How To Determine Malleable Pipe Fitting Thread Type and Pipe Fitting...

For malleable cast iron (malleable iron) pipe fittings produced in China, the threads must be 55° sealing pipe threads (R/Rp/Rc, BSPT), conforming to GB/T 7306. Pipe fitting types are categorized by function: elbows, tees, crosses, couplings, reducers, unions, plugs/caps, etc., which can be quickly determined by appearance and markings.

I. How to Determine the Thread Type (Core: 55° Sealing Pipe Thread)

1. First, check the standard (the only mainstream standard in China):

GB/T 3287: National standard for malleable cast iron pipe fittings, stipulating that the threads must conform to GB/T 7306 (55° sealing pipe thread).

GB/T 7306.1: Cylindrical internal thread Rp tapered external thread R (most commonly used).

GB/T 7306.2: Tapered internal thread Rc tapered external thread R.

GB/T 7307: 55° non-sealing G thread (only for non-sealing parts such as union nuts).

2. Quick Visual Identification (3 Steps)

Thread Angle: 55° (measuring the thread crest width with calipers can aid in judgment).

Taper: External thread 1:16 (smaller towards the tip); Internal thread: Rp (straight), Rc (tapered).

Markings: R, Rp, Rc, or BSPT are marked on the end face/side of the fitting.

Specifications are marked as 1/2", 3/4", 1" (inches, non-metric).

3. Differentiate from NPT (American Standard 60°) (to avoid confusion)

NPT: 60°, taper 1:14, sharper thread tip, no rounded transition.

BSPT(R): 55°, taper 1:16, rounded thread tip, the only standard for malleable iron parts in China.

4. Simple tool confirmation

Thread gauge: 55° thread profile gauge 1:16 taper ring gauge/plug gauge, matching indicates R/Rp/Rc.

Calipers formula: Taper = (large end diameter − small end diameter) / length, ≈1:16, then it is BSPT.

II. Quick determination process (for on-site use)

1. Check the material: Black/galvanized, thick end, malleable cast iron (malleable iron) → the thread must be 55° BSPT.

2. Check the thread: 55°, 1:16 1. Taper, inch specifications → R/Rp/Rc (GB/T 7306).

2. Check shape: Elbow / T-junction / Cross / Pipe clamp / Reducer → Classify directly by appearance.

3. Check markings: End face marked GB/T 3287, R, DN15 (1/2") → Confirm standard and specification.