74390 – Textron Lycoming OEM Exhaust Rocker Arm
Available now through the AirParts Aero marketplace.
Overview Section
The Lycoming 74390 is a high-strength Exhaust Rocker Arm designed for various Lycoming four- and six-cylinder engines. Unlike intake rockers, exhaust rockers like the 74390 are subjected to significantly higher thermal stress due to their proximity to the exhaust gas flow and the heat conducted through the exhaust valve stem.
Forged from premium aircraft-grade steel and heat-treated for maximum durability, the 74390 translates the upward motion of the pushrod into the downward force required to open the exhaust valve. It features a specialized geometry to maintain precise valve timing and lift, which is critical for scavenging spent combustion gases and maintaining engine cooling.
Key Features Section
Forged Alloy Steel: Engineered with a superior grain flow structure to resist fatigue and cracking under high-frequency reciprocating loads.
High-Lead Bronze Bushing: Includes a precision-machined, factory-installed bushing that ensures a smooth, low-friction interface with the rocker shaft.
Thermal Fatigue Resistance: Specifically heat-treated to withstand the high temperatures of the exhaust valve environment without losing hardness or structural integrity.
Pressurized Lubrication Paths: Features internal oil porting to deliver a constant supply of engine oil to the rocker shaft, pushrod socket, and valve tip.
Hardened Wear Surfaces: The valve-contact "toe" and pushrod "socket" are induction-hardened to prevent pitting and wear, ensuring consistent dry tappet clearances.
Textron Lycoming OEM Standard: Manufactured to the exact weight and metallurgical tolerances required to maintain the engine's original type certification.
Technical Specifications Section
| Specification | Detail |
| Part Number | 74390 |
| Manufacturer | Textron Lycoming |
| Component Type | Exhaust Rocker Arm |
| Material | Forged Alloy Steel |
| Bushing Type | Bronze (Typically P/N 71140 or equivalent) |
| Engine Application | O-320, O-360, IO-360, IO-540 series |
| Service Standard | Inspect/Replace per Lycoming Table of Limits |
Aircraft Compatibility Section
The 74390 is a foundational valve train component found in Lycoming engines powering:
Cessna 172 Skyhawk: Essential for the O-320 and IO-360-L2A engine variants.
Piper Cherokee / Archer: Standard hardware for the O-320 and O-360 powerplants.
Beechcraft Bonanza / Baron: Found in the IO-540 and O-540 series engines.
Grumman Tiger / Cheetah: Utilized in the high-output O-360-A4K configurations.
Experimental Aircraft: The primary exhaust rocker choice for Vans RV and Glasair builders using Lycoming engines.
Technical Applications & Safety
The exhaust rocker arm is a critical link in the engine's cooling and breathing cycle. Because the exhaust valve is the hottest part of the engine, the 74390 must operate without "cocking" or "binding." If the rocker bushing is worn, it can side-load the exhaust valve stem, leading to "Morning Sickness" (sticky valves) or a catastrophic broken valve guide.
Safety Warning: Never interchange intake and exhaust rockers unless the parts manual specifically lists them as identical for your engine model. The 74390 is often weight-matched to the exhaust assembly. During inspection, check the "toe" for pitting or spalling. If the hardened surface is compromised, the rocker will rapidly wear the valve tip, leading to a loss of compression and potential power loss.
Technical Engineering Insight
The engineering value of the 74390 lies in its Oil-Film Retention. The internal bushing is finished with a specific surface roughness that allows a microscopic film of oil to stay trapped between the shaft and the rocker, even during the "dry start" period. This prevents the initial metal-on-metal contact that causes most rocker failures.
By choosing the genuine 74390 through AirParts Aero, you ensure that the "socket" geometry matches the radius of the Lycoming pushrod perfectly. Aftermarket rockers with slightly different radii can cause "point-loading" on the pushrod ball, leading to the pushrod eventually punching through the rocker arm socket.
Installation & Maintenance Addendum
Bushing Fit: When installing on a new rocker shaft (P/N 75444), verify the clearance is within the Lycoming Table of Limits (typically .0005 to .0020 inch).
Dry Tappet Clearance: Always measure the clearance between the rocker toe and valve tip with the lifter collapsed. This must fall within .028 to .080 inch for most models.
Shaft Orientation: Ensure the rocker shaft is installed with the oil holes facing the correct direction as specified in the overhaul manual to ensure the 74390 receives adequate pressure.
Description: Exhaust Rocker Arm
Part Number: 74390
Alternate Part Numbers: 17F21187, 71265
Product Overview:
A metal item designed to pivot on a shaft, ball stud, or the like, and transmit the intermittent motion derived through linkage, from the cam lobe of a camshaft or cam ring to the poppet valve of an internal combustion engine. It may have an adjusting screw, roller, and/or bearing(s).