Metric Hexagon Socket Screw Plug with Collar MF — DIN 908 — Carbon Steel, Zinc CR3

FULLERKREG DIN 908 MF metric fine 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. Zinc CR3 plating provides significantly superior corrosion resistance compared to black oxide — meeting RoHS and ELV directives by eliminating hexavalent chromium (Cr6+) — making these plugs suitable for general industrial, automotive, and outdoor applications where enhanced corrosion protection is required.

The integrated collar (flange) provides a wider sealing bearing surface and positive stop for consistent seating depth. The metric fine (MF) thread delivers superior vibration resistance and precise torque control. The internal hexagon (Allen key) drive allows installation in recessed or confined port locations.

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Product Specifications

Standard	DIN 908 (MF — Metric Fine Thread with Collar)
Thread Type	Metric fine (MF) straight thread per ISO 261 / DIN 13
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 copper or aluminium sealing washer (DIN 7603) or bonded 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	M10 × 1 — M52 × 2 (fine pitch)
Drive Tool	Internal hexagon (Allen key) per ISO 2936

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Dimensional Data — DIN 908 MF Socket Screw Plugs with Collar

All dimensions in millimeters (mm). MF = Metric Fine thread. Data per DIN 908 standard.

Thread (d)	Pitch (mm)	s — A/F (mm)	e — A/C (mm)	k — Head Height (mm)	t — Socket Depth (mm)	D — Collar Ø (mm)	Sealing Ø (mm)
M10	1.0	17	19.6	4	3	16	10
M12	1.5	19	21.9	5	4	18	12
M14	1.5	22	25.4	5	4	20	14
M16	1.5	24	27.7	6	5	22	16
M18	1.5	27	31.2	6	5	25	18
M20	1.5	30	34.6	7	6	28	20
M22	1.5	32	36.9	7	6	30	22
M24	2.0	36	41.6	8	6	34	24
M26	1.5	36	41.6	8	6	34	26
M27	2.0	41	47.3	8	6	38	27
M30	2.0	46	53.1	9	8	42	30
M33	2.0	50	57.7	10	8	46	33
M36	2.0	55	63.5	10	8	50	36
M38	1.5	55	63.5	10	8	52	38
M42	2.0	65	75.1	12	10	60	42
M45	2.0	70	80.8	12	10	64	45
M48	2.0	75	86.6	14	12	68	48
M52	2.0	80	92.4	14	12	72	52

* s = across flats (A/F); e = across corners (A/C); k = head height; t = minimum socket depth; D = collar outer diameter. All dimensions per DIN 908. Zinc CR3 plating adds ~5–8μm per surface — does not affect thread fit class.

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Allen Key (Hex Key) Size Reference

Thread	M10	M12	M14	M16	M18	M20	M22	M24	M27	M30	M33	M36	M42	M48	M52
Hex Key (mm)	14	17	19	22	24	27	30	32	36	41	46	50	55	65	70

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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 degrades above this)
RoHS / ELV	Compliant — no hexavalent chromium (Cr6+)
Magnetic	Yes

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Zinc CR3 vs Black Oxide — Surface Finish Comparison

Property	Zinc CR3 (This Product)	Black Oxide
Corrosion resistance	Good — ≥120h salt spray to white rust	Mild — requires oil for protection
Coating thickness	5–8 μm zinc layer	1–2 μm (negligible)
RoHS compliance	Yes — Cr3+ trivalent	Yes (black oxide is RoHS compliant)
Appearance	Bright silver / light blue iridescent	Matte black
Temperature limit	+120°C	+150°C
Outdoor use	Suitable with periodic inspection	Not recommended without oil coating
Automotive / OEM	Preferred — meets most OEM zinc plating specs	Less common for external use
Best for	General industrial, automotive, outdoor, humid environments	Indoor, oil-wetted, hydraulic systems

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Sealing & Installation Notes

• Thread type: Metric fine (MF) straight thread — does NOT self-seal. A sealing washer or bonded seal is required.
• Recommended sealant: Copper or aluminium sealing washer (DIN 7603), or bonded seal (Dowty washer) for hydraulic applications.
• Port compatibility: MF fine thread — ensure the mating port is tapped to the same fine pitch. Do NOT install in standard metric coarse-tapped ports.
• Zinc plating & thread fit: Zinc CR3 plating (5–8μm) does not affect standard thread fit class — plugs install normally in MF-tapped ports.
• Torque: Apply recommended torque per thread size. The collar provides a positive stop — do not over-torque beyond collar contact.
• Re-use: Replace sealing washer on each re-installation. Inspect zinc surface for damage or red rust before re-use in critical applications.
• Drive tool: Use a correctly sized hex key or hex bit socket. Worn keys risk rounding the socket.

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Typical Applications

• 🔧 Hydraulic manifolds & valve blocks — sealing unused MF-tapped ports where enhanced corrosion protection vs black oxide is required
• 🚗 Automotive & OEM equipment — zinc CR3 meets most automotive OEM surface finish specifications
• ⚙️ Machine tools & industrial machinery — coolant and lubrication circuit port closures in humid or wash-down environments
• 🏡 Outdoor & exposed installations — where black oxide would corrode without oil protection
• 💨 Pneumatic systems — closing unused ports in pneumatic valve manifolds with fine-thread ports

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Frequently Asked Questions

Q: What is Zinc CR3 and why is it RoHS compliant?
A: Zinc CR3 refers to zinc electroplating with a trivalent chromium (Cr3+) passivation layer. Unlike older hexavalent chromium (Cr6+) yellow zinc plating, CR3 contains no hexavalent chromium — making it fully compliant with RoHS (Restriction of Hazardous Substances) and ELV (End of Life Vehicles) directives.

Q: How does Zinc CR3 compare to black oxide for corrosion resistance?
A: Zinc CR3 provides significantly better corrosion resistance — typically ≥120 hours to white rust in salt spray testing (ISO 9227), compared to black oxide which requires oil to provide any meaningful protection. For outdoor, humid, or wash-down environments, Zinc CR3 is the preferred choice.

Q: Does the zinc plating affect thread fit?
A: The zinc CR3 layer is typically 5–8μm thick — thin enough that it does not affect standard thread fit class. Plugs install normally in MF-tapped ports without modification.

Q: What does MF mean and can I use this in a standard metric port?
A: MF = Metric Fine thread. The pitch is finer than standard metric coarse. MF plugs are NOT interchangeable with standard metric coarse-tapped ports — always verify the port thread pitch before ordering.

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