Will this plug work?
http://www.mcmaster.com/#50925k337/=x62y2n
Thread Size 3/8"-24
Maximum psi @ 72° F 7,500
Additional Specifications UN/UNF Hex-Head Plugs
Use with air, water, hydraulic oil
RoHS Not Compliant
Heavy duty forged or bar stock construction gives these little fittings a lot of might. Made of zinc-plated steel for corrosion resistance. Meet ASTM A108 or A576.
Connections: NPTF (Dryseal), which are compatible with NPT threads, or UN/UNF straight threads.
Plugs are hollow and have a Buna-N O-ring for a tight seal.