Magneto Timing on the A&P Practical: Internal and External Timing
The Airframe and Powerplant (A&P) oral and practical (O&P) test includes hands-on work. For A&P practical magneto timing, separate the magneto’s internal setting from its timing to the engine. This guide explains the general sequence; every engine- or magneto-specific setting, connection, and procedure must come from the manufacturer’s instructions.
On this page
- What does the O&P practical evaluate?
- What must I establish before moving the engine?
- What does internal timing establish?
- How do I establish the engine timing position?
- How do I time the magneto to the engine?
- How should I interpret the timing light?
- Which timing mistakes should I catch?
- How should I rehearse before test day?
What does the O&P practical evaluate?
The Federal Aviation Administration (FAA) Airman Certification Standards (ACS) identify the skills below. A Designated Mechanic Examiner (DME) evaluates the assigned project against its stated performance standard, including your selection of appropriate tools and data. Each project has a skill element and two practical questions. The project is pass/fail: the skill demonstration must succeed on the first attempt, and both practical questions must be answered successfully. These study topics do not promise a particular test assignment.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, para. 5-5-11-7C3), p. 16
| ACS skill | Exact published skill |
|---|---|
| AM.III.H.S1 | Set magneto internal timing. |
| AM.III.H.S2 | Time magneto to engine. |
FAA-S-ACS-1, printed 52, PDF 57
Follow the applicable manufacturer’s instructions and approved data for the assigned work. The ACS safety appendix calls for current maintenance publications, safe tool handling, equipment preparation, safety glasses, hearing protection, and any other required personal protective equipment (PPE). Disregard for safety will result in failure. If a project action would jeopardize safety, the evaluator will ask you to simulate that portion.
FAA-S-ACS-1, printed A-3, PDF 67
What must I establish before moving the engine?
Treat the propeller as a live ignition hazard. If a magneto ground lead (the P lead) is open, that magneto is not shut off even with the ignition switch in the off position, and turning the propeller can start the engine with personnel in the propeller arch. The timing check needs the magnetos switched on: when a timing light is used to check a magneto in a complete ignition system installed on the aircraft, the ignition switch must be turned to BOTH, or the lights do not indicate breaker point opening. With the switch on, the magnetos are not shut off, so turning the propeller during the check carries the same risk of the engine starting. Before you turn the propeller or connect timing equipment, establish and follow the manufacturer’s safety procedure with the person supervising the work. A switch position alone is not proof of a safe condition. This guide does not provide an engine-starting or run-up procedure.
FAA-H-8083-32B, Powerplant, printed 4-9, PDF 168 · FAA-H-8083-32B, Powerplant, printed 4-31, PDF 190
What does internal timing establish?
Internal timing sets the magneto’s own relationship between rotor position, breaker-point opening, and distributor position. The E-gap angle is how far a pole of the rotor magnet is past its neutral position when the breaker points open. The manufacturer determines that angle for each magneto model to obtain the strongest spark at the instant of breaker point separation. A degree value or point gap shown for one model is not a setting for another.
- Identify the magneto model and use its manufacturer’s internal-timing instructions.
- Use the prescribed marks, gauge, or fixture to establish that model’s E-gap position and breaker-point setting.
- Confirm the prescribed distributor-to-No. 1 relationship before timing the internally adjusted magneto to the engine.
FAA-H-8083-32B, Powerplant, printed 4-26, PDF 185 · FAA-H-8083-32B, Powerplant, printed 4-29, PDF 188
How do I establish the engine timing position?
Use the engine manufacturer’s timing method and specified advance. Establish No. 1 cylinder on the compression stroke, identify the correct timing marks, and position the crankshaft at the prescribed degrees before top dead center (TDC). View marks straight on to avoid parallax. TDC is not the same as top center. The piston does not move while the crankshaft turns through a small arc (the no-travel zone), and at TDC the piston is in the center of that zone. A piston in top center has little timing value because the corresponding crankshaft position can vary.
- Locate the engine’s specified timing references and instructions.
