Magneto Timing

Magneto Timing
• The two areas of timing a magneto are
internal, and external.
• A number of things must occur at the
same time, or in a well orchestrated
sequence for the engine to function.
Magneto Timing
• Within the magneto the magnetic rotor
must be just past neutral axis (E-Gap).
• The points must be just opening.
• And the distribution rotor must be
pointed at the correct cylinder.
Magneto Timing
• Within the engine the intake and
exhaust valve must be closed.
• The piston/cylinder that is to fire must
be close to the top of its compression
• It must have a fresh air/fuel charge.
Magneto Timing
• The selected wire(s) from the
magneto(s) distributor must be
connected to this cylinder.
• And the crankshaft/magneto must be
Magneto Timing
• If all of these conditions are met, the
cylinder will fire.
Magneto Timing
• Timing is initially set internally in the
• It is then set to the engine.
• Both “setting procedures” may call for
initial gross settings, followed by some
means for fine adjustment.
Magneto Timing
• Usually the first thing to be set is the
distributor/rotor alignment.
• Depending on the magneto, the
distributor will be indexed to the rotor
shaft during initial assembly of the
magneto upper and lower housing.
Magneto Timing
• This calls for correct alignment of the
drive and driven gears.
• There are indicator marks for this
• Most can be set for either left (CCW) or
right (CW) hand rotation.
Magneto Timing
• The next step is to install the
rotor/points cam. On some this can be
indexed infinitely, and others it will be
one or two positions depending on
Magneto Timing
• Then for most magnetos, set the rotor at
the points opening position (E-Gap)
• Adjust the points to just open.
• Move rotor to maximum opening to
verify the points range of operation.
Magneto Timing
• If the points don’t have the correct
range of operation, even though they
are in time, the coil will not charge
• Or the points will wear out prematurely.
Magneto Timing
• Once E-Gap and range are within limits,
auxiliary points may be adjusted.
• Adjustment of points will require the use
of a feeler gauge, a screw driver and a
degrees of rotation indicator.
Magneto Timing
• The points are secured with two screws,
where one attachment is slotted. By
carefully moving the slotted end with the
other screw snug it can be set, and
Magneto Timing
Magneto Timing
Aircraft Magneto Operation
• Critical Safety Note:
• Treat all aircraft engines as if they were
a loaded Gun
• MAGNETO grounding systems fail
• Stay clear of the Propeller Arc
Aircraft Magneto Operation
• Aircraft magnetos are attached to the
engine case with adjustable hold down
clamps or fasteners
• Aircraft engines use two independent
Aircraft Magneto Operation
• The gear drive must be correctly aligned
to the engine gears
• They have a spark lead for each
• To maintain proper timing each lead
must be attached to the correct cylinder
Aircraft Magneto Operation
• Each magneto must be set so that they
spark simultaneously
• They must do this when the crankshaft and
camshaft are in the correct position
Installation and Adjustment
• Upon initial installation they must
inserted with the drive gears correctly
meshed to the engine
• This is done by rotating the propeller
to No.1 cyl on compression stroke
• A timing mark on the crankshaft or ring
gear is aligned with the case split
Installation and Adjustment
Cyl. No. 1 on
compression stroke
Front of engine
Case Split
Timing marks
Installation and Adjustment
• The magneto is then set to No. 1
spark lead just firing
– This can be done by locating either a
chamfered internal tooth, or some use an
alignment pin inserted into the housing
Installation and Adjustment
Timing gear view hole
Chamfered tooth
Timing pin
Timing mark
Installation and Adjustment
• Do not rotate the crankshaft or
the magneto until all alignment
jigs have been removed
Installation and Adjustment
• Each magneto is now installed
carefully maintaining alignment
• Once they are both clamped snugly in
place a buzz box can be used to “fine
tune” the setting
• The buzzer will change tone when the
internal switch just opens
Installation and Adjustment
• The buzzer has three leads, one for each
mag, and one for ground
• The buzzer leads are attached to the Pleads outputs on the mag
• Lightly tap the magneto housing with a
knuckle for fine movements.
