MS35335-33 – Lycoming Engines OEM External Tooth Lock Washer
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Overview
The MS35335-33 is a military-specification, external tooth lock washer, specified by Lycoming Engines for use in flight-critical applications on their piston engines. This is not a common hardware store washer; it is a precision-engineered locking device designed to provide positive, anti-rotation security for small fasteners in the extreme, high-vibration environment of an aircraft engine. The MS35335-33 part number designates a washer for a #4 screw, fabricated from heat-treated, cadmium-plated carbon steel.
The locking action of this washer is twofold. First, the helically twisted teeth act as small springs, providing a degree of tension in the bolted joint. More importantly, the hardened, sharp edges of the external teeth are designed to bite into the bearing surface of both the screw head and the joint material. This creates a powerful mechanical interference that provides the maximum possible torsional resistance to loosening. For securing small components, electrical connections, and accessories on a Lycoming engine, the use of this certified MS35335-33 washer is a non-negotiable requirement for an airworthy assembly.
Key Features
Mil-Spec Certified: Manufactured and tested in strict accordance with MS35335 (supersedes AN936), guaranteeing material, dimensional, and hardness specifications.
OEM Specified by Lycoming: A fully traceable part, certified by the engine manufacturer for use in their powerplants.
External Tooth Design: The external teeth are positioned at the maximum possible radius, providing a mechanical advantage that delivers the highest torsional resistance to loosening compared to other washer types.
Heat-Treated Spring Steel: Fabricated from high-carbon steel and then hardened and tempered to create a tough, spring-like quality, ensuring the teeth can effectively bite into the mating surfaces without flattening.
Cadmium Plated: Coated with a Type II cadmium plating for superior corrosion resistance in the harsh engine compartment environment.
Technical Specifications
Part Number / Specification: MS35335-33
Component Type: Washer, Lock, Tooth, External
Screw Size (-33): For use with a #4 size screw.
Material: Carbon Steel, Heat-Treated.
Finish: Cadmium Plated, Type II (Yellow Chromate) per QQ-P-416.
Governing Specification: MS35335 / NASM35335.
Aircraft Compatibility
Lycoming Engines: This is a standard hardware component used across the entire range of Lycoming piston engines (e.g., O-235, O-320, IO-360, IO-540 series) where a #4 external tooth lock washer is specified in the parts manual.
Universal Aerospace Application: Used throughout the aerospace industry for securing small fasteners in high-vibration areas.
Note: The correct washer for any application must be verified against the specific engine or airframe Illustrated Parts Catalog (IPC).
Applications
Engine Accessories: Securing small covers, brackets, clamps, and control cable attachments.
Electrical Grounding: The sharp teeth are highly effective at cutting through surface primers and oxide layers to create a positive, low-resistance path for grounding wires.
Instrument & Panel Fastening: Providing vibration resistance for small screws used to mount instruments and electrical components.
Why Choose MS35335-33?
In the severe vibratory environment of a piston engine, a fastener that is not positively locked will fail. The MS35335-33 is an engineered locking device. Using a non-certified lock washer from a hardware store is a critical safety risk. Such parts are typically made from soft, un-heat-treated steel; their teeth will flatten instantly under torque, providing zero mechanical locking action and leaving the fastener free to vibrate loose. This can lead to the failure of an electrical ground, the loss of an access panel, or the detachment of a critical component. The genuine, Lycoming-specified MS35335-33 is guaranteed to be properly hardened to perform its safety-critical locking function.
The engineering principle that makes the MS35335-33 external tooth lock washer highly effective is its generation of high torsional resistance through mechanical interference. The effectiveness of a toothed washer is a function of the resistive torque it creates against loosening. This torque is a product of the frictional force and the radius at which that force acts (Torque = Force × Radius). The external teeth of the MS35335 design are located at the maximum possible radius for a given screw size. This larger radius provides a significant mechanical advantage, allowing the biting force of the teeth to generate a higher resistive torque compared to an internal tooth washer of the same size. This superior torsional resistance, combined with the spring tension from the helically twisted teeth, makes the external tooth design the preferred locking method for small fasteners in the most critical, high-vibration applications.
Note: The MS35335-33 is a flight-critical piece of hardware. In many critical applications, these washers are considered single-use, as the teeth are designed to deform and may lose their spring and biting capability upon removal. The washer is designed to mar the surfaces it contacts; this is part of its locking function. Installation must be performed by a certified A&P/IA mechanic.
Documentation
MS35335 (Standard): The military standard sheet defines all dimensional, material, and performance requirements for this series of washers and is publicly available through various online mil-spec libraries such as the Defense Logistics Agency ASSIST database.
Part Number: MS35335-33
Alternate Part Numbers: AN936B416, SH1114-05, 114604, AN936-B416, 193-8, 121753, 453475, 1114-00, 0076574, 76574, 402350207, 1114-00, 1114-13, 70178-33, MS35335-19, 0740794, 853-1013, 10351369, 853074, 373-5042-00, 373-5042-000, 69103, 104-3S416B
Product Overview
A washer is usually round with a centrally located hole. Its edges and two bearing surfaces are so designed that applied pressure brings into play the spring tension and/or frictional locking principle which materially resists any tendency toward turning. It may be made of one or two pieces.
Provide efficient locking by means of the spring tension and biting action of tapered twisted teeth. MS35333 and MS35335 supersede AN936A and AN936B washers respectively.
Item Specification
| Hole Diameter | 0.256 inches minimum and 0.267 inches maximum | |
| Outside Diameter | 0.494 inches minimum and 0.510 inches maximum | |
| Material Thickness | 0.023 inches minimum and 0.028 inches maximum | |
| End Item Identification | W/s:aircraft, (AC-130H, MC-130H, EC-130E, HC-130); compressor, reciprocating M4; airborne mobile dir air spt ctl (2ID), (02M JUL83)(AN/UYQ-3A); aviation engine test systems (aets); support equipment, B-1 aircraft; central message switch, AN/TYC-39A; boile; medium mine protected vehicle - mmpv; armored multi-purpose vehicle (ampv); A4 bradley fighting vehicle; A3 bradley fighting vehicle | |
| Hardness Rating | 40.0 rockwell C minimum and 50.0 rockwell C maximum | |
| Material | Steel comp 1050 or steel comp 1551 or steel comp 1552 or steel comp 1053 or steel comp 1055 or steel comp 1059 or steel comp 1060 or steel comp 1561 or steel comp 1064 overall | |
| Material Document and Classification | 66 fed std all material responses overall | |
| Surface Treatment | Cadmium and chromate overall | |
| Surface Treatment Document and Classification | QQ-P-416, ty 2, cl 3 fed spec single treatment response overall | |
| Style Designator | Flat external teeth |
Fits in
- Bombardier Challenger 601
- Bombardier Learjet 60 - all variants
- Dassault Falcon 7X
- Textron Aviation Beechcraft King Air 100 - all variants
- Textron Aviation Beechcraft King Air 200 - all variants
- Textron Aviation Beechcraft King Air 90 - all variants
- Textron Aviation Cessna 560 Citation V-Ultra-Encore-Excel-XLS
- Textron Aviation Cessna 650 Citation III-IV-VI-VII