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Ring Gear Tech

 

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:

 

122 Tooth 149 Tooth
Note the "V" geometry between teeth Note the "U" geometry between teeth

 

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:

235 - 122 Tooth

290- 122 Tooth

320 - Varies.  Older aircraft seem to have an increased likelihood of having a 122-Tooth ring gear while more recent aircraft might have a 149-Tooth ring gear on 320-series engines.

360 - Varies but usually 149 Tooth

540 - 149 Tooth (but there are exceptions)

720 - 122 Tooth

 

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

 

On left:  Proper 149-tooth Gear (PN 72556 or 10551) shrunk onto

proper ring gear support assembly (PN 74414, 76628, 10552, or 11837)

 

On Right: Proper 122-tooth Gear (PN 60882) shrunk onto

proper ring gear support assembly (PN 68867 or 76899, or 76944) 

 

What happens when a 149-tooth gear is shrunk onto a PN 6867 ring gear support assembly.

See how the ring gear face sits further aft (upward in the photo) than the support assembly?  

Proper 122-tooth Gear (PN 60882) shrunk onto

proper ring gear support assembly (PN 68867).  

See how the face of each is flush with the other? 

 

Proper 149-tooth Gear (PN 72556) shrunk onto

proper ring gear support assembly (PN 74414).  

See how the face of each is flush with the other? 

Why it's a problem:  Note the difference in:

a) The flange thickness, and

b) The depth of each type ring gear.

Another view of the difference between the two assemblies: 

149-tooth ring gear/support assembly on left

122-tooth ring gear/support assembly on right.

 

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...

 

Ring Gear and Support Assembly Part No.

Gear Tooth Pitch

Ring Gear and Support Assembly Part No.

Gear Tooth Pitch

Ring Gear and Support Assembly Part No.

Gear Tooth Pitch

68867

10/12

31M22194

12/14

LW-11572

12/14

74321

10/12

31M22246

12/14

LW-12226*

12/14

74460

10/12

31M22350

12/14

LW-12227

12/14

75550

10/12

31M22782

12/14

LW-13381*

12/14

76173

10/12

31M22972

12/14

LW-13382*

12/14

76944

10/12

31M22993

12/14

LW-14167

12/14

78652

10/12

77236

12/14

LW-15882

12/14

LW-11380

10/12

72245

12/14

LW-16064

12/14

LW-13675

10/12

72899

12/14

LW-16470

12/14

LW-14011

10/12

74329

12/14

LW-16471

12/14

LW-18695

10/12

74414

12/14

LW-16581

12/14

LW-19151

10/12

74977

12/14

LW-16635

12/14

31M19420

12/14

75030

12/14

LW-16795

12/14

31M19717

12/14

75221

12/14

LW-16904

12/14

31M19861

12/14

76628

12/14

LW-18506*

12/14

31M21308

12/14

74977

12/14

LW-18633

12/14

31M21951

12/14

LW-10184

12/14

LW-18736

12/14

31M21994

12/14

LW-10552*

12/14

LW-18768

12/14

31M22028*

12/14

LW-11151

12/14

LW-19163

12/14

31M22045

12/14

LW-11519*

12/14

--------------

--------

* Reverse rotation

 

 

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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