Precision Magneto Timing
Here is our mag timing procedure that we optimized for the Cirrus SR22 aircraft. Also included is the form that we’ve developed to help you do the calculations.
- Ensure magneto switch is off. Remove key.
- Remove top cowl.
- Remove battery ground strap.
- Remove all top spark plugs—be sure to keep track of copper gaskets.
- Find TDC of #1 cylinder.
- Identify prop blade face which will be convenient for viewing the digital protractor you will attach in the next step.
- Install protractor with double-back tape and retention tape at the end of the prop blade selected in Step 6. (Double-back tape helps to remove any associated jiggle when “bumping” the prop while checking the timing until point “open” lights.) (See picture below.) (Buy Digital Protractor here.)
- Turn prop backwards until TDC finder plug fits.
- Using the digital protractor, locate “normal” rotation stop point, noting the location angle #1. Reverse prop direction and locate stop point, noting location angle #2. Remove stop pin. Do not move prop. Perform math (#1 angle – #2 angle)/2 = TDC angle. Rotate prop to the mag-to-engine timing degree setting. Ensure protractor reading is “normalized” and note this angle number. This angle number is the BTDC angle of mag-to-engine timing. Further check that the impulse couplings have released, and install the timing indicator light tool to both mag “P” leads. DO NOT REMOVE THE “P” leads from the mags. Install the key in the mag switch and turn the key to the “BOTH” position. Install and “activate” the timing indicator unit and check the “point open” light as the propeller is rotated. Note the starting initial timing indications on the digital protractor, i.e. 22.5 degrees left mag and 21.2 degrees right mag. (Log these numbers by mag to evaluate future mag-to-engine timing drift.) Adjust one mag to proper timing of 22 degrees by using the digital protractor indication. Time the second mag to the first mag and check with the digital protractor. NOTE: Common digital protractors have .1 degree resolution. Timing to .1 degree or less is possible in “mag-to-mag” as well as “mag-to-engine” timing. Most digital protractors automatically “shutdown” after a period of inactivity. You can reactivate (turn “on”) the digital protractor to view without worrying about losing the “angle.”
- When the timing is set to your satisfaction, remove the protractor, remove the timing indication tool, ensure the mag switch is “off,” and remove the key. Check and service the spark plugs for proper gap, lubricate the threads, replace the copper gaskets, install and torque to proper specs, clean the lead ends with isopropyl alcohol, ensure springs and contact buttons in spark plugs are clean of any corrosion, debris, etc, and install. Install the ground strap to the primary battery, and install the top cowling with all the hardware (screws in front!). Perform a ground run and test the function of the mag switch, engine operation, and mag drop/split. Perform the required documentation for approval for return to service.
Note: The BTDC timing may vary by engine model. Check latest information for correct timing settings!
Download the Timing Adjustment Worksheet.