Metallics MSW23M Washers 1/4 x 5/8 Mstr Seal Galv (Box 1000)

The MSW23M master seal washer is sized for 1/4 inch bolts and screws, with a 1/4 inch inside diameter and a 5/8 inch outside diameter. That 5/8 inch OD gives you a bearing surface wide enough to compress the seal around the fastener head without needing an additional flat washer underneath.

A master seal washer is a sealing washer, not a plain flat washer. It goes under the head of a 1/4 inch fastener where the penetration has to be weathertight or kept clean of moisture, and Metallics lists the MSW23M for electrical, plumbing, and heating and air conditioning work. Typical use is fastening strut, clamps, enclosures, or sheet metal where water would otherwise track down the screw.

They are galvanized steel, so they hold up in damp and exterior-exposed work far better than plain steel. Galvanizing is a sacrificial zinc coating, not stainless, so for salt spray, coastal exposure, or continuous wet service a stainless sealing washer is the better call. For ordinary rooftop, outdoor mechanical, and unconditioned-space fastening, galvanized is the normal choice.

The MSW23M ships as a box of 1,000 washers, weighing 3.25 lbs for the full box. That is a bulk pack intended for crews doing repeat 1/4 inch penetrations, not a job-lot handful, so order one box per truck rather than per job.

Yes for the same 1/4 inch bolt size, but understand what changes. A plain flat washer only spreads load; the MSW23M master seal washer also closes off the hole, which is what you want on any penetration through a roof, enclosure wall, or duct. Where you are only distributing clamp load on an interior dry connection, a standard galvanized flat washer does the job and the sealing type is unnecessary.