A servo arm must match the spline count, the case size, and the throw you need. Standard servos often use a 25-tooth spline, while micro servos use smaller counts. A longer arm gives more throw but less force. A bench tester centers a servo without a radio. This guide shows how to match horns, extensions, and testers to your build.
How do you match a horn to a servo spline?
Count the spline teeth on the output shaft before you order horns. A standard servo often has 25 teeth, and the horn must match that count.
A 25T metal arm in 1.5 inch length fits Futaba, TowerPro, and Power HD standard servos. A horn for a 21-tooth micro servo will not press onto a 25-tooth shaft.
What arm length should you use?
A 1.5 inch arm gives fast response with less throw on a 25T servo. It suits ailerons and elevators on sport planes.
A 2.25 inch arm gives more throw for large control surfaces. It cuts force at the linkage, so check that the servo still has torque to spare.
What does a servo tester do?
A bench tester drives a servo through its full travel without a receiver or radio. It centers the servo so you can set the horn at 90 degrees.
A tester runs on 4.8 to 6 V and tests one servo at a time. It shows a dead servo in seconds, before you install it in a model.
Which servo extension length do you need?
A 6 inch extension reaches servos in a wing root on most foam planes. It uses 26AWG wire, which suits standard servos.
A 12 inch extension reaches tail servos on larger planes. Use 22AWG wire for long runs and high-torque servos to cut voltage drop.
How do you set up linkages with horns and arms?
Set the servo horn at 90 degrees to the pushrod at neutral. That gives equal throw in both directions from center.
A control linkage kit adds horns, stoppers, and pushrods in one pack. It fits RC airplane control surfaces with standard servo arms.
How do you test a servo before flight?
Plug the servo into a tester and sweep it from end to end 3 times. Listen for grinding, which means stripped gears.
Check centering after each sweep: a good servo returns to the same spot. A servo that drifts needs replacement before it flies.
Frequently asked questions
What spline does a standard servo use?
Most standard servos use a 25-tooth spline. Micro servos use smaller counts, so count the teeth before ordering horns.
Can I use a JR extension with a Futaba servo?
Yes, if the plug fits your receiver. Universal extensions fit both JR and Futaba style connectors.
Do I need a servo tester if I have a radio?
A tester is still useful. It centers a servo on the bench without powering the whole radio system.
How long can a servo extension be?
A 12 inch extension works for most planes. For runs past 18 inches, use 22AWG wire to limit voltage drop.
Parts used
25T Extended Metal Servo Horn, $7.99. Extended 25T metal servo horn arm for RC car, boat, and airplane servos. Red or black. Ships from NY, USA.
4-Point Aluminum Servo Arm Horn Set with Rudder Angle Ball Link, RC Aircraft, 2-Pack, $8.37. 4-point aluminum alloy servo arm horn set with rudder angle ball link for RC aircraft, racing drones and airplanes. Two triangle horns plus glass fiber plate, ball link and hardware. Ships from NY, USA.
FPV Pan/Tilt Servo Gimbal Mount (SG90 / MG90S), $8.09. Nylon dual-axis FPV PTZ gimbal for RC planes and drones. Anti-vibration, fits 9-12g servos. With or without SG90/MG90S servos. Ships from NY, USA.
2-1/4in Metal Servo Arm, 25T, $11.99. 2-1/4in metal servo arm for 25-tooth standard servos. Maximum throw for big control surfaces. Fits Futaba and compatible brands.
RC Servo Tester, $15.99. Bench servo tester for RC servos.
Control Linkage Hardware Kit, $21.95. Control horns, pushrod linkage stoppers, and push rods for RC airplane control linkages.
JR-Style Servo Extension 6in, 26AWG, 5-Pack, $5.70. Five 6in servo extensions with JR/Hitec/Spektrum male connectors on 26AWG wire. For JR-style servo leads.
Universal Servo Extension 6in, 26AWG, 5-Pack, $7.20. Five universal 6in servo extensions on 26AWG wire. Works with Futaba and JR style connectors.
Universal Servo Extension 12in, 26AWG, 5-Pack, $8.40. Five universal 12in servo extensions on 26AWG wire. Reaches servos in wings and tail sections.
Heavy-Duty Universal Servo Extension 12in, 22AWG, 5-Pack, $5.99. Five heavy-duty 12in universal servo extensions on 22AWG wire. Thicker wire for long runs and high-torque servos.
Servo Arms, Extensions and Testers
Related buying guides on Quality RC Parts
How to Fit Servo Arms, Horns, and Testers for RC Use
How to Size Servo Extensions, Arms, and Testers for RC
How to Choose the Right Servo for Your RC Plane or Car
Browse all the guides on the Quality RC Parts Drone Guides hub, or start at the Quality RC Parts store.
Sources
Quality RC Parts stocks parts in New York, USA and ships in-stock orders in 1-2 business days.
Last updated: 2026-11-10.