- Identify No. 1 compression, then establish the timing position with the prescribed method.
- Make the final approach in the normal direction of engine rotation to take up gear backlash.
FAA-H-8083-32B, Powerplant, printed 4-26, PDF 185 · FAA-H-8083-32B, Powerplant, printed 4-27, PDF 186
How do I time the magneto to the engine?
With internal timing complete, follow the engine and magneto manufacturers’ instructions to place the magneto at its No. 1 firing position and engage the drive at the prescribed engine position. Connect the timing equipment only in the required test configuration. Adjust the installed magneto so its breaker points just open at the specified crankshaft position. Secure the mounting as instructed, then recheck the timing using the prescribed approach in normal rotation. Do not substitute a generic degree value for the engine’s specification.
FAA-H-8083-32B, Powerplant, printed 4-30, PDF 189
How should I interpret the timing light?
Check the instructions for the timing-light model before interpreting its indication. Some lights turn on when the breaker points open; other models turn off. A universal instruction such as “the light must come on” can therefore give the wrong result. Weak batteries can also produce erroneous readings. Identify what the instrument is indicating, rather than judging timing from an unexplained light change.
FAA-H-8083-32B, Powerplant, printed 4-28, PDF 187
Which timing mistakes should I catch?
Watch for the wrong cylinder stroke, the wrong timing mark, parallax, a final approach that leaves backlash in the wrong direction, an incorrect interpretation of the timing light, and a failure to recheck after securing the magneto. Ignition too far advanced can oppose the rising piston and contribute to power loss, overheating, detonation, or preignition. Timing too late leaves inadequate time for combustion and can also reduce power.
FAA-H-8083-32B, Powerplant, printed 4-26, PDF 185 · FAA-H-8083-32B, Powerplant, printed 4-27, PDF 186 · FAA-H-8083-32B, Powerplant, printed 4-28, PDF 187 · FAA-H-8083-32B, Powerplant, printed 4-30, PDF 189
How should I rehearse before test day?
Our study suggestion: work through magneto timing with an instructor, name the applicable source before selecting tools or values, and inspect the finished work against that source. Keep a note of the step you need to repeat in supervised practice. Rehearse the explanation separately from the physical task.
Frequently asked questions
May the DME assess an O&P practical project remotely?
No. The examiner must personally observe each practical project in its entirety, and all testing must be conducted in person. Use browser-based oral rehearsal to practice explanations, and obtain supervised hands-on practice for the physical task.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, para. 5-5-11-7C3), p. 16
Do both magnetos always fire at the same crankshaft angle?
No. Some early engines use staggered timing, with each magneto timed to a different crankshaft position. Use the timing specified for each magneto by the engine manufacturer rather than assuming simultaneous opening is correct for every engine.
FAA-H-8083-32B, Powerplant, printed 4-30, PDF 189
Are built-in timing marks the only way to set the crankshaft position?
No. Most reciprocating engines have built-in timing reference marks, and the engine manufacturer’s instructions give their location. Another timing device is the timing disc: most mount to the crankshaft flange and use a timing plate with a scale numbered in crankshaft degrees. Use the device and method the engine manufacturer specifies, not an improvised mark or a guessed advance.
FAA-H-8083-32B, Powerplant, printed 4-26, PDF 185 · FAA-H-8083-32B, Powerplant, printed 4-27, PDF 186
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Related guides
- Ignition and Starting Systems Oral Exam Questions and Answers
- Powerplant Oral Exam Questions and How to Answer Them
- Engine Inspection Oral Exam Questions and Answers
Sources
FAA sources checked October 19, 2026.
- FAA-S-ACS-1
- FAA Order 8900.1, Volume 5, Chapter 5, Section 11, Administer an Oral and a Practical Test: Mechanic Applicant Under Part 65 Subpart D (CHG 929, 9/17/2024), para. 5-5-11-7C3)
- FAA-H-8083-32B, Powerplant
About this guide: drafted with AI from the FAA sources listed on this page and checked claim by claim against them. Not reviewed by an A&P mechanic or DME. How we make our guides
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