I'm running about 50' of 1" CSST natural gas tubing and want the best flow possible before I branch out and step down. I see there are two choices for connections;

1" MIP (3/4" FIP) x 1" CSST
1" FIP x 1" CSST

If I select the first fitting, am I effectively reducing my entire 1" tubing run to 3/4" since it can fit 3/4" MIP threads even if I used the 1" MIP side of the fitting?

Saying it another way, does fitting number two have a better throughput than the first regardless of how I connect the first?

  • So you're converting to iron pipe downstream of CSST?
    – isherwood
    Mar 29, 2022 at 13:00
  • Having a short section of smaller ID pipe between two long sections of larger ID does not change the effective rating of the entire upstream section to the diameter of the short smaller section. The flow resistance of a section of pipe is proportional to the length of the section and the introduction of a short section of slightly smaller ID should be negligible. Mar 29, 2022 at 17:57

1 Answer 1


I think you would need to look at the inside of each fitting to determine this. In gas adapter fittings, it's not unusual to see a 1" MIP fitting also be internally threaded for 3/4" (and 3/4" MIP fittings internally threaded for 1/2"). Obviously such a fittings have slightly smaller internal diameter than a pipe of the same size, due to increased wall thickness required for the internal threaded bore; however, such fittings often have a restriction that's even smaller at the bottom of the bore.

They are not intended to be considered in capacity/flow calculations because they are typically made for use at the end of the line, and line sizing calcs would take them into account. Most gas appliances have regulators/restrictions/orifices that are smaller yet. Your question seems to imply that you need max flow, so one would assume an alternative (non appliance) use which you did not reveal.

I think the solution is to visually inspect the two components, or upsize the entire line so the concern is moot.

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