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FULLERKREG DIN 908 imperial pipe thread hexagon socket screw plugs with collar are manufactured from 35K carbon steel to Grade 6.8, with a black oxide 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. The black oxide finish provides a clean, low-reflectance appearance and mild corrosion protection when oil-wetted — making these plugs well-suited for hydraulic manifolds, machine tool circuits, and indoor industrial fluid systems. The internal hexagon (Allen key) drive allows installation in recessed or confined port locations.
| 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 | Black Oxide (Blackened) |
| Black Oxide Thickness | 1–2 μm (does not affect thread tolerances) |
| Size Range | G1/8 — G2 (BSPP) |
| Drive Tool | Internal hexagon (Allen key) per ISO 2936 |
| Best For | Indoor, oil-wetted hydraulic systems; hydraulic manifolds; machine tool circuits |
| [Product Photo 1] | [Product Photo 2] | [Product Photo 3] | [Product Photo 4] |
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. Black oxide coating (1–2μm) does not affect thread fit.
| 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 |
| 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 | Black Oxide (alkaline blackening / hot black oxide process) |
| Black Oxide Thickness | 1 – 2 μm (negligible — does not affect thread fit) |
| Corrosion Resistance | Mild — oil-wetted service recommended. Apply hydraulic oil or anti-corrosion grease for protection. |
| Appearance | Matte black — low reflectance, clean industrial finish |
| Temperature Limit | −30°C to +150°C (black oxide service range) |
| RoHS | Compliant |
| Magnetic | Yes |
| Property | Black Oxide (This Product) | Zinc CR3 |
|---|---|---|
| Corrosion resistance | Mild — requires oil for meaningful protection | Good — ≥120h salt spray to white rust |
| Coating thickness | 1–2 μm (negligible) | 5–8 μm zinc layer |
| Appearance | Matte black — low reflectance | Bright silver / light blue iridescent |
| Temperature limit | +150°C | +120°C |
| Oil-wetted service | Excellent — black oxide is oil-retentive | Good |
| Outdoor / humid use | Not recommended without oil coating | Suitable with periodic inspection |
| Hydraulic manifolds (indoor) | ✅ Preferred — oil-wetted, clean appearance | ✅ Suitable |
| Outdoor / wash-down | ❌ Not recommended | ✅ Preferred |
| Higher temperature (>120°C) | ✅ Suitable to +150°C | ❌ Not recommended above +120°C |
Q: What is BSPP (G thread) and does it self-seal?
A: BSPP (British Standard Pipe Parallel) is a straight parallel thread — the thread diameter is constant along its length. Unlike BSPT (tapered pipe thread), BSPP does NOT self-seal through thread interference. A bonded seal (Dowty washer) or O-ring face seal must be used to achieve a pressure-tight connection.
Q: Why choose black oxide over Zinc CR3 for hydraulic applications?
A: Black oxide is the traditional choice for hydraulic manifold plugs used in continuously oil-wetted environments. The oil-retentive surface works synergistically with hydraulic fluid to provide adequate corrosion protection indoors. Black oxide also has a higher temperature limit (+150°C vs +120°C for zinc) and a clean matte black appearance preferred in many hydraulic assemblies. For outdoor or humid environments, Zinc CR3 is the better choice.
Q: Can I use PTFE tape to seal a BSPP plug?
A: No. PTFE tape is designed for tapered threads (BSPT/NPT). 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)?
A: A bonded seal 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: Does black oxide affect thread fit?
A: No. Black oxide is an extremely thin conversion coating (1–2μm) — it does not add measurable thickness to the thread and does not affect standard thread fit class. Plugs install normally in BSPP-tapped ports.
Q: How do I identify if my port is BSPP or BSPT?
A: 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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