How to Choose RC Servo