I assume you are referring to the curved "splines" of the engine starter's
Bendix Drive. I further assume that when you engage the starter all you hear is
a whirring sound of the starter motor spinning without the Bendix drive engaging
the starter ring gear.

Assuming the drive is fairly new and corrosion isn't a problem you can normally
get it to work, and keep working, by spraying the drive with silicone spray. DO
NOT use other lubricants. Silicone is the recommended lubricant, mostly because
is dries after being applied and doesn't attract dirt and grit which can cause
problems.

Good luck,

Ted Stanley - A&P-IA