Maintenance Matters

Timing Slick Magnetos.


One set of timing marks is on the front side of the starter ring gear. Line these marks up with the 1/16-inch hole in the starter housing. As an option, a small pin can be inserted in the hole to make it easier to line up with the marks.

Many Experimental/Amateur-Built airplanes have Lycoming engines, and most Lycoming engines come with Slick magnetos, so learning the basics of timing Slick magnetos will be useful for many amateur builders. At least once each year, you will need to check their timing and possibly adjust it, or you will need to have someone do it for you. This task is not too difficult, but it may seem daunting to someone who has never done it. With this article as a guide, and a little help from a more experienced friend the first time through, you should be able to check and adjust the external timing of your Slick magnetos without any trouble. However, just because it is fairly easy to do, does not mean that it should be regarded casually. Improper timing can lead to overheating and detonation, destroying your expensive engine. Do learn how to do it, but do be sure you have competent help the first few times you try it. Checking magneto timing is a standard checklist item for your yearly condition inspection, but there are times when you may wish to check it during the year, too. Poor mag checks point to magnetos that are possibly out of sync with each other. Decreased performance with no other apparent cause might be due to retarded timing. Increased cylinder head temperatures could be caused by timing that is too advanced. In each case a quick check of magneto timing is in order.

A pointer made from ½-inch aluminum angle points to the timing marks on the back side of the starter ring gear.

External Timing

The timing process covered here is called external timing. There is also such a thing as internal timing or E-gap setting, which will be covered in another article. External means that we are only going to be working with the external parts of the magneto, not delving into its inner workings. Checking the timing is easy to do when the cowling is off, and we’ll cover the steps to do that first. If you determine that the timing needs to be adjusted, or there is a need to remove and replace a mag, we’ll cover those points later in this article.

The engine data plate will tell you the recommended timing for your Lycoming engine. Look for it on the sump on the right side of the engine.


Here are the steps for checking the external timing of your Slick magnetos:

1. Remove the engine cowl. Most likely you will need to remove both the upper and lower sections of the cowl.

2. Remove the top spark plugs from all cylinders. Since you will be turning the engine over with the magnetos on, it is vitally important that no cylinders can possibly fire while you are doing this work. While you are at it, this is a good time to remove all the plugs and clean and gap them.

3. Put your thumb over the #1 spark plug hole (front-right) and rotate the engine in its normal running direction (clockwise as seen by the pilot). When you feel the pressure building in the cylinder, look for the markings on the back side (towards the engine) of the starter ring gear. If the engine baffles block your view of the timing marks on the back side, there is a second set on the front side that should be visible. When the #1 cylinder is at the top of its stroke, the TC (top center) mark on the ring gear should line up with the split in the engine case halves. If you are using the front marks, it will line up with the tiny hole in the starter housing. You are now oriented as to the position of the crankshaft.

Connect the red and green leads from the timing light to the left and right magnetos, then connect the black (ground) lead to any solid, grounded metal where you can make a good contact.

4. If you have not already done so, now is a good time to hook up the magneto timing light. These are readily available for under $50 from your favorite aircraft parts and tools vendors such as Aircraft Spruce, Wick’s and others. To connect the timing light, attach the red lead to the left magneto P-lead, the green lead to the right magneto P-lead, and the black (ground) wire to any grounded metal where you can make good contact. You can leave the timing light off for now if you like.

Turn the starter switch to the “Both” position to time the magnetos. Only do this after all the top plugs have been removed from the engine. Be careful not to turn the key to “Start.”

5. Many people like to make a pointer to make it easier to see where the case split lines up with the timing marks. One can be made easily from a piece of half-inch aluminum angle. Different engines may call for different pointer designs, and some people get quite creative with laser pointers, but a simple device will do the job. If baffling blocks access to the rear- facing marks, you can still use the front timing marks and simply line them up by eyeball with the timing hole in the starter housing.

6. With everything now in place, turn the starter switch to “Both” to unground both magnetos. Be careful not to turn the switch to “Start”! Be sure to have the propeller area clear when you turn the switch, just to be safe. Spinning props are dangerous things. As an alternative, you can disconnect the P-leads from both magnetos, making sure the timing light leads are still connected to the P-lead posts on the mags. However, simply switching on both mags is the preferred course of action.

Make sure that the alligator clip has a good grip on the P-Lead attaching screw—this can be tricky on the right mag, which is upside down.

7. Rotate the engine counter-clockwise past the timing marks a bit and then slowly turn the engine back in the clockwise direction until you hear a sharp snap sound. This is the impulse coupling snapping through the starting cycle. Immediately stop turning the prop and once again back it up a little past the timing marks (25_23_ etc.). Now be sure to turn on the timing light if you have not already done so.

8. Very carefully turn the engine in the clockwise direction until you hear the timing light beep and/or see one of the magneto timing indicator lights go off. Note the timing as shown by the pointer or the alignment of the front marks with the timing hole in the starter. Then keep turning the engine until the second beep starts and/or the second light goes off Again, note the timing.

This timing light works by turning the lights off and making a tone when the points open for each magneto. Some timing lights work differently. Be sure to read the instructions for your timing light.

9. Attention! Some timing lights will have the indicator lights go off when the points open and others will have the lights turn on. Be sure to read the instructions that come with your timing light and adjust your interpre- tation of the instructions in this article accordingly.

10. Look at the engine data plate, typically on the right side of the engine case, to see what the factory- recommended timing is for your engine. It will be somewhere between 20 and 25 degrees. Compare this to the actual timing of the mags you noted in Step 8. In a perfect world, the timing of both magnetos and this number would all be the same, but they probably aren’t. So now you have some work to do.

