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MS20002C4 – Countersunk Aerospace Washer (1/4" Thread Size)
Available now through the AirParts Aero marketplace.
Overview Section
The MS20002C4 is a high-strength, Countersunk Precision Washer manufactured to Military Standards (MS).1 Specifically designed for use with high-tensile, internal-wrenching bolts (such as the MS20004 series), this washer features a unique countersunk (beveled) internal diameter. This recess is engineered to accommodate the large under-head fillet radius of high-strength fasteners, ensuring the bolt sits flush and the load is distributed evenly across the mating surface.
The "C" in the part number denotes a Corrosion-Resistant Steel (CRES) construction, typically 300-series stainless steel, making it ideal for applications exposed to moisture, exhaust gases, or hydraulic fluids. The "4" indicates compatibility with 1/4-inch diameter fasteners. This washer is a critical component in structural joints where high clamping forces and fatigue resistance are paramount.
Key Features Section
Countersunk Interior Diameter: Features a 90-degree (nominal) internal bevel designed to clear the "radius" or "fillet" found under the heads of high-strength bolts, preventing stress risers.
High-Strength CRES Construction: Fabricated from stainless steel to provide excellent tensile strength and superior protection against environmental and galvanic corrosion.
Load Distribution: Provides a hardened, smooth bearing surface that prevents the bolt head from "marring" or digging into softer airframe materials like aluminum or composites.
Precision Dimensional Control: Manufactured to strict MS20002 tolerances to ensure consistent thickness and parallelism, which is vital for maintaining proper fastener preload.
Fatigue Resistance: By allowing the bolt head to seat perfectly, it ensures that the tension loads are purely axial, significantly extending the fatigue life of the entire joint.
Technical Specifications Section
| Specification | Detail |
| Part Number | MS20002C4 |
| Material | Corrosion Resistant Steel (CRES / Stainless) |
| Fastener Size | 1/4 Inch (0.250") |
| Inside Diameter (ID) | 0.255" – 0.275" (Top of bevel) |
| Outside Diameter (OD) | 0.490" – 0.510" |
| Thickness | 0.057" – 0.067" |
| Countersink Angle | Designed for 90° – 100° bolt fillets |
| Finish | Passivated per AMS2700 |
Aircraft Compatibility Section
The MS20002C4 is a specialized structural fastener frequently found in the heavy-maintenance manuals of airframes in the AirParts Aero inventory:
Engine Pylon Mounts: Used to secure engine mounting brackets to the wing spar where high-tensile internal-wrenching bolts are required.
Landing Gear Trunnions: A common component in the pivot assemblies of Cessna Citation and Beechcraft King Air landing gear.
High-Load Control Surfaces: Found in the hinge assemblies of elevators and rudders for high-performance jets.
Primary Structural Spars: Utilized in "critical joints" where standard AN960/NAS1149 washers do not provide sufficient hardness or fillet clearance.
Technical Applications & Safety
The MS20002C4 is far more than a standard flat washer. Because high-strength bolts (like the MS20004) have a large radius where the head meets the shank to prevent cracking, a standard flat washer would "ride up" on that radius. This would cause the bolt to bend and eventually snap under load. The MS20002C4 provides the necessary "relief" for that radius.
Safety Warning: Never substitute an MS20002 countersunk washer with a standard flat washer. If a bolt head is not seated flat against the washer due to a missing countersink, the joint will lose its preload and the bolt is highly likely to fail due to fatigue or shear stress.
Technical Engineering Insight
The engineering value of the MS20002C4 lies in its Joint Integrity Management. In high-torque applications, the washer acts as a hardened "race" between the rotating bolt head and the static airframe. The CRES material is specifically chosen because it has a high "yield strength," meaning it will not "cup" or deform even when the 1/4" fastener is torqued to its maximum limit.
Furthermore, the Passivation of the MS20002C4 is a chemical process that removes "tramp iron" from the surface, leaving a chromium-rich layer that is highly resistant to oxidation.3 This is critical in areas where the washer might be in contact with aluminum; without passivation, the stainless steel could accelerate the corrosion of the aluminum airframe through a galvanic cell. Choosing the genuine MS-standard washer through AirParts Aero ensures your structural repairs meet the original design safety factors.
Installation & Maintenance Addendum
Orientation: The beveled (countersunk) side must face the bolt head. Installing the washer upside down will cause the joint to fail.
Cleanliness: Ensure the countersink is free of debris. Any dirt trapped between the bolt fillet and the washer bevel will result in incorrect torque readings.
Reuse Policy: Inspect for "galling" or circular scoring. If the bearing surface is rough, replace the washer to ensure accurate preload during re-torquing.
