DIN 433 Small OD Flat Washers — A4 Marine Stainless Steel (SUS316), Whitened & Passivated

FULLERKREG DIN 433 A4 flat washers are manufactured from A4 stainless steel (SUS316) with a whitened and passivated surface finish. DIN 433 specifies a small outer diameter washer designed for use with countersunk screws, socket head cap screws in counterbored holes, and confined-space applications where standard DIN 125A washers are too large.

A4 (SUS316) contains 2–3% molybdenum (Mo), giving a PREN of ~24–26 vs ~18–20 for A2 (SUS304) — providing significantly superior resistance to chloride pitting and crevice corrosion. This makes A4 DIN 433 washers the definitive choice for marine, offshore, coastal, and chemical environments where A2 may suffer pitting corrosion.

Product Specifications

Standard	DIN 433 (Small OD flat washer) ≡ ISO 7092
Material	A4 Stainless Steel / SUS316 / 1.4401
Surface Finish	Whitened & Passivated
Size Range	M1.6 – M20
OD Characteristic	Small OD — fits within counterbores and confined spaces
Hardness	≤ 200 HV
Tensile Strength	≥ 515 MPa (A4-70)
PREN	~24–26 (vs ~18–20 for A2 SUS304)
Corrosion Resistance	Excellent — superior chloride pitting & crevice corrosion resistance vs A2
Magnetic	Slightly magnetic after cold working
Related Standard	ISO 7092  |  DIN 125A A4 (standard OD)

Product Gallery

[Photo 1]

[Photo 2]

[Photo 3]

[Photo 4]

Dimensions — DIN 433 / ISO 7092

All dimensions in mm. Dimensions identical for A2 and A4 grades.

Nominal Size	d1 — Inner Ø (mm)	d2 — Outer Ø (mm)	h — Thickness (mm)
M1.6	1.7	3.5	0.3
M2	2.2	4.5	0.3
M2.5	2.7	5.0	0.5
M3	3.2	6.0	0.5
M3.5	3.7	7.0	0.5
M4	4.3	8.0	0.5
M5	5.3	9.0	1.0
M6	6.4	11.0	1.6
M8	8.4	15.0	1.6
M10	10.5	18.0	1.6
M12	13.0	20.0	2.0
M14	15.0	24.0	2.5
M16	17.0	27.0	2.5
M20	21.0	34.0	3.0

* d1 = inner diameter; d2 = outer diameter; h = thickness. All per DIN 433 / ISO 7092.

A4 (SUS316) vs A2 (SUS304) — Why Choose A4?

Property	A4 — SUS316 (This Product)	A2 — SUS304
Molybdenum (Mo)	2.0 – 3.0%	None
PREN	~24–26	~18–20
Chloride pitting resistance	Excellent	Good
Crevice corrosion resistance	Very good	Moderate
Marine / offshore suitability	Yes — preferred grade	Limited in high-chloride
Tensile strength	≥ 515 MPa	≥ 500 MPa
Cost	Higher (Mo content)	Standard
Best for	Marine, coastal, offshore, chemical, food-grade	General outdoor, architectural, mild corrosive

DIN 433 vs DIN 125A — Small OD vs Standard OD

Property	DIN 433 (This Product)	DIN 125A (Standard OD)
OD characteristic	Small — fits within counterbores	Standard — general purpose
M8 outer diameter	15.0 mm	16.0 mm
M10 outer diameter	18.0 mm	20.0 mm
M12 outer diameter	20.0 mm	24.0 mm
Best for	Countersunk screws, counterbores, confined spaces	General fastening, load distribution
ISO equivalent	ISO 7092	ISO 7089

Material Properties — A4 / SUS316

Designation	A4 / SUS316 / 1.4401 / 0Cr17Ni12Mo2
Cr Content	16.0 – 18.0%
Ni Content	10.0 – 14.0%
Mo Content	2.0 – 3.0% — key differentiator vs A2
Tensile Strength	≥ 515 MPa (annealed)
Hardness	≤ 200 HV
PREN	~24–26
Density	7.98 g/cm³
Temperature Range	−50°C to +300°C
Magnetic	Slightly magnetic after cold working

Typical Applications

• 🚢 Marine & offshore fastening — A4 is the mandatory grade for marine countersunk fastening; small OD fits within countersink recesses on boat fittings, deck hardware, and offshore structures
• 🌊 Coastal & salt-air environments — superior chloride resistance for coastal architectural hardware, cladding, and outdoor structures in high-salinity areas
• 🔩 Countersunk screws in marine applications — small OD allows flush seating of countersunk screws in marine timber, GRP, and composite panels
• 🍼 Food & pharmaceutical equipment — A4 SUS316 preferred grade for hygienic applications; small OD for compact fastening in equipment panels and covers
• 🧲 Chemical plant fastening — superior resistance to chloride-containing process environments; small OD for confined-space fastening in chemical equipment
• 🏭 Precision machinery — corrosive environments — small OD for counterbored and confined-space fastening in machinery exposed to corrosive media

FAQ

Q: Why choose A4 over A2 for DIN 433 washers?
A: A4 (SUS316) contains 2–3% molybdenum, giving PREN ~24–26 vs ~18–20 for A2 (SUS304). In practice, A4 resists chloride pitting and crevice corrosion significantly better — making it the preferred choice for marine, coastal, offshore, and chemical environments. Choose A4 when the application involves salt water, salt air, or chloride-containing media.

Q: What is the difference between DIN 433 and DIN 125A?
A: DIN 433 has a smaller outer diameter than DIN 125A for the same bolt size (e.g. M10: DIN 433 d2 = 18 mm vs DIN 125A d2 = 20 mm). DIN 433 is designed for countersunk screws and confined spaces; DIN 125A is the general-purpose standard washer. Both have the same inner diameter for the same bolt size.

Q: What is the ISO equivalent of DIN 433?
A: DIN 433 is equivalent to ISO 7092 (small washer series). DIN 433 is the German national standard; ISO 7092 is the international equivalent.

Q: Are A4 DIN 433 washers suitable for seawater immersion?
A: A4 (SUS316) provides significantly better seawater resistance than A2. For splash zone and above-waterline marine applications, A4 is the correct choice. For continuous seawater immersion, even A4 can suffer crevice corrosion in stagnant conditions — specify Duplex 2205 for fully immersed applications.