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MAGNETOS: TIMING, REMOVAL AND REPLACEMENTFrom CP55, Page 9 (April, 1988)
here at RAF is confined almost exclusively to the Slick magnetos due to Slicks being
easier to fit in the confines of an EZ, also due to their being lighter in weight. Since
most EZ flyers will have Slicks installed, this discussion will concern only the Slick
We will start out with the installation of the magneto since this
may be the most confusing area, leading to the most starting problems, etc. based on the
calIs and letters we receive. The older style, 4050 and 4051, "throw away"
models probably should be traded in on the newer 4250 and 4251 rebuildable mags. They are
more reliable, more readily obtainable, and are easily repaired or rebuilt even by
the owner/buiIder. Basically the differences between the original "throw away"
and rebuildable mags is size. The "throw away" being smaller than the
rebuildable. Also, when timing the magneto, prior to installing on the engine, you must
"spark out" the "throw away" model by spinning the timing gear to set
the magneto on cylinder number one. On the newer, rebuildable magnetos, Slick supplies a
little "pin", a T-118 timing pin, which is used to set the magneto timing to the
number one cylinder (See photo).
With the distributor cover off,
look into the forward end (on a EZ!) of the mag, you will see two holes in the
plastic molding, the top one marked for left hand rotation (L), the bottom on for right
hand rotation (R). Look on the data plate on the body of the mag for its direction of
rotation. Left rotation is normal for a Lycoming 0-235, 0-320, or 0-360. Now, gently
push the timing pin into the hole marked (L) until it bottoms. Rotate the timing gear on
the shaft of the mag opposite the direction of normal rotation until you feel the
pin drop into a hole.
If you have to rotate the magneto very far, you will feel the
timing pin trip over a bump inside the mag. Don't force it to rotate. Gently back the
timing pin out a 1/4" or so to clear the bump, rotate the shaft and gently push the
pin back in. Continue rotating until the pin locates in the hole. The magneto is now
internally set on cylinder number one. It is not a bad idea to tape the pin in place with
a piece of masking tape. In any case, the pin must remain in this position, without the
distributor cover installed, until
the magneto is actually in place on the accessory case.
Now, you must set your engine at 25 degrees before top dead
center on the number one cylinder (or whatever angle your data plate calls out, 28' for
0-235-L2C). Remove the top spark plug from the number one cylinder, hold your thumb over
the hole and rotate the engine in the direction of normal rotation until you feel pressure
under your thumb. Continue rotating the crankshaft until the advance timing mark (20
degrees, 25 degrees, or 28 degrees, check your data plate) is exactly opposite the small
hole located at the 2 o'clock position on the front face of the starter housing. (This is
for Lycoming engines with a starter and starter ring gear installed.)
NOTE: If the prop is accidentally turned in the direction
opposite normal rotation, you must repeat the above procedure since accumulated backlash
in the timing gears will make the final timing incorrect.
At this point, the engine is ready for assembly of the magnetos.
With the timing pin still in place, carefully fit the magneto into its hole. When it sits
flush on the machined surface of the accessory case, pull the timing pin out (rotating the
prop at this point may shear the timing pin off) and, while holding the mag firmly
in place, install the toe clamps, flat washers, lock washers and nuts and tighten until
finger tight. Repeat for the other magneto, being certain that the prop has not moved.
Use a battery powered magneto timing light such as a model E50
from Eastern Electronics (available from Spruce). Connect it to a convenient engine case
bolt (ground) and to each magneto terminal (the same stud your mag switch is connected
to). If the mag switches are wired up, you will have to make both mags "hot"
(mag switches to the normal engine running position even though the distributor cover is
not yet installed). Make sure the fuel valve is off and the mixture is at idle cut off,
and always treat the prop as you would a loaded gun!)
Rotate each magneto in its housing until the timing light comes
on. Now slowly turn it in the opposite direction until the light goes out. Slowly turn the
magnetos forward again until the timing light just goes on. Tighten the nuts a
Now, back the prop off enough to turn both timing lights off.
Slowly bring the prop back in the direction of normal rotation until both lights come on.
They should come on simultaneously, or very close to it. Now check and see if. the
appropriate timing mark on the starter ring gear is in perfect alignment with the hole in
the starter housing. if it is, tighten the magneto hold-down nuts firmly (maximum torque
is 150 inch/lbs., minimum is 110 inch/lbs.). Recheck that the timing lights come on
together at the proper time and you are ready to install the distributor caps. If you are
working on a Long-EZ, this is the hard part! The distributor covers are so close to the
firewall that a 90' screwdriver must be used on the standard Slick screws. Believe it or
not, this can take an hour or more to do! The Allen head screws Mike called out in CP 54
make this job easy (less than 30 seconds per screw) and he still has a supply of stainless
steel Allen head screws suitable for this job. Send $1.00 plus a SASE for 6 screws.
There you have it! If your airplane has a Lycoming engine and
no starter or starter ring gear installed, you will need a timing indicator such a model E25 and a top dead center locator (both available from
Aircraft Spruce) or an equivalent protractor-type indicator.
This type indicator fits onto the spinner or prop (does not need
to be centered) and has a weighted pendulum-type pointer. Use the top dead center
finder in the top spark plug hole on cylinder number one, set the protractor indicator so
the pointer points at O' or top dead center (TDC), then turn the prop backwards to about
35' before TDC, then come slowly forward to 25' (or 28') to be certain to get rid of all
If you have a Continental engine with Slick mags (0-200 the main
difference is that the crankshaft flange on Continental engine is marked every 2 degrees
from 24 degrees to 32 degrees. You must look on your data plate to determine which to use
(0-200 is 281 BTDC). There is also a mark for TDC. It is a line across the edge of the
prop flange between the letters TC.
You will need to make a triangular aluminum pointer on which you
must scribble an index line that is perpendicular to the base and passes through the apex.
The base of this metal pointer should be placed on the machined front surface of the
crankcase with the index line exactly on the split in the crankcase halves. Rotate the
prop in the normal direction of rotation until the index line points at the 28' mark
(0-200A). This sets the engine with the number one cylinder at 28' before top dead center
which is the point at which you install the Slick magneto (which is also timed to the
number one cylinder) per the instruction for the Lycoming.
If you have an older 4051 Slick mag that needs to be
"sparked out", remove the bottom vent plug. The distributor cover must be
installed and you must find the high tension lead marked TI or BI on the spark plug nut.
Hold the lead wire spring 1/16" to 1/8" away from the magneto body and turn the
impulse coupling one "click" at a time until you see a strong spark jump between
the spring and the magneto body. Stop turning the shaft right at the point where the
impulse trips and the spark occurs. You may have to do this several times to get it
correct. It will not shock you if you do it right!
Now , reverse-the rotation about 25 degrees until you can see the
timing pin hole through the vent plug hole. Insert the timing pin which will hold the
rotor and line the pin up with the center of the vent plug hole. Now install the magneto
onto the accessory case.
On a 4050 Slick mag with no impulse, you must turn the shaft vigorously
counterclockwise (LH rotation) until a strong spark snaps from the spring to the magneto