Multiple severe kickbacks really made a mess of this ring gear. A successful engine start will have to wait for a replacement and a careful examination of the timing and wiring.
Despite the best efforts of amateur builders, a kickback on the first engine start is more common than you might think. The damage is usually seen in two places-the starter main body casting gets broken or bent, or the starter ring gear gets a few teeth knocked off. Either way, you have a repair job to deal with before you are going to get your engine running. Of course, a starter kickback can happen at any time during the life of an engine, but first startup seems to be the most likely time to have a problem. Another way to wreck your starter ring gear is to mismatch the teeth on the starter and the ring gear. This can happen because Lycoming engines come with either a 122-tooth ring gear or a 149-tooth ring gear. The starter gear must match the tooth pitch or disaster will follow.
If the starter is damaged, your best bet is to simply remove it and send it back to the manufacturer for repair. Especially if your starter is new, this makes much more sense than buying a new starter and saves about half the cost. The good news is that the latest versions of lightweight starters are less vulnerable to kickback damage.
Use a Dremel tool or cutoff wheel to cut the old ring gear. Cut as deep as possible without cutting into the aluminum carrier.
If your starter is fine, but you have broken a few teeth off the starter ring gear, that thing some people call the flywheel, you have two choices. You can buy a new carrier with the ring gear installed, or you can replace the ring gear yourself. Since there is no good reason to spend a bunch of money replacing good parts, we will take a look at replacing just the ring gear. To get an idea of the dollars involved, my local engine shop, Corona Aircraft Engines, offered to sell me a new ring gear for $200 and install it for an extra $50. I found one online for $177, but I would have to add shipping to that, so $200 looks like a fair price. I searched for the complete ring gear and carrier assembly and found one on eBay for $625. After that I didn’t even want to see what a new one from Lycoming would cost. With those numbers in mind, it looks like we can rule out buying a complete ring gear/carrier assembly. We are still left with the option of paying the engine shop $50, but it isn’t that hard to do the work yourself, so let’s get to it.
The first step in the process is to buy a new ring gear. There are two choices, as mentioned before, so you need to know if you have a 122-tooth or a 149-tooth ring gear. Trying to start an engine with a 149-tooth ring gear and a 122-tooth starter motor will have you going back to the parts store to say goodbye to even more of your hard-earned money. Check the model number of your starter and count the teeth on your ring gear twice. No need to add to your frustration by mismatching parts.
With a new ring gear in hand, it is time to remove the old ring gear from the carrier. Needless to say, this means removing the propeller and the ring gear carrier assembly from the plane. Using a Dremel tool or an abrasive cutoff wheel of some sort, carefully cut into the steel ring gear as far as you can without touching the aluminum carrier. The front side of the carrier has a flange that can be damaged if you try to cut the front side as deeply as the back side. Go slowly. Check often. Do not get impatient. Do not cut into the carrier.
When you have cut the old ring gear as far as you dare, get a cold chisel and a hammer. This is one of the very few times when a big hammer is the proper tool. Place the cold chisel so it will cut into the edge of the ring gear at the point where you have already cut it, and give it several good whacks. With luck the ring gear will snap open and fall off the carrier. If it does not, you may need to work up the courage to cut the ring gear some more, again working slowly and carefully. It is also better to use a heavier hammer rather than a lighter one, and don’t forget to wear safety glasses. It will eventually work, and the ring gear will come off.
The bare carrier should now be placed in your freezer for 20 to 30 minutes. While you are doing this, you need to put the new ring gear in your oven for a similar amount of time at about 450. The chamfers leading into the gear teeth should be on the side facing up, so it will be ready to place on the carrier. This will shrink the carrier and expand the ring gear enough to overcome the interference fit that the parts have at room temperature. While you are waiting, be sure to prepare your work area for the mating of your parts, and make sure you have a clear path from your oven to the work area. Place two blocks of wood to support the edges of the ring gear and get your tools ready. You will need a medium-sized hammer and a pair of pliers or hot pads for carrying the heated ring gear.
The new ring gear is ready to replace the damaged gear on the ring gear carrier assembly. Make sure the new ring gear is perfectly clean before heating it up. Any remaining residue on the ring gear will really make a bad smell in your oven.
