Lycoming O-320 Firing Order: 1‑3‑2‑4 Fully Explained
🔢 Cylinder Numbering & Orientation (Critical for Understanding)
Before diving deeper, proper cylinder identification is essential. Lycoming uses a standard numbering system viewed from the pilot’s seat (looking forward):
- Cylinder #1 – Right front (passenger side, closest to propeller flange)
- Cylinder #2 – Left front (pilot side, front)
- Cylinder #3 – Right rear (passenger side, aft)
- Cylinder #4 – Left rear (pilot side, aft)
The firing order 1 → 3 → 2 → 4 therefore alternates between right and left banks, providing excellent primary balance. This contrasts with some inline automotive engines that may have two consecutive firings on the same bank.
❓ Why 1-3-2-4? The Engineering Rationale
The why firing order matters centers on reducing torsional vibration and maintaining crankshaft durability. The O-320 crankshaft has two crankpin throws: one serving cylinders 1 & 2 (opposed), and another serving cylinders 3 & 4. By sequencing power strokes at 180° intervals (#1 at 0°, #3 at 180°, #2 at 360°, #4 at 540°), the net twisting force on the crank is evenly distributed. This prevents resonant vibration peaks that could lead to fatigue cracks. Additionally, the even firing order allows optimal intake and exhaust tuning, contributing to the engine’s renowned reliability.
🛠️ How to Check & Verify Firing Order (Step-by-Step)
How to check firing order on a Lycoming O-320: the most reliable method is to inspect the ignition harness routing from each magneto (or electronic ignition) to the spark plugs. Follow this procedure:
- Remove the top cowling for access.
- Locate the right magneto (typically behind cylinder #1) and left magneto.
- Identify the distributor block terminals; they are numbered 1,2,3,4 (or marked with cylinder positions).
- Verify that terminal #1 connects to cylinder #1 (right front) spark plug.
- Terminal #3 must connect to cylinder #3 (right rear).
- Terminal #2 → cylinder #2 (left front), terminal #4 → cylinder #4 (left rear).
- Repeat for the second magneto (both must follow the same 1-3-2-4 pattern).
Additionally, you can perform a power check: run the engine at 1800 RPM, switch to each magneto individually. If roughness or RPM drop exceeds 125 RPM difference, suspect incorrect firing order or timing.
⚡ Types of Firing Orders in Aviation vs. O-320 Specifics
Most four-cylinder horizontally opposed engines use either 1-3-2-4 or 1-2-4-3. The Lycoming O-320 uses the former. Continental O-200 and O-300 also use 1-3-2-4, while some Lycoming O-235 early versions use the same. The type of firing order is determined by the camshaft grind. No other firing order is permissible for the O-320 without major internal re-engineering.
✅ Advantages & Disadvantages of the 1-3-2-4 Sequence
| Advantages | Disadvantages / Risks (if altered) |
|---|---|
| ✔️ Perfect primary balance – minimal airframe vibration ✔️ Even torque pulses reduce crankshaft fatigue ✔️ Compatible with dual magneto redundancy ✔️ Enhances engine longevity (TBO 2000+ hrs) | ❌ Only disadvantage emerges from incorrect wiring: severe misfire, backfire, loss of power, possible valve/piston damage. ❌ Non-standard orders impossible without custom cam. |
| ✔️ Allows simple magneto timing (both mags fire same sequence) ✔️ Smooth idle & transition to full power | ❌ Misdiagnosed firing order can be mistaken for carburetor issues. |
🛡️ Is It Safe? Safety Analysis & Operational Integrity
Is it safe? Yes — the Lycoming O-320 firing order is inherently safe and has been proven in millions of flight hours. However, safety is contingent on correct ignition lead routing and periodic verification. According to FAA Advisory Circular 43.13-1B, mechanics must confirm firing order after any ignition maintenance. An incorrect order can lead to detonation, overheating, or in-flight engine failure. Always perform a magneto check before takeoff: any abnormal roughness indicates a firing order issue or timing discrepancy.
