Update: Lycoming Service Instruction 1141_A has a wonderful collection of useful information regarding Lycoming ring gears. The instructions detail the replacement procedure but there's also a nice table on the second page that lists all ring gear P/N's and their compatible ring gear support assemblies.  DO NOT mix and match ring gears and ring gear support assemblies.  Why?  Read on...

Ever wonder how to tell the difference between the two Lycoming ring gear types without having to count every tooth?

Here's how to tell at a glance:

Occaisionally, an experimental aircraft builder, given a choice of ring gears, will inquire as to which one is "better" generally wondering if the reduced number of teeth of the 122 would allow a faster cranking RPM, etc.  Well the answer is "absolutely not."  You must also consider the number of teeth on the starter drive pinion will be different when matched to each ring gear and effectively, the difference ends in a near perfect wash.  

The rhyme or reason as to why Lycoming chose one ring gear over the other for particular engines remains a complete mystery to all.  One might attempt to make the argument that the sturdier teeth on the 122 tooth gear was selected for the 720 models.  But the same person would be at a loss to explain why the same ring gear was also used nearly exclusively on O-235 and O-290 engines and occasionally on -320 and -360 motors as well. 

Here's what we have experienced broken down by engine series:

The bottom line:  COUNT 'EM!  We sell a starter for either gear.  But just because your aircraft (or engine) is SUPPOSED to have a certain ring gear doesn't necessarily mean it does.  You need to count them to confirm and order the starter to match.  The first three numbers of a Sky-Tec Lycoming starter model will always be the number of teeth on the ring gear it is compatible with.

Why you cannot shrink a 149-tooth ring gear on a hub designed for a 122-tooth gear

So What's compatible with what?

We have yet to find a comprehensive list of ring gear/support assembly compatibilities.  SI1141_A does list the following but these appear to be a mash-up of assemblies of the sub-assemblies (ring gear + ring gear support assembly) P/N's (not terribly helpful in this context).  Why are there so many!?  Can't say either.  But for what it's worth...

Why it's a problem with SOME starters, but not necessarily OTHERS:

When Sky-Tec designed the 149-NL starters, we worked very closely with Lycoming to "toe the line" - that is, what we refer to as the "DMZ" - the imaginary line a starter pinion should never cross.  Sky-Tec holds the forward location of all NL starter pinions RIGHT AT (within  0.010") of that line.  Doing so ensures the NL engages AS MUCH ring gear as possible when it is engaged.  All Bendix-type starters and some other Sky-Tec starters (LS/PM models for instance) do not toe that line as closely.  So in some cases, the problem of a mismatched ring gear/ring gear support assembly is ONLY noticed after switching to an NL-type starter.  Solutions:  1) Use the proper combination of ring gear/ring gear support assembly for your engine, 2) use a starter that allows more forgiveness with regard to compensating for the mismatch.

Why this problem is PREVALENT with Superior Ring Gear Assemblies:

It would appear that Superior reverse engineered the 60882-type ring gear support assembly (for 122-tooth ring gears) when they manufactured and PMA certified Superior Ring Gear Support Assembly PN SL77579-1 Rev.11 (for 149-tooth ring gears).  Unfortunately, when pairing those ring gear support assemblies with 149-tooth ring gears, the ring gear rests further aft than Lycoming specification and therefore tends to crash the front face of NL-type starters by a mere couple thousandths of an inch.  Other starter models tend to stand back sufficiently from the ring gear so the mismatch is not made known.  But the NL "hugs the line" defined by Lycoming and the mismatched ring gear/support assembly causes a microscopic conflict as a result.

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