When sufficient time has passed, get the carrier out of the freezer and place it on the blocks on your workbench. Double check to see that the mating surface is perfectly clean. Now the moment of truth. Retrieve the hot ring gear with the side that has the chamfered cuts leading into the gear teeth facing up. (It should have already been placed in the oven that way.) If the chamfered cuts end up on the front side, your starter will not properly engage into the ring gear teeth. That means that you will have to cut off your new ring gear and start the whole process over.
Quickly but carefully carry the ring gear over to the carrier and drop it into place. A few taps of your hammer should help to set it firmly against the flange. It is all over in a few seconds. Double check around the perimeter of the gear to make sure it is fully seated into position. With the new gear in place let the assembly cool for a few minutes before you move it.
Use some pliers or hot pads to handle the ring gear after getting it out of the 450 oven. Work quickly but carefully. There is no second chance to get the ring gear on properly.
Now that you have a new ring gear, it’s time to put it back on your plane. Be sure to line up the “0” on the carrier with the similar mark on the crankshaft flange. You can double check by moving #1 cylinder to top dead center and verifying that the “TC” mark on the carrier lines up with the timing hole in the starter on the front side or the case split on the back side. The engine will start and run just fine if the carrier is installed with the wrong orientation, but you will have no way to set the timing of your engine in the future. Be sure to check this out before you install the prop.
The propeller can be installed once the ring gear assembly is in place. Consult your propeller manufacturer for proper installation instructions including proper torque for prop bolts. Be sure that you have positioned the alternator belt before you bolt on the prop. There is no way to get the belt on without removing the prop if you forget. Set the belt tension after the prop is installed.
Now that everything is back together, the task of figuring out why the engine kicked back presents itself. The most likely culprit is improper ignition timing, but there are some other potential causes, especially when dealing with a first engine start. In any case check your ignition timing before doing anything else.
When reinstalling the ring gear assembly, be sure to line up the “0” on the carrier with the mark on the crankshaft flange.
Your ignition switch could be grounding out the wrong magneto, or it could not be grounding either magneto. Standard practice dictates that the right magneto is grounded out, so the left magneto-the one with the impulse coupling-can start the engine with the timing retarded to top dead center instead of the typical 20 to 25 degrees. Check your wiring to make sure this is properly configured. If you have the standard ACS ignition switch, be sure that the jumper is installed between the R terminal and the adjacent GND terminal. If you have electronic ignition, be sure to go over the installation of your ignition components and follow the manufacturer’s recommendations for setting the timing properly.
Poor technique can sometimes be the culprit. Be sure to firmly engage the starter switch and do not release pressure until you are finished cranking. Sometimes people will be rather tentative when engaging the starter and will allow it to momentarily stop cranking and then re-engage. This is an open invitation to starter trouble. This same problem of tentative or intermittent engagement can be traced to problems with the starter circuit wiring or the starter relay. Any poor contact in the relay or the circuit could cause poor starter engagement and a possible kickback.
If you have the standard ACS ignition switch and are using two magnetos, be sure to install the jumper between the R terminal and the adjacent GND terminal. This way the right magneto gets grounded out when starting. Consult the manufacturer of any electronic ignition system for their installation instructions.
Lightweight, or what some people call low-inertia, propellers are a secondary contributor to kickback problems. They do not directly cause kickbacks, but their low inertia does not put up much resistance to a kickback. There is no suggestion here to avoid these props. Just be aware of the heightened need to be careful about avoiding things that could cause a kickback if you use a lightweight wood or composite prop.
Proper timing, proper wiring, and good technique should eliminate starter kickbacks entirely, but if you are still concerned, there are starters designed specifically to reduce kickback damage. Sky-Tec makes the HT-series of starters with built-in kickback protection. This feature will add about $50 of extra cost and 10 oz. of weight compared to one of their standard LS-series starters. Hartzell also has a starter line with kickback protection called the E-Drive series.
Of course, ring gears do sometimes need to be replaced because they simply wear out. Lycoming Service Instruction 1141A specifically addresses this issue and includes basic information for removing and reinstalling the ring gear. We hope that you can enjoy your plane long enough to wear out a ring gear. But for new builders facing their first engine start, please make sure you have done what you can to avoid a damaging kickback at initial start-up.