STD-1874 – Lycoming OEM Screw .250-20X.81 Long
Available now on the Airparts.aero Marketplace.
Overview
The Lycoming STD-1874 is a genuine OEM, flight-critical crankcase screw designed specifically for assembling the main crankcase halves of numerous Lycoming reciprocating engines. This is not a common hardware store fastener; it is a precision-engineered screw manufactured from high-tensile strength steel alloy, designed to achieve a specific clamping force when torqued. The STD-1874 plays a fundamental role in maintaining the structural rigidity of the engine block, ensuring the precise alignment of the crankshaft and camshaft bearings, and preventing oil leaks between the case halves.
Key Features
- High-Tensile Steel Alloy: Fabricated from a specialized steel alloy, heat-treated to achieve the high tensile strength required to withstand the constant stress, vibration, and thermal cycling of engine operation.
- Precision Rolled Threads: Features clean, rolled .250-20 UNC threads for maximum strength and fatigue resistance. The rolling process creates a superior grain structure that is far more durable than standard cut threads.
- Engineered Clamping Force: Designed to provide a precise and even clamping force when torqued to the Lycoming-specified value. This is essential for preventing case fretting and maintaining main bearing bore alignment.
- Corrosion-Resistant Finish: Plated with a durable, corrosion-resistant finish (typically cadmium) to protect the fastener from the harsh environment within the engine cowling and prevent seizure.
- Genuine Lycoming OEM Part: As a genuine factory part, it guarantees conformance to Lycoming's stringent material specifications, manufacturing processes, and quality control standards, ensuring the highest level of airworthiness.
Technical Specifications
- Part Number: STD-1874
- Component Type: Crankcase Assembling Screw
- Head Style: Hex Head Cap Screw
- Material: High-Tensile Steel Alloy
- Finish: Cadmium Plated (or equivalent protective finish)
- Thread Specification: 1/4-20 UNC-3A
- Nominal Diameter: 1/4 inch (0.250 inches)
- Threads Per Inch: 20 (Coarse Thread)
- Length: 0.812 inches
- Recommended Torque: 108 in-lbs / 9 ft-lbs (Must be verified in the current Lycoming Overhaul Manual for the specific engine model)
Engine Compatibility
The Lycoming STD-1874 screw is one of the most common crankcase fasteners used across a wide range of Lycoming's most popular direct-drive engines. Verifying the correct part number in the engine's official Illustrated Parts Catalog (IPC) is always a mandatory step.
- Common Engine Series:
- O-235
- O-320, IO-320
- O-360, IO-360, AIO-360, AEIO-360
- O-540, IO-540, AEIO-540 (in certain locations)
Applications
The STD-1874 screw has a singular, flight-critical application:
- Crankcase Assembly: Used in multiple locations around the crankcase parting line to fasten the two case halves together, forming the main structural component of the engine.
- Mandatory Inspection Item: These screws must be carefully inspected during any engine overhaul and replaced if there is any sign of corrosion, thread damage, or stretching.
Why Choose STD-1874?
The crankcase is the foundation of the entire engine. The integrity of the fasteners holding it together is non-negotiable. Using a genuine Lycoming STD-1874 OEM screw is the only way to ensure the fastener has the correct material strength, ductility, and manufacturing quality to perform its job safely. Substituting a non-certified commercial-grade bolt risks improper clamping force, fastener fatigue, and catastrophic failure of the engine case. For any professional engine builder or A&P mechanic, using genuine Lycoming hardware is a fundamental requirement for a safe, reliable, and airworthy engine.
Order Your STD-1874 Lycoming Screws Now
Ensure the structural integrity of your Lycoming engine with genuine OEM crankcase hardware. Visit Airparts.aero to order your STD-1874 screws today.
The Lycoming STD-1874 is a critical component in what is known as a "clamped joint." The science behind this joint relies on stretching the screw into its elastic region to create a clamping force on the crankcase halves that is greater than any separating force the case will experience during operation. This high level of static preload prevents any microscopic movement or "fretting" between the case halves, which would otherwise lead to wear, oil leaks, and potential fatigue failure of the case itself. The specified torque value is carefully calculated to achieve this optimal preload without yielding the screw. This is why the use of a calibrated torque wrench and adherence to the multi-stage, sequenced tightening procedure outlined in the Lycoming overhaul manual is absolutely essential.
Note: This is a flight-critical component. Installation must be performed by a certified A&P mechanic in strict accordance with the current Lycoming Overhaul Manual for the specific engine model. Always use a calibrated torque wrench and follow the specified tightening sequence
