Just read about the number of teeth in various servo brands and thought it way be worth sharing.
this is copied from a few posts in RC groups.
What I know for sure is that Futaba standard size servos use splines with 25 teeth. With a 4-arm servo horn it is easy to find an arm that would be perpendicular to the pushrod without using much subtrim, if any.

Hitec uses spline with 24 teeth. So no matter how you turn the servo horn it is always offset to the same degree. Their newest servos use a 25-tooth spline. I have no idea if it is identical in size to Futaba's spline.

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Hobbico and Tower Hobbies servos are either rebranded Hitec or Futaba servos, so the above said regarding Hitec and Futaba servos apply, which one depends on the OEM.
JR servo spline has 23 teeth and it is nearly compatible with the older Airtronic servos. There is may be a slight size difference but usually JR and and Airtronic servo arms are interchangeable.
So, a standard JR servo spline has 23 teeth.
Most Hitec servos' spline has 24 teeth (standard, mini, micro).
A standard Futaba servo spline has 25 teeth.
A Hitec HS-55 servo spline has 15 teeth  Also compatible with Futaba S3154 and other S31xx (all S31xx, or only some of them?).

The take home message is that if you use incompatible servo arms, the teeth will strip under stress leading to a crash.
So be wary of MIXING & misMATCHING.
Avijit

Yep, seen it cost someone dearly 

http://www.rcindia.org/servos-gyros-and-all-electronics/the-importance-of-servo-arms/

Quote from: avijit17basu on October 20, 2009, 03:37:45 PM
Just read about the number of teeth in various servo brands and thought it way be worth sharing.
this is copied from a few posts in RC groups.
...
...
The take home message is that if you use incompatible servo arms, the teeth will strip under stress leading to a crash.
So be wary of MIXING & misMATCHING.
Avijit

Good Information. Thanks