We had a bit of a snowstorm last night so when I pushed the Cub into the hangar she was still frosty. I needed to do some maintenance and I was thankful for the heated hangar Mike let me use. Every 500 hours of engine time it is mandatory that we overhaul our magnetos (mags). The magneto is the apparatus that creates spark in the engine in order to ignite the gas mixture in the cylinder to create power. It is important that the spark occurs at precisely the right moment in the up and down motion of the piston. This key location is referred to as 25 degrees before top dead center (25 BTC). This is the procedure to ensure the magneto is installed on the engine at precisely the correct orientation in relation to the other moving parts. I will be as brief and clear as possible.
First of all, I removed the engine cowl, and loosened the nose bowl. The prop and spinner can stay in place. Then I loosened the bolts on the left side of the engine mount and removed the bolts on the right side in order to swing the engine. Swinging the engine allows enough room to unbolt and remove the existing magnetos. Make certain that primer lines, wires, control cables, etc. have enough play to swing the engine appropriately.
You can see that by swinging the engine this becomes a rather painless job with lots of room to work. Remove the ignition harness and p-leads from the back of the magnetos.
Now, remove all the lower spark plugs and find the compression stroke for the #1 cylinder (front right on a Lycoming) by placing your thumb over the #1 spark plug hole and swinging the prop around by hand until you hear air blowing out around your thumb. Continue the rotation of the prop until you hear the impulse coupling click. Then, stand directly in front of the starter drive and move the prop until the line indicating 25 BTC is perfectly aligned with the small dot on the starter. Now cylinder #1 is at 25 BTC which is the reference point for installing the new magnetos. It is very important that the prop does not get moved until the new mags are correctly installed.
Remove both of the old magnetos and ensure that the mating surfaces are clean from any gasket material.
This photo shows the left mag. You can tell because of the impulse coupling located below the gear (compare to right mag picture). Impulse couplings are only installed on the left mag on this engine. Impulse couplings help increase spark intensity to help during start up. This is why we always start our engines on the left mag. Remove the small plastic cap from the side of the left magneto and spin the exposed gear until a red tooth appears in the window. Center the red tooth in the window and don’t allow it to move. Lube the new gasket with engine oil and install.
This is the right Mag. (No impulse coupling)
Now, ensure the prop is still located so that Cylinder #1 is at 25 BTC and install the magnetos to the engine without allowing the gear to spin. This is easier said than done, but you can always check your accuracy by looking through the side window and observing if the red tooth is still centered, after the mag is mated to the engine. It may take a couple of tries before you manage to mate the magneto to the engine and not move the tooth. Install washers, lock washers, and nuts but only snug the nuts because you will need to loosen them again to fine tune the timing with a timing light.
It’s difficult to explain the next step with the timing light, so I made a short video. The timing light enables the mags to be timed to exactly 25 BTC. So far we have gotten the mags relatively close, but looking at a red tooth through a window is not exactly precision. I made this short video demonstrating the timing light process. I usually have to re-adjust the mags several times to get them perfect. This video shows my first attempt, it took me three more tries before I got it perfect. The fine tune adjusting is made by simply loosening the mag so that it can be spun CW or CCW a couple of degrees. Hopefully somebody will find this abbreviated mag installation helpful.