The cylinder assembly of a reciprocating aircraft engine is one of the most mechanically demanding environments in all of engineering. Each cylinder endures thousands of combustion events per minute, extreme temperature swings, and high mechanical loads from combustion pressure and valve actuation. Because of this environment, a careful visual inspection of the cylinder barrels, heads, valve train components, and associated hardware is a critical step during any engine teardown, top overhaul, or time-between-overhaul (TBO) evaluation. Identifying a crack, abnormal wear pattern, or corroded valve stem early can prevent a catastrophic failure in flight.
This article covers what the FAA's powerplant maintenance guidelines teach about visually examining cylinders and the valve train—what you are looking for, how to interpret what you find, and why each finding matters for airworthiness.
The Cylinder Assembly: Components and Their Roles
Before beginning an inspection, a technician must understand what makes up the cylinder assembly. The cylinder barrel is the tubular steel section that houses the piston. Its inner surface, called the bore or cylinder wall, must be smooth, properly dimensioned, and free of corrosion or scoring. The cylinder head, usually cast from aluminum alloy, is threaded or shrunk onto the barrel. It contains the combustion chamber, intake and exhaust ports, spark plug bosses, and the valve seats. The valve train encompasses every component involved in opening and closing the intake and exhaust valves: the camshaft, cam followers (tappets or lifters), push rods, rocker arms, rocker shafts, valve springs, spring retainers, keeper locks, and the valves themselves.
Inspecting the Cylinder Barrel
After cleaning, the first visual task on the barrel is an overall assessment of the exterior. Look for cracks, especially at the base flange where the cylinder attaches to the crankcase, and around the cooling fins. Cooling fin damage—broken, bent, or missing fins—reduces heat transfer and can lead to localized overheating. FAA guidance specifies maximum allowable fin damage limits; fins that are cracked at their base or missing more than the allowable area must be evaluated for rejection or repair.
Inside the bore, examine the cylinder wall for scoring (longitudinal scratches caused by piston ring or foreign object contact), corrosion pits, glazing (an overly polished surface that prevents oil retention and proper ring seating), and rust. Rust on a steel barrel often occurs from prolonged storage with moisture present. Light surface rust may be removable; deep pitting is cause for rejection. Scoring from a broken ring or ingested debris is typically cause for rejection or cylinder boring to the next allowable oversize. Chrome-plated and nitrided cylinders have specific inspection and repair requirements distinct from plain steel barrels, and the technician must consult the manufacturer's overhaul manual for these.
The choke bore (also called taper bore) design—where the barrel diameter is slightly smaller at the top than at the bottom—is intentional in many aircraft cylinders to compensate for thermal expansion. Dimensional inspection with inside micrometers verifies that taper and out-of-round measurements remain within limits, but the visual inspection sets the stage by flagging obvious defects before precision measurement begins.
Inspecting the Cylinder Head
The cylinder head exterior should be examined for cracks, especially between the spark plug bosses, around the exhaust port flange, and in the fins. Cracks in an aluminum head are particularly serious and almost always result in rejection of the component. The exhaust port and the surrounding area are high-stress, high-temperature zones; a technician should look for erosion, burning, and carbon buildup indicating exhaust gas leakage past the valve seat.
The valve seats—the precision-ground surfaces against which the valve faces seal combustion pressure—must be inspected for pitting, burning, recession, and width. A seat that is too wide allows carbon to collect and prevents proper heat transfer from the valve head into the seat and head structure; one that is too narrow may allow blow-by. Burned or eroded seats are typically reground to specified angles and widths or replaced. The valve guides, pressed or cast into the head, must be checked for wear (excessive side clearance) and carbon deposits. A worn guide allows a valve to rock, leading to accelerated seat wear and potential valve stem failure.
Inspecting the Valves
Valves receive some of the harshest treatment in the engine. The exhaust valve in particular operates at extremely high temperatures and depends on contact with its seat to transfer heat. Visually inspect each valve for:
- Burning or erosion of the valve face or margin—a thin or absent margin indicates heat damage and the valve must be rejected.
- Pitting or corrosion on the stem, especially near the keeper groove area where fatigue cracks can initiate.
- Cracks, even hairline, at the junction of the stem and head (the fillet area)—this zone is a classic fatigue failure location.
- Carbon deposits on the stem, which can cause a valve to stick in the guide (a sticky valve), preventing full closure and leading to burning.
