STZ Industries 1-inch black malleable iron 90-degree elbow with female NPT threads

Black Pipe Fittings: Malleable Iron Sizing, Threading, and When to Use Them

Black pipe fittings get their name from the black oxide finish on unfinished malleable iron, and that finish signals what they're built for: gas and general low-pressure piping, not potable water. Mixing that up - or getting threading and sizing wrong - is where most black pipe jobs go sideways.

Black vs. galvanized: match the coating to what's running through it

Black malleable iron fittings are the standard for gas line installation and repair - the threading and fitting design are built to gas piping standards, and black iron is not rated the same way for potable water. Galvanized malleable iron fittings have a zinc coating that resists corrosion, and they're the choice for water lines and other applications where rust is the concern rather than gas service. Check which one an existing run uses before extending it - black and galvanized fittings look similar enough at a glance to grab the wrong one.

Threading: NPT, and MIP vs. FIP

Malleable iron fittings use standard NPT (National Pipe Taper) threads, sized by nominal pipe size - 1/2 in., 3/4 in., 1 in., and up. MIP (male iron pipe) and FIP (female iron pipe) describe which side of the connection a fitting provides; matching a male fitting to a female counterpart of the same nominal size and thread is what makes the joint. Hand-tighten first, then use a pipe wrench to bring it to roughly 2-3 turns past hand-tight for a properly seated joint - overtightening can crack malleable iron, which is more brittle than it looks.

Common fitting types

  • 90° elbow: changes direction of the run
  • Tee: splits the line into a branch
  • Coupling: joins two straight threaded ends in-line
  • Union: a three-piece fitting that lets a section come apart for service without unthreading the whole run
  • Cap and plug: terminate a line - a cap covers a pipe end, a plug threads into a fitting
  • Floor flange: mounts a threaded pipe to a flat surface
  • Reducing coupling: steps down from one pipe size to a smaller one
  • Nipple: a short threaded pipe segment (as short as a "close nipple") used to connect two fittings directly

Sealant matters as much as the fitting

Gas connections require pipe thread sealant or tape rated specifically for gas service - a general-purpose or water-rated sealant is not a safe substitute on a gas line, even if the joint looks tight. If you're at all unsure about a gas line connection, that's a job worth having a licensed professional verify before it goes into service.

Buying to match your actual usage

Need a size not listed here? Request a quote and we'll get you pricing.

Quick buying checklist

  • Use black malleable iron for gas lines, galvanized for water and general corrosion-prone use
  • Confirm nominal size and MIP/FIP orientation before ordering
  • Hand-tighten, then wrench roughly 2-3 turns past hand-tight - don't overtighten malleable iron
  • Use a union anywhere the joint needs to be serviceable later, not a straight coupling
  • Always use gas-rated thread sealant on gas connections, never a general-purpose sealant
  • Have a licensed professional verify any gas line connection you're unsure about
Back to blog