comprising a magnet at the end of the male-threaded portion.

16. The drain plug of claim 15 , the magnet comprises a rubber magnet or a rubber coating.

17. The drain plug of claim 11 , wherein the square cross-section of the washer is maintained within the annular groove when the washer is compressed.