- Stem wear—a valve stem worn beyond service limits will develop excessive guide clearance, causing oil consumption and guide wear.
Sodium-filled exhaust valves, common in many aircraft engines, must never be cut, machined without proper precautions, or subjected to impact that could breach the stem, as the sodium filling is a hazardous material. Any valve with a cracked or questionable stem must be rejected and properly disposed of per the manufacturer's instructions.
Inspecting Springs, Retainers, and Keepers
Valve springs maintain proper valve seating force and control valve motion. Visually check each spring for pitting from corrosion, cracks, coil contact (coils that have collapsed onto each other indicating a fatigued spring), and overall squareness. Springs that lean more than allowable when stood on a flat surface are rejected. Dual-spring assemblies (an inner and outer spring) provide redundancy; if one breaks, the other typically prevents a catastrophic dropped valve, but both should be replaced as a set.
The spring retainer (or spring cap) and keeper locks (split-cone keepers) are small but critical parts. Inspect keepers for cracks, rounded grooves, and corrosion. A worn keeper groove can allow the valve to drop into the cylinder—a catastrophic event. When reassembling, ensure keepers seat fully in the valve stem groove; they must not be reused if worn or damaged.
Inspecting Rocker Arms, Rocker Shafts, and Push Rods
The rocker arm converts camshaft motion from the push rod into downward force on the valve stem tip. Inspect the rocker arm pad—the hardened surface that contacts the valve stem tip—for excessive wear, pitting, or spalling. Inspect the rocker shaft bore for wear and scoring. A loose rocker arm on its shaft produces side loading on the valve stem and accelerates guide wear.
Check the push rods for straightness by rolling them on a known-flat surface; a bent push rod causes erratic valve timing and uneven wear throughout the valve train. Also inspect the push rod ends—the ball ends or socket ends—for wear, pitting, and cracks. Hollow push rods serve as oil passages in many engine designs; blockage from sludge or carbon can starve the rocker assembly of lubrication.
Why This Inspection Matters
The consequences of missing a defect during cylinder and valve train inspection range from elevated oil consumption and rough operation to a complete engine failure. A dropped valve—caused by a broken keeper or failed stem—can destroy a piston, connecting rod, and crankcase in milliseconds. A cracked cylinder head can cause sudden loss of compression, fire, or cockpit fume contamination. Technicians working on certificated aircraft must document their findings and comply with the applicable manufacturer's overhaul manual, FAA Airworthiness Directives, and 14 CFR Part 43 requirements for maintenance records.
Key Numbers and Rules
- Valve margin (the rim of material between the valve face and the top edge of the valve head) must meet the minimum thickness specified by the manufacturer—typically at least 1/32 inch; a knife-edge margin is automatic cause for rejection.
- Valve stem-to-guide clearance limits vary by manufacturer but excessive clearance (often more than 0.005 inch for exhaust valves) requires guide replacement or reaming to oversize.
- Cooling fin area limits: most manufacturers specify a maximum percentage of the total fin area that may be damaged on any given cylinder before rejection.
- Spring free length and load at a specified installed height must be within manufacturer limits—typically checked with a spring tester, but visual collapse and corrosion are the first screening steps.
- All maintenance on certificated aircraft cylinders must comply with 14 CFR Part 43 and be documented with the required return-to-service statement.
Common Test Traps
- Confusing choke bore with a defect. The intentional slight taper of an aircraft cylinder barrel is normal design, not a flaw. Test questions may describe a barrel that is larger at the bottom than the top and ask if it is serviceable—it may well be, if within limits.
- Sodium-filled valve handling. Test questions emphasize that sodium-filled exhaust valves are hazardous and must not be cut or disposed of as ordinary scrap; special procedures apply.
- Glazed versus scored bore. Glazing (smooth, shiny bore) and scoring (scratched bore) look different and have different causes. Glazing prevents ring seating; scoring indicates mechanical damage. Mixing these terms on the exam is a common error.
- Keeper wear and valve drop. Examinees sometimes underestimate how critical keeper inspection is. A worn keeper groove is an airworthiness-critical defect even if the valve itself looks fine.
- Push rod oil passages. Not all push rods are solid—many are hollow oil passages. Mistakenly assuming all push rods are solid can lead to missed lubrication system checks during inspection.