The data plate on the side of the magneto will show if it is a right- or left-hand rotation mag.

11. If one of the magnetos is timed properly there is nothing more to do with that mag, but often both mags need to be retimed. Loosen the magneto mounting nuts with a half-inch wrench or socket, whatever fits. Loosen the nuts a little bit, just enough to allow the mag to rotate in its mount. Set the engine so that the timing pointer is pointing at the correct timing number. Always be sure to approach the timing mark by turning the engine in a clockwise direction. If you go past the mark, back it up and re-approach it in a clockwise direction again. Do not approach the timing mark in the counter-clockwise direction.

12. With the pointer pointing at the correct timing mark, carefully move the magneto until the timing indicator light and/or beeper goes off. Then tighten the mag nuts. Recheck the timing after tightening the nuts to be sure the mag didn’t move.

With the spark plug wire cap removed, you will see the holes for the locking pin, shown here in the L hole for a left-hand rotation mag.

13. Repeat the operation in Step 11 for the other magneto. With both timing light wires connected and both magnetos properly timed, both lights and/or beepers should go off together exactly when the pointer is pointing at the correct timing. Always approach the timing marks from the high side and move the prop in the direction of normal engine rotation (clockwise) so that any slack is taken out of the system. It may take several passes and the readjustment of one or both magnetos to get everything just right. With experience the process will get easier.

The spark plug wires are held onto the magneto with a cap and three screws. They are easier to see with the magneto off the airplane, but you will probably want to detach them from the magneto before removing it. You need to know this if you have to use the locking pin to install the magneto.

Seal it!

In the unfortunate event that someone prior to you has used some sort of gasket cement on the magneto gaskets, you may have to remove one or both magnetos and replace the gaskets. On a new engine, you will be spared this bother, but to stop some older engines from leaking, mechanics will often use something like Permatex to seal the mating surfaces. You will know if this is the case if, even after loosening the magneto nuts, you simply cannot move the magneto to reset the timing. There will also likely be signs of gasket cement visible around the base of the magneto. If this is your situation, you will have to remove the mounting nuts; use enough force to break the magnetos loose from the engine and replace the gaskets.

You will often need to get a new magneto gasket, which is readily available from many aircraft parts vendors. To remove and replace the magneto you will also want to purchase a magneto locking pin (Slick part #T-118).

If you are very careful and a little bit lucky, when you remove each magneto, you can simply pull it out, clean the mating surface, and put it back in, exactly as it came out. For the rest of us, it will be necessary to first remove the spark plug wire harness by unscrewing three screws on the cap. With the plug wires disconnected, you can remove the magneto from the airplane and have free access to work on cleaning the magneto base and the engine accessory case. If you mark the spark plug wire cap and the magneto before you remove the cap, it will make it easier to correctly position the cap when you put it back. The screws won’t go in if it is not in the correct position, so marking the cap is optional, but it saves time if you have a mark to guide you.

When cleaning the engine accessory case opening, it is a good idea to first place a clean rag in the hole to prevent gasket material or other foreign objects from falling into the engine. Also be careful when scraping away gaskets or cement. The accessory case is aluminum, as is the magneto housing, so it is easy to gouge the metal, which is sure to cause a leak later. The safest way to remove gaskets that have been cemented in place is with Permatex Gasket Remover. This eliminates almost all the scraping. Most auto parts stores will have this product.

With everything cleaned up, it is time to reinstall the magnetos. Get new gaskets, making sure you have the correct ones. These are Lycoming parts, not Slick parts. The most popular one is #LW12681, but there are others. You can install the gasket dry, but it is easier if you lightly coat it with DC-4 dialectic grease and stick it to the magneto housings before you reinstall the mags. If you are concerned about leaks, you can use Permatex Aviation Gasket Sealer, but you will have to go through the removal and cleaning drill again every time you retime the mags. However, it may be necessary to deal with leaks.

Stabbing the Mag

Before you reinstall the magnetos, you will need to be sure the engine is set to the firing position for #1 cylinder (20 to 25 degrees before TDC), and you will need to lock each magneto into the proper position with a magneto locking pin (Slick T-118) inserted into the top of the mag. You will notice that there are three holes to choose from, one marked “R,” another marked “L,” and a third marked “X.” Check the magneto data plate to see if you have a left-hand rotation magneto or a right-hand one. Then insert the pin in the appropriate hole by carefully turning the mag until the pin drops all the way into the hole. There are bumps along the way that can fool you, but none of them will allow the pin to drop down nearly as far as the correct one. With the mag locked down and the engine at the firing position for #1 cylinder, you can now reinstall the magneto. Remove the locking pin, go through the timing steps previously mentioned, reinstall the spark plug wire cap and re-attach the P-lead.

You have now set the external timing of your magnetos, helped to insure smooth, safe operation of your engine, and eliminated timing as a possible source of any problems you may be having. Your best reference for working on Slick magnetos is the Slick L-1363F 4300/6300 Series Magneto Maintenance and Overhaul Manual. You can view the manual online at various sites or purchase it for $23.50.

Dave Prizio is a Southern California native who has been flying since 1973. Born into a family of builders, it was natural that he would become a contractor and spend his leisure time building airplanes. He has completed three: a GlaStar, Glasair Sportsman and Texas Sport Cub. He shares his love of aviation by flying Young Eagles and serving on the EAA Homebuilt Aircraft Council.

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Dave Prizio
Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed four—two GlaStars, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and was a member of the EAA Homebuilt Aircraft Council for six years.


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