Starter damage, ring gear and broken teeth.

 Gremlins of a particular species like to break off ring gear teeth and kill starters.

Improper clearance between the starter and ring gear.  This is attributed to a known issue with new Superior ring gears and the Skytec 149NL starters having zero clearance. Destroys starters.

Flipped trigger wire on an EI coil.  This caused the wasted spark to occur on the bottom of the intake stroke causing a kick back.

Starting the engine without sufficient fuel pressure/momentarily bumping the starter switch.

Timing not retarding or advanced when starting and light weight props.

Only missing one tooth this time.

The first issue was resolved by replacing the starter.  Oddly, the issue didn't seem to occur until after several initial engine test runs.  I'd started the engine, was wearing a non-ANR headset and could hear a clattering ring ring sound.  I shut the engine down and quickly found the starter had not disengaged.  It shelled a bearing.  Although it had only been used a few times, it was out of a two year warranty.   A call to tech support at Skytec quickly diagnosed the issue was due to the dimensions that Superior and Skytec used to make these components resulted in no clearance between the gears.  A shim would be required to increase the clearance so that the starter would disengage properly.  

Aeroperformance aka Aircraft Spruce near me had starters in stock so I ordered one.  The price had gone up considerably since I'd purchased one several years ago  There is also a refundable core charge that is dependent on Skytec inspecting the core.  

I made a pattern from the starter's mating surface and fabricated a .020" thick shim out of 2024T3.  The starter was mounted and clearance checked based on instructions from some hot rod web sites.   The starter then disengaged properly after that.  

Shim for starter

So, back to first engine runs... The engine would start fine but then one day when starting....a nasty crunch & kickback sound indicated there were still gremlins.  Ultimately, this cause was attributed to one of the EI coil trigger wires being transposed causing a spark at the bottom of the intake stroke.  

All was well for months, but then one day when for some reason (distraction), I'd neglected to turn on the fuel pump(s) and I'd usually also been able to start the engine by just bumping the starter switch. This is my theory, but apparently when the fuel pressure is low and the injectors fire, only a minuscule amount of fuel enters the intake and if the cylinder that's on the bottom of the its stroke, finally accumulates enough fuel to fire, the engine kicks back.   Since that episode I ensure the fuel pumps are operational before starting and I keep the starter engaged until the engine cranks up.  It is odd that the shear pin in the starter didn't let go before breaking a teeth off the ring gear.  Fortunately others with the same EI/EFI and similar engines haven't had this problem.

The flywheel and ring gear have to pulled off the engine.  I asked some local pros if they'd done this before but no luck.  Searching the interwebs for replacing automotive ring gears found several methods.  One was to heat the ring gear with a hot torch to expand it and it would either fall off or with a few pops with a hammer, come off.  I tried this initially but after paint started blackening decide to try the cold chisel method.  Since this method breaks the ring gear, it made sense to help it out by cutting a slice as deep as possible without damaging the surface under the ring gear.  There is a lip that the ring gear is against so a round cut off wheel can only go so deep, but weaken the gear.  The assembly needs a very solid surface so that the chisel strikes are effective.  Place the ring gear retaining lip up so that the ring gear is driven away from it when it is struck from above. The chisel head is placed in the slot that was cut. A 3 lb. hammer, large cold chisel, gloves and eye protection are prudent.  I tried multiple light strikes initially but didn't make any progress.  Experience revealed on the last two ring gears, that it took three solid whacks to break and depart the ring gear. The ring usually moves a little on the first whack, more on the 2nd and breaks on the third. It comes off faster than a scared cat so heavy gloves are needed to protect your hands.  

Cold chisel and slit cut in the ring gear.

  

3 whacks later.

Installing the new gear is easy.  It's important to verify the tooth count is the same.  The U shaped teeth are on the 149 count gears and V shaped are on the 122 count gears.  The ring gears teeth are machined so it's important to mount the gear with the machined side towards the starter oriented away from the retaining lip.  Make sure all of the mating surfaces are clean and free of burs.  Heat the new gear (oven works fine) to 375 - 400F.  Put the other part in the freezer.  When you're ready...and wear gloves to handle the hot ring,  drop the ring gear on.  It is so loose it spins around but in less than a minute, it will have shrunk and secured itself.

The ring gear will fit in a countertop oven.

Fortunately I haven't had any issues for awhile but will keep watch for those gremlins.

Update - I installed a TVS diode across the starter motor relay contacts, suspecting that the starter motor EMF was causing a spike that caused an ignition spark on the bottom of an intake stroke.  The kick back seems to happen when the starter switch is released.