Ring Gear Tech

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.

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