Home » Imperial Hexagon Socket Screw Plug with Collar, Pipe Thread (DIN 908) — Carbon Steel, Zinc CR3

Imperial Hexagon Socket Screw Plug with Collar, Pipe Thread (DIN 908) — Carbon Steel, Zinc CR3

Imperial Hexagon Socket Screw Plug with Collar, Pipe Thread (DIN 908) — Carbon Steel, Zinc CR3

FULLERKREG DIN 908 imperial pipe thread hexagon socket screw plugs with collar are manufactured from 35K carbon steel to Grade 6.8, with a bright Zinc CR3 (trivalent chromium) electroplated surface finish. These plugs feature British Standard Pipe Parallel (BSPP / G thread) — the straight parallel pipe thread widely used in hydraulic and pneumatic systems across the UK, Europe, Asia, and Commonwealth markets.

The integrated collar (flange) provides a wider sealing bearing surface and positive stop for consistent seating depth. Unlike tapered pipe threads (BSPT/R), the BSPP straight thread requires a bonded seal (Dowty washer) or O-ring face seal to achieve a pressure-tight connection. The internal hexagon (Allen key) drive allows installation in recessed or confined port locations.

Product Specifications

Standard DIN 908 (Imperial Pipe Thread — BSPP with Collar)
Thread Type BSPP — British Standard Pipe Parallel (G thread) per ISO 228-1 / BS EN ISO 228
Thread Profile Whitworth 55° thread form, parallel (straight) — does NOT self-seal
Head Type Flat head with collar (flange) and internal hexagon (Allen key) drive
Collar Function Wider sealing face, positive stop, consistent seating depth
Sealing Method Collar face — requires bonded seal (Dowty washer) or O-ring face seal
Material Carbon Steel 35K
Grade 6.8
Surface Finish Zinc CR3 — Bright Zinc Electroplated, Trivalent Chromium Passivation
RoHS / ELV Compliance Yes — Cr3+ trivalent chromium, no hexavalent chromium (Cr6+)
Zinc Layer Thickness Typically 5–8 μm
Salt Spray Resistance ≥ 120 hours to white rust (per ISO 9227)
Size Range G1/8 — G2 (BSPP)
Drive Tool Internal hexagon (Allen key) per ISO 2936

Product Gallery

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Dimensional Data — DIN 908 BSPP Imperial Pipe Thread Socket Plugs with Collar

All dimensions in millimeters (mm) unless noted. Thread sizes in BSP (G) designation. Data per DIN 908 standard.

Thread (G) TPI Thread Ø (mm) s — A/F (mm) e — A/C (mm) k — Head Height (mm) t — Socket Depth (mm) D — Collar Ø (mm)
G 1/8 28 9.728 14 16.2 4 3 16
G 1/4 19 13.157 19 21.9 5 4 22
G 3/8 19 16.662 22 25.4 5 4 27
G 1/2 14 20.955 27 31.2 6 5 34
G 5/8 14 22.911 30 34.6 6 5 36
G 3/4 14 26.441 32 36.9 7 6 41
G 7/8 14 30.201 36 41.6 7 6 46
G 1 11 33.249 41 47.3 8 6 50
G 1 1/8 11 37.897 46 53.1 8 6 55
G 1 1/4 11 41.910 50 57.7 9 8 60
G 1 1/2 11 47.803 55 63.5 10 8 68
G 1 3/4 11 53.746 60 69.3 10 8 75
G 2 11 59.614 65 75.1 12 10 80

* s = across flats (A/F); e = across corners (A/C); k = head height; t = minimum socket depth; D = collar outer diameter; TPI = threads per inch. All dimensions per DIN 908 / ISO 228-1. Zinc CR3 plating (5–8μm) does not affect thread fit.

Allen Key (Hex Key) Size Reference

Thread G1/8 G1/4 G3/8 G1/2 G3/4 G1 G1¼ G1½ G2
Hex Key (mm) 12 17 19 24 30 36 46 50 60

Material & Surface Finish Properties

Property Value
Base Material Carbon Steel 35K (medium carbon steel)
Property Class Grade 6.8
Tensile Strength (Rm) ≥ 600 MPa
0.2% Proof Stress (Rp0.2) ≥ 480 MPa
Hardness 190 – 250 HV
Surface Treatment Zinc electroplating + Trivalent Chromium (CR3) passivation
Zinc Layer Thickness 5 – 8 μm (typical)
Passivation Type Trivalent chromium (Cr3+) — RoHS & ELV compliant
Salt Spray Resistance ≥ 120 hours to white rust / ≥ 240 hours to red rust (ISO 9227)
Appearance Bright silver / light blue iridescent
Temperature Limit Up to +120°C (zinc plating service limit)
RoHS / ELV Compliant — no hexavalent chromium (Cr6+)
Magnetic Yes

BSPP (G Thread) vs BSPT (R Thread) — Which Pipe Thread?

