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262 USING SERVO MOTORS

misplace any parts. If necessary, make a drawing of the gear layout so you can replace
things in their proper location.
3. Remove the retainer clip at the bottom of the output gear. Doing this disengages the
potentiometer from the gear, so that the pot no longer turns when the gear does. On
some servos there is no retainer clip; the servo engages into a spline molded into the
bottom of the gear. On these you need to carefully drill out the bottom of the gear.
4. Reassemble the case.

In the following section you’ll find detailed modification instructions for the Hitec HS-
422—the same instructions apply to many other servos, such as the venerable Futaba S- 148
and the GWS S03. With the same or minor variations, the steps that follow can be applied to
similarly designed servos.

TOOLS YOU NEED

You’ll need the following tools to complete the conversion process:
• #0 Phillips screwdriver
• 1/8″ or smaller flat- bladed screwdriver
• “Nippy”: cutters, X- Acto blade, or razor saw
• Small, flat jeweler’s file

CHOOSING A SERVO TO MODIFY
The best servos, from a standpoint of easy modification:

Use a lower ball bearing or bushing that supports the output gear. At the very least,
the output gear should be supported by a molded- in ledge, rather than directly on the
potentiometer shaft.
Use a removable potentiometer shaft clip. The clip can be readily removed in order to
disengage the output gear from the potentiometer shaft. Servos that lack a removable
clip will use instead a molded- in channel that the potentiometer fits into. This is the case
of the Hitec HS- 311 and the Futaba S- 3003. If your servo is so constructed, you’ll need
to carefully drill out the bottom of the output gear in order to remove the spline, as
detailed later in this chapter.
Use a plastic output gear, rather than a metal gear. The better, heavy- duty servos use
a metal output gear. While the metal is able to handle increased stress, it’s significantly
harder to modify. If your servo of choice uses a metal output gear, a motorized hobby
tool such as the Dremel will make the job of grinding down the nub much easier.
Are normal sized or larger. Mini and micro servos are more difficult to modify. You may
ruin one or two before you get the hang of it.

STEPS FOR MODIFYING A HITEC HS- 422
The Hitec HS- 422 is a good middle- of- the- road servo: low price, available anywhere, and well
made. It employs both a top and bottom oil- impregnated brass bushing and a removable clip
on the underside of the output gear. Perfect!
Throughout the following, use care to avoid wiping off or absorbing through your skin too

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