17F21187 – Lycoming OEM Exhaust Rocker Arm Assembly
Available now through the AirParts Aero marketplace.
Overview Section
The Lycoming 17F21187 is a premium Exhaust Rocker Arm Assembly specifically engineered for Lycoming’s modern roller-tappet engine series. As a critical component of the overhead valve train, this assembly translates the pushrod's upward motion into the force required to open the exhaust valve, allowing spent combustion gases to exit the cylinder.
Because the exhaust valve is subjected to significantly higher temperatures than the intake valve, the 17F21187 is manufactured to withstand intense thermal cycling and high-frequency reciprocating loads. It is a core component in Lycoming’s effort to reduce internal friction and increase the reliability of the valve train compared to legacy flat-tappet designs.
Key Features Section
Modern Roller-System Optimization: Designed to work exclusively with Lycoming’s roller-cam technology, providing smoother valve actuation and reduced camshaft wear.
Forged Steel Durability: Forged from high-alloy steel to ensure superior grain flow and fatigue resistance, preventing cracks in the rocker body under high-stress conditions.
High-Lead Bronze Bushing: Includes a factory-installed, precision-reamed bushing that provides a stable, low-friction interface with the rocker shaft.
Advanced Lubrication Porting: Features an integrated internal oil gallery that directs pressurized engine oil to the rocker shaft, pushrod socket, and valve tip for maximum cooling.
Induction-Hardened Toe: The "toe" (valve contact surface) is hardened to a specific Rockwell depth to prevent "mushrooming" or pitting from constant contact with the exhaust valve stem.
Textron Lycoming OEM Certified: Meets the exact weight-balance and metallurgical specifications required to maintain the engine's original type certificate.
Technical Specifications Section
| Specification | Detail |
| Part Number | 17F21187 |
| Manufacturer | Lycoming Engines (Textron) |
| Component Type | Exhaust Rocker Arm Assembly |
| Material | Forged Alloy Steel |
| Bushing Type | Specialized Heavy-Duty Bronze |
| Engine Application | IO-360, IO-390, IO-540, TIO-540 (Roller-Tappet) |
| Service Standard | Replace/Inspect per Lycoming Table of Limits |
Aircraft Compatibility Section
The 17F21187 is the standard exhaust rocker for the latest generation of Lycoming engines found in the AirParts Aero inventory:
Cirrus SR20 / SR22: Essential for the IO-390 and IO-540 roller-tappet engine variants.
Cessna 172S (Skyhawk SP): Used in the modern IO-360-L2A configurations.
Diamond DA40 / DA42: Found in the high-efficiency Lycoming IO-360-M1A powerplants.
Piper Archer DX / Seminole: Standard hardware for the newer fuel-injected Lycoming models.
Beechcraft G36 Bonanza: Critical for the IO-540-AB1A5 engine maintenance.
Technical Applications & Safety
The exhaust rocker operates in the harshest environment of the cylinder head. The 17F21187 must maintain perfect alignment to prevent "side-loading" the exhaust valve stem. If the bushing becomes worn, the rocker can tilt, leading to "stuck valves" or broken valve guides—a leading cause of engine failure in modern piston aircraft.
Safety Warning: Do not interchange the 17F21187 with legacy flat-tappet rockers (like the 74390). The geometry and weight are specific to the roller-tappet valve train. During installation, always verify the Dry Tappet Clearance (typically $0.028$ to $0.080$ inches) to ensure the hydraulic lifter can properly compensate for the expansion of the exhaust valve as it heats up.
Technical Engineering Insight
The engineering value of the 17F21187 lies in its Thermal Dissipation Design. Because the exhaust valve stem can reach temperatures exceeding 1200°F, the rocker arm acts as a secondary heat sink. The 17F21187 is designed with a specific mass profile that allows it to absorb heat from the valve tip and dissipate it into the circulating engine oil without the steel losing its "temper" (hardness).
By sourcing this assembly through AirParts Aero, you ensure that the bushing-to-bore fit is an OEM "interference fit." Aftermarket rockers often have looser bushings that allow oil to bypass the rocker shaft, leading to reduced oil pressure at the valve tip and premature wear of the exhaust valve.
Installation & Maintenance Addendum
Pre-Oil: Submerge the assembly in clean MIL-L-6082 or MIL-L-22851 oil before installation to prevent a "dry start" on the bushing.
Shaft Inspection: Always inspect the Rocker Shaft (P/N 75444) for "step-wear." If you can feel a ridge with your fingernail, the shaft must be replaced to prevent the 17F21187 from binding.
Valve Tip Inspection: Ensure the exhaust valve tip is flat and free of "mushrooming" before installing the new rocker.
Description: Exhaust Rocker
Part Number: 17F21187
Alternate Part Numbers: 74390, 71265
Product Overview:
A rocker arm is a valvetrain component in internal combustion engines. 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.
AirParts stocks thousands of engine and engine overhaul parts. Cranks, cases, cams, cylinders, and more. Most available on exchange programs.