Feature BSPP — G Thread (This Product) BSPT — R Thread
Thread form Parallel (straight) — constant diameter Tapered — diameter reduces along length
Self-sealing No — requires bonded seal or O-ring face seal Yes — taper interference provides seal (+ PTFE tape)
Standard ISO 228-1 / BS EN ISO 228 ISO 7-1 / BS 21
Designation G 1/4, G 1/2, G 1 etc. R 1/4, R 1/2, R 1 etc.
Common markets UK, Europe, Asia, hydraulics & pneumatics UK, Asia, plumbing, general fluid systems
Sealing hardware Bonded seal (Dowty washer) or O-ring face seal PTFE tape or thread sealant
Re-usability Excellent — replace bonded seal each time Good — re-apply PTFE tape on re-use
Interchangeable? No — BSPP and BSPT are NOT interchangeable despite similar thread pitch

Sealing & Installation Notes

  • Thread type: BSPP (G thread) is a parallel thread — it does NOT self-seal through thread interference. A bonded seal (Dowty washer) or O-ring face seal is mandatory for pressure-tight installation.
  • Recommended sealant: Bonded seal (Dowty washer) sized to match thread designation — e.g. G1/2 bonded seal for G1/2 plug. O-ring face seal (ORFS) is an alternative for high-pressure hydraulic applications.
  • Do NOT use PTFE tape: PTFE tape is designed for tapered threads (BSPT/NPT). It is not effective as a primary sealant for BSPP parallel threads.
  • Port compatibility: BSPP (G) plugs fit BSPP-tapped ports only. Do NOT install in BSPT (R thread) tapered ports — the threads will engage but will not seal correctly.
  • Torque: Apply recommended torque per thread size. The collar provides a positive stop — do not over-torque beyond collar contact.
  • Re-use: Replace bonded seal on each re-installation. Inspect zinc surface for damage before re-use in critical applications.
  • Drive tool: Use a correctly sized hex key or hex bit socket. Worn keys risk rounding the socket.

Typical Applications

  • 🔧 Hydraulic systems — sealing unused BSPP ports on manifolds, cylinders, and valve blocks — the dominant pipe thread standard in European and Asian hydraulics
  • 💨 Pneumatic equipment — closing unused G-thread ports on pneumatic valves, actuators, and fittings
  • ⚙️ Machine tools & industrial equipment — coolant and lubrication circuit port closures with BSPP ports
  • 🚗 Automotive & OEM — sealing G-thread ports in gearboxes, power steering, and hydraulic units
  • 🌊 Marine & offshore (non-immersion) — Zinc CR3 provides adequate corrosion protection for above-waterline hydraulic port sealing

Frequently Asked Questions

Q: What is BSPP (G thread) and how does it differ from BSPT?
A: BSPP (British Standard Pipe Parallel) is a straight parallel thread — the thread diameter is constant along its length. BSPT (British Standard Pipe Taper) is a tapered thread that self-seals through interference. BSPP requires a bonded seal or O-ring face seal; BSPT uses PTFE tape or thread sealant. They are NOT interchangeable despite similar thread pitch.

Q: Can I use PTFE tape to seal a BSPP plug?
A: No. PTFE tape is designed for tapered threads (BSPT/NPT) where it fills the taper gap. For BSPP parallel threads, use a bonded seal (Dowty washer) or O-ring face seal — these are the correct sealing methods for G-thread connections.

Q: What is a bonded seal (Dowty washer) and where do I get one?
A: A bonded seal (also called a Dowty seal or bonded washer) consists of a metal ring with a vulcanised rubber O-ring on the inner face. When the plug is tightened, the rubber compresses against the port face to create a pressure-tight seal. They are available in sizes matching each G-thread designation (G1/8 through G2).

Q: Is Zinc CR3 suitable for hydraulic fluid environments?
A: Yes. Zinc CR3 is compatible with most mineral-based hydraulic fluids, water-glycol fluids, and synthetic esters. For aggressive fluids or high-temperature applications, consult your fluid supplier for compatibility data.

Q: What is the purpose of the collar on this plug?
A: The collar provides a wider bearing surface for the bonded seal, acts as a positive mechanical stop for consistent seating depth, and helps centre the plug during installation — improving sealing reliability and assembly consistency.

Q: How do I identify if my port is BSPP or BSPT?
A: Use a thread gauge or measure the thread diameter at multiple points along the thread length. BSPP threads maintain a constant diameter; BSPT threads taper (reduce in diameter) along the thread length. When in doubt, consult the equipment manufacturer’s documentation or contact FULLERKREG for assistance.

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