8.2 Horizontally Opposed and Radial Engine Components

Cylinder Arrangements and Firing Order

Cylinder layout is a favorite identification topic. The main arrangements are:

• Horizontally opposed (flat): cylinders lie in two banks across the crankcase. Compact, light, easily streamlined, and air-cooled; the standard for modern general aviation (Lycoming, Continental).
• Radial: cylinders are spaced evenly around the crankcase in one or more rows; always an odd number per row (5, 7, 9...) for even firing. Great power-to-weight, large frontal area; common on warbirds and round-engine classics.
• Inline: cylinders in a single row; small frontal area but harder to cool the rear cylinders. Rare today.
• V-type: two banks set in a V; high power for its size, used historically (e.g., Allison, Merlin).

Firing order is fixed by crankshaft and cam design and spaces power impulses evenly to reduce vibration. Memorize the common opposed orders: a four-cylinder Lycoming fires 1-3-2-4; a four-cylinder Continental fires 1-4-2-3; a six-cylinder Continental/Lycoming fires 1-6-3-2-5-4. A single-row radial fires all odd-numbered cylinders in sequence and then the even-numbered ones (e.g., a nine-cylinder fires 1-3-5-7-9-2-4-6-8). Top dead center is the reference position used for timing, valve work, and cylinder installation — locate it with the proper cylinder and method, never by eyeballing prop position.

Radial Rod Geometry and Component Inspection

Radial engines use a master rod that rides the crankpin, with articulating rods (link rods) pinned to the master rod flange to drive the other pistons. This geometry produces slightly different piston travel per cylinder and changes inspection concerns. Because oil can drain into the lower cylinders of a shut-down radial, hydraulic lock is a real hazard: rotating the engine on a locked cylinder can bend a rod or blow a cylinder. Always clear suspect cylinders per the engine procedure before turning the prop or starting.

A cylinder assembly includes the barrel, head, cooling fins, intake and exhaust valves, guides, springs, rocker arms, piston, rings, and wrist pin. A cylinder that still makes power can be unairworthy if limits are exceeded. Connect each name to a consequence and an inspection emphasis:

Cylinder Work and Decision Discipline

Cylinder replacement on an opposed engine is procedural: check surfaces, install piston and pin, orient ring gaps, fit base seals, follow the torque sequence, lubricate, set the valve train and pushrods, reinstall baffles, and run post-maintenance checks. An exam answer that skips torque sequence or approved limits is weak.

When studying components, always tie name to symptom: a worn bearing lowers oil pressure or makes metal; a burned exhaust valve drops compression with exhaust leakage; a cracked crankcase leaks oil or loses structural support; a broken cooling fin may or may not exceed limits depending on its location and the manufacturer's rejection criteria. The mechanic inspects, measures, compares to approved data, and decides — habit and appearance are never the basis for an airworthiness call.

Material Selection and Why Components Are Built the Way They Are

Understanding component construction helps you answer "why" questions on the test. Crankshafts are forged alloy steel, nitrided on the journals for hardness, and counterweighted (often with dynamic dampers — pendulum-type weights that absorb torsional vibration at the firing frequency). Connecting rods are forged or machined alloy steel with a precision split big-end carrying a steel-backed insert bearing.

Pistons are typically forged or cast aluminum alloy for light weight and good heat conduction; they carry compression rings (sealing combustion) and an oil control ring (metering oil on the cylinder wall). The wrist pin is usually a full-floating design retained by aluminum plugs or circlips so it can rotate in both the piston and the rod bushing.

Cylinder barrels are steel (sometimes nitrided or chrome-plated), while heads are aluminum alloy cast onto or screwed and shrunk onto the barrel for light weight and rapid heat rejection through the cooling fins.

Valves earn special attention: the exhaust valve runs far hotter than the intake and is often sodium-filled in the hollow stem. The sodium melts in operation and sloshes, carrying heat from the hot valve head up the stem to the guide, lowering valve-head temperature and resisting burning. Intake valves run cooler because incoming charge cools them, so they are usually solid. Knowing material and cooling logic lets you reason about failure modes — a burned exhaust valve, a stuck valve from carbon ("morning sickness"), or a cracked head from thermal shock after a rapid power reduction.

Finally, the camshaft and valve train differ by layout. Opposed engines use a single block-mounted camshaft with hydraulic or solid lifters, pushrods, and rocker arms; radials use a slow-turning cam ring (a single ring with lobes that operates all cylinders in a row) rather than a conventional camshaft. Correct valve clearance (lash) matters: too much clearance shortens valve-open duration and causes noise and poor breathing, while too little can hold a hot exhaust valve off its seat and burn it.

Hydraulic lifters automatically maintain zero lash, but solid lifters require periodic adjustment to the manufacturer's cold or hot clearance. These construction differences are exactly why inspection access and emphasis shift between the two families even though the combustion cycle is identical.