Installation and Adjustment
• Be sure to rotate the magneto in a
fashion that eliminates any gear or
impulse coupling freeplay
• Set the second magneto identical to
the first unless otherwise specified
• Clamp each unit down and rotate
propeller to verify accuracy
Installation and Adjustment
• Often tightening magnetos will change
their setting
• Make sure P-leads are attached and
• Once this is done attach spark plug
leads in the correct locations
Spark Plug Firing Order
• Engine firing order is the sequence of
spark events that must occur for all
cylinders to produce power.
• It is established by the engine’s crank &
Spark Plug Firing Order
• Magneto firing order is always 1, 2, 3, 4,
? Etc.
• #1 mag will always go to #1 cylinder,
then #2 mag goes to #3 or #4
depending on the manufacturer.
Spark Plug Firing Order
• Cylinder numbering is set by the
• Lycoming is with #1 in front odds on the
• Continental is #1 rear odds on the right
Spark Plug Firing Order
• Radials put #1 on top, rear bank if twin
• Odds to the rear and evens to the front
in direction of crankshaft rotation.
Spark Plug Firing Order
• For all Lycoming and Continental 4 and
6 cyl engines the firing order of the
engines are the same.
• This is due to the difference in
numbering and firing order.
Spark Plug Firing Order
• Lycoming 4 Cyl
2 1
4 3
• Firing order = 1, 3, 2, 4
• Continental 4 Cyl
4 3
2 1
• Firing order = 1, 4, 2, 3
Spark Plug Firing Order
• Lycoming
2 1
4 3
6 5
6 5
4 3
2 1
• 1, 4, 5, 2, 3, 6
1, 6, 3, 2, 5, 4
Spark Plug Firing Order
• Five, seven, and nine cylinder radials
fire odds the evens.
• Twin banks fire opposite side/opposite
bank, roughly.
High Voltage Distribution
• Generally most ignition systems
develop high voltage in the magneto
then distribute it via high tension wires.
• These wires are positively mechanically
High Voltage Distribution
• They will have an inner core of stranded
• An outer core of shielded wire.
• The attachments should ground the
shielding on both ends.
• The high voltage shielding is provided
by a 4mm thick layer of insulation.
High Voltage Distribution
• The leads going from the magneto
points to the kill switch will also be
shielded but not of the high tension
• The problem with high tension voltage is
it produces large amounts of EMF
Low Voltage Distribution
• This system is used for engines
operated at high altitude.
• The coils are suspended on the end of
the spark plugs.
Low Voltage Distribution
• The mag has a primary coil that feeds a
second primary coil at the plug end. It is
wound with the secondary coil.
• The high voltage tends to easily leak at
low atmospheric pressures
Starting Devices
• There are several types of units
designed to assist engine starting.
• The magneto is not setup very well for
the low RPM situation found during
Starting Devices
• The most common is a impulse coupler.
• This device works by having fly weights
that catch stop pins at slow RPM.
Starting Devices
• The pin stops the mag for about 20°
then lets it fly back to original timing.
• This gives it retarded timing and faster
mag speed.
Starting Devices
Starting Devices
Starting Devices
• One problem these have is the new
generation of starters and light weight
high current wiring.
• This allows the starter to spin the
engine faster than the coupler is
designed to work.
Starting Devices
• They are typically installed on the left
mag only.
• The ignition switch should account for
this and disable the right one during
Starting Devices
• Another type is the induction vibrator.
• It provides a rapid firing battery powered
spark during the timing event.
Starting Devices
Starting Devices
• The Bendix S-200 series use an
additional “retard” breaker assembly.
• This actuates an additional battery
powered ignition circuit timing to fire
very late.
Starting Devices
Starting Devices
• The vibrator section will only operate
when the starter circuit is on, the retard
points are open and the main points are
• Power comes from the starter circuit,
and the other two points will ground the
Combined Ign/strt sw
Spark plugs
Spark plugs