📈 Use Cases & Applications of O-320 Firing Order Knowledge
Understanding the use of firing order extends beyond mechanics: pilots use it to interpret engine analyzer data (e.g., EGT peaks sequence). During troubleshooting, a mechanic may perform a cylinder cutout test; knowing the firing order helps identify which cylinder is misfiring. Moreover, homebuilders installing aftermarket electronic ignition (e.g., Lightspeed, SureFly) must configure the ignition timing curve to respect 1-3-2-4.
🔧 Advanced: Magneto Synchronization & Firing Order Interaction
The two magnetos on a standard O-320 fire the same cylinder at slightly different times (typically both fire at 25° BTDC, but internal timing may differ by 1-2°). However, the firing order remains identical for both magnetos. If one magneto’s distributor is mis-indexed, it will fire the wrong cylinder at the wrong time, causing a massive power loss. Always time magnetos according to Lycoming Service Instruction 1326A, and verify the firing order using a timing light and the top dead center (TDC) mark on the starter ring gear.
📋 O-320 Variants: Is Firing Order Always the Same?
Yes, across all Lycoming O-320 series — including O-320-A, -B, -C, -D, -E, -H, -L, and military versions like O-320-B1A — the firing order is invariable: 1-3-2-4. The differences between variants involve compression ratio, accessory case, propeller flange, or cylinder material (steel vs. nitrided), but the camshaft timing remains constant.
🧰 Maintenance Best Practices & Firing Order Verification Checklist
To maintain safety and performance, include these steps in every annual/100-hour inspection:
- Visual trace of each spark plug lead from magneto to cylinder.
- Use an ohmmeter to confirm harness continuity and correct terminal assignment.
- Perform a differential compression test and note any cylinder that runs cooler (could indicate incorrect firing).
- After maintenance, ground run: engine should accelerate smoothly, with max RPM drop per mag ≤125 RPM and no roughness.
- If replacing a magneto, verify the distributor gear orientation matches the firing order per Lycoming manual.
❓ Extended FAQ – All Your Questions Answered
What is the firing order for Lycoming O-320?
The firing order is 1-3-2-4. This is non-negotiable and applies to all O-320 models.
How does firing order affect engine vibration?
1-3-2-4 gives evenly spaced power pulses (every 180° crank rotation) that cancel primary vibrations, resulting in smooth operation and less stress on engine mounts.
What are symptoms of incorrect firing order on O-320?
Rough idle, backfiring through carburetor, loss of power, high CHT on some cylinders, and uneven EGT readings. Severe cases cause engine failure.
Is it safe to run the engine if firing order is wrong?
Absolutely not. It is unsafe and can cause internal damage, valve burning, and crankshaft stress. Immediate correction required.
Can electronic ignition change the firing order?
No. Electronic ignition (EI) systems must follow the same 1-3-2-4 sequence. The difference is spark timing curve, not order.
How to remember firing order easily?
Mnemonic: “One, Three, Two, Four — smooth as never before.” Or note that after #1 fires, it jumps to the opposite bank rear (#3), then to front left (#2), then to rear left (#4).
What is the difference between firing order and ignition timing?
Firing order is which cylinder fires; timing is when (degrees before TDC). Both must be correct for proper engine operation.
Does propeller selection affect firing order?
No. Propeller choice (fixed-pitch, constant-speed) does not interact with firing order. However, severe vibrations from wrong order can damage prop hub.
📊 Technical Data Table: O-320 Firing Order Summary
| Parameter | Value / Description |
|---|---|
| Engine family | Lycoming O-320 (horizontally opposed, 4-cylinder) |
| Firing order | 1-3-2-4 |
| Crank angle between firings | 180° (uniform) |
| Camshaft rotation relative to crank | 1:2 (cam turns once per two crank revs) |
| Ignition system | Dual magneto (or optional electronic), both follow same order |
| Typical magneto timing | 25° BTDC at idle / 20-25° BTDC full advance |
| Consequence of crossed leads | Rough running, power loss, possible engine failure |
