Ring gears - What's up with 122/149 teeth?

[RViter]

What is the purpose or application of the different starter ring-gear teeth count? Why 122 or 149? Is this optional for some reason? or related to a specification? (12 vs 24v, for example). Are they either/or? Does the starter have to 'match' the gear tooth shape?

I have a prop-strike O-360-A3A to repair, evaluate, and the starter housing was cracked, too. Planned to replace it with a SkyTec or B&C lightweight, so it's not a factor. The engine was salvaged without the prop or flywheel/ring-gear, so there is no indication of which flavor it had (off Cherokee 180). So, is the 122/149 decision a factor or is there a pre-defined specification to follow on acquisition of these replacement parts? otherwise, why?

THANKS!!

gary christensen
Mesa AZ
RV-6A Finishing Kit
O360-A3A (repairable)
N234GB (reserved)
www.mykitlog.com/garyc

[Toobuilder]

IIRC, the 122 version was dropped some years ago. To answer your question directly, either ring gear will work fine on pretty much any engine, but the starter pinion should match. A mismatch between the starter and ring gear will cause accelerated wear, but beyond that it's not a huge deal.

[RViter]

Originally Posted by Toobuilder

IIRC, the 122 version was dropped some years ago. To answer your question directly, either ring gear will work fine on pretty much any engine, but the starter pinion should match. A mismatch between the starter and ring gear will cause accelerated wear, but beyond that it's not a huge deal.

Excellent clarification - about what I suspected but matching the gear configuration makes perfect sense to keep the teeth face properly aligned. I would speculate that mismatched one way, less of a factor, but the other way would be significantly more. The contact surface would shift.

THANKS - I'll be looking for a flywheel with 149 teeth on the ring gear, and specify same configuration for the new starter.

Probably too much to hope that almost all of the O-320/360 flywheels are identical as it would appear to have no reason to modify the design, and the ring-gears would be applied, as desired, over the decades. Essentially making the flywheel totally common and the gear ring selectable.

THANKS Again - Gary

[Toobuilder]

Just as a data point - the ring gear is available by itself. It can be pressed off the "ring gear support".

[pmnewlon]

Originally Posted by RViter

Probably too much to hope that almost all of the O-320/360 flywheels are identical as it would appear to have no reason to modify the design, and the ring-gears would be applied, as desired, over the decades. Essentially making the flywheel totally common and the gear ring selectable.

THANKS Again - Gary

My engine has three different ring gear support assemblies available. Lycoming has parts catalogs available for download for all their engines, which specify the different support assy and ring gear options. PM sent.