Contamination is hazardous and expensive. To protect hydraulic systems from contaminants, use protective closures. Two types of protective metal closures are approved for sealing hydraulic equipment. They are caps and plugs conforming to appropriate military specifications. Guidelines for selection and use of protective closures for hydraulic equipment are as follows: Use caps and plugs of the proper size and material. Never blank-off openings with wooden plugs, paper, rags, tape, or other unauthorized devices. Use closures of metal construction conforming to specifications listed in table 2-2 for sealing hydraulic system equipment, lines, tubes, accessories and components.

Table 2-2.Protective Caps and Plugs

Figure 2-52.Typical blank-off plates.

Figure 2-52 shows typical blank-off plates. In all cases where there is a choice between an internal or external installation, use the external type of closure. Use metal protective closures to seal open ports of all hydraulic lines and accessories. Use metal protective closures to seal new and reusable hydraulic tubing and hose assemblies. Plastic closures may be used to seal electrical fittings and receptacles or other nonfluid openings where contamination is not considered a problem. Keep all protective closures clean, sorted by size, properly identified, and stored in readily accessible bins. Check protective closures visually for cleanliness, thread damage, or sealing deformation before using. 

Rubber, plastic, or unthreaded type of protective closures designed to fit over open ends of bulk hose and tubing should be used in accordance with design function only. Do not use this type of protective closure as a plug for insertion into open lines, hoses, or ports of hydraulic equipment. Remove protective closures before installing equipment. If an opening normally requiring protection is found uncovered, the part or assembly should be cleaned and checked before installation or assembly.