As a rule of thumb, MTM Hydro 1/4" fittings are normally found at the end of the pressure washing system on the spray wand or outlet of the spray gun. When purchasing plugs and couplers the Male model has exposed threads while the Female version has threads on the interior of the fitting. See the image for reference.
All MTM Stainless Plugs are National Pipe Thread (NPT) which means they have a tapered Male or Female thread (depending upon the model). NPT is mainly used in the Western Hemisphere and was developed on the concept of interference by the third thread. When installing NPT fittings to another NPT thread we recommend that you use 4-5 wraps of teflon tape wound clockwise, or 2-3 drops of the Loctite thread sealant.
British Standard Pipe Thread (BSP) is found in the European market and has a different thread pitch than NPT. BSP is also considered a 'straight thread' which means it has no taper at the end. When installing NPT on BSP or vice versa, use 6-7 wraps of Teflon or 2-3 drops of Loctite and tighten accordingly. When tightening opposing thread styles (BSP and NPT) you will experience more interference sooner and you will not be able to tighten the fitting as far as like-threads. You will still have a tight seal, just make sure to use the recommended amount of Teflon Tape or Loctite.