17F21188 – Lycoming OEM Intake Rocker Arm Assembly
Available now through the AirParts Aero marketplace.
Overview Section
The Lycoming 17F21188 is a high-precision Intake Rocker Arm Assembly engineered for Lycoming's advanced "roller-tappet" series engines.1 Unlike legacy flat-tappet rocker arms, the 17F21188 is part of a modernized valve train system designed to reduce internal friction, decrease wear, and improve the overall mechanical efficiency of the engine.
This assembly acts as the critical mechanical link between the pushrod and the intake valve. By translating the vertical lift of the camshaft into the downward force required to open the intake valve, the 17F21188 ensures precise fuel-air mixture delivery into the combustion chamber. It is manufactured from high-strength forged steel and features a precision-reamed bushing to ensure smooth operation over thousands of flight hours.
Key Features Section
Forged Steel Strength: Forged to ensure optimal grain flow, providing superior fatigue resistance against the constant reciprocating loads of the valve train.
Roller-Tappet System Compatibility: Specifically designed to work in harmony with Lycoming’s newer roller-cam technology, providing more consistent valve timing.
High-Lead Bronze Bushing: Features a factory-installed, high-durability bushing that ensures a low-friction interface with the rocker shaft, reducing the risk of "seizing" or galling.
Induction-Hardened Toe: The valve-contact surface (the toe) is specially hardened to prevent "mushrooming" or pitting, maintaining correct valve clearances longer than aftermarket alternatives.
Precision Oil Porting: Includes an integrated lubrication path that directs pressurized oil from the pushrod socket to the rocker shaft and valve tip, ensuring the entire assembly remains cool and lubricated.
OEM Lycoming Certification: Guaranteed to meet the weight-balance and metallurgical standards required for certified aircraft engines.
Technical Specifications Section
| Specification | Detail |
| Part Number | 17F21188 |
| Manufacturer | Lycoming Engines (Textron) |
| Component Type | Intake Rocker Arm Assembly |
| Material | Forged Alloy Steel |
| Bushing Material | Specialized Bronze Alloy |
| Engine Application | IO-360, IO-390, IO-540, and TIO-540 (Roller-Tappet Models) |
| Service Limit | Replace per Lycoming Table of Limits / EMM |
Aircraft Compatibility Section
The 17F21188 is the standard intake rocker for most late-model Lycoming engines found in the AirParts Aero inventory, including:
Cessna 172S Skyhawk SP: Essential for the IO-360-L2A engine variants.
Piper Archer III: Used in the modern O-360/IO-360 configurations.
Cirrus SR20: Found in the high-performance Lycoming IO-390 and IO-360 powerplants.
Beechcraft G36 Bonanza: Standard hardware for the IO-540 series engines.
Diamond DA40/DA42: Utilized in the IO-360-M1A and related Lycoming variants.
Technical Applications & Safety
The transition to the 17F21188 style rocker arm was part of Lycoming’s effort to eliminate the "spalling" issues associated with older flat-tappet designs. Because the roller-tappet system creates different harmonic loads, using the correct OEM rocker arm like the 17F21188 is vital for maintaining the structural integrity of the valve train.
Safety Warning: During a "Top Overhaul" or cylinder replacement, always inspect the Rocker Shaft (P/N 75444) for wear. A worn shaft will allow the 17F21188 to "cock" to one side, leading to uneven wear on the valve stem and eventually causing a "stuck valve." Never attempt to regrind the toe of a 17F21188; if the hardened surface is breached, the rocker will fail rapidly.
Technical Engineering Insight
The engineering value of the 17F21188 lies in its Moment of Inertia optimization. To allow for the higher RPMs and increased valve lifts of modern fuel-injected engines, Lycoming optimized the mass distribution of this rocker arm. By reducing mass at the extremities while maintaining stiffness in the center, the 17F21188 reduces "valve float" and ensures the valve follows the camshaft profile precisely.
This item replaces Lycoming part number: 71266
By choosing the genuine 17F21188 through AirParts Aero, you ensure the bushing is reamed to the exact "interference fit" required. Aftermarket rockers often have loose bushings that allow oil pressure to bleed off, resulting in "lifter clatter" and inadequate lubrication to the valve tips.
Installation & Maintenance Addendum
Initial Lube: Always submerge the 17F21188 in clean engine oil prior to installation to ensure the bushing is not "dry" during the first critical seconds of engine start.
Clearance Check: Verify the dry tappet clearance per Lycoming Service Instruction 1068. Correct clearance ensures the hydraulic lifter operates within its optimal range.
Rocker Box Sealing: Always use new gaskets (like the 75456 or silicone equivalent) when closing the rocker box to prevent oil leaks onto the exhaust manifold.
Description: ROCKER ASSEMBLY, INTAKE
Part Number: 17F21188
Previous Part Number: 71266
Product Overview
A rocker arm is a valvetrain component in internal combustion engine. As the arm is acted on by a camshaft lobe, it pushes open either an intake or exhaust valve. This allows fuel and air to be drawn into the combustion chamber during the intake stroke or exhaust gases to be expelled during the exhaust stroke.
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