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Honda Spare Parts Sizes

90452323000	WASHER, SEALING, 5MM	Aluminium, 5.2 x 10.0 mm, 1.0 mm thick.
9390334480	SCREW, TAPPING, 4×16	3.9 x 16 mm, head 9 mm diameter x 2.5 mm height, 1.4 mm pitch.

The 9390334480 screw seems to follow Japanese Industrial Standards (JIS): the thread pitch is rather coarse - 1.4 mm - instead of the more fine 0.7 mm pitch of the M4 ISO metric standard:

The imperial system for self-tapping screws contains the #8 gauge, which has 4.20 mm diameter and 18 threads per inch (1.41 mm pitch), i.e. a screw #8-18 x 5/8” should fit, although the diameter is 0.3 mm larger.