A two-step reduction in a single bush, taking 3/8 inch NPT down to 1/8 inch NPT. Reach for it when a casting or manifold gives you a generous 3/8 inch port but all you need to install is a small sender, switch or take-off line. Typical uses include oil pressure switches in a large block port, boost and vacuum references off an inlet manifold, and blanking down an unused fuel or oil port to a serviceable size. Bear in mind that the small outlet is a genuine restriction, so keep it for sensing duties rather than main flow paths. Seal the tapered threads with PTFE tape or liquid sealant and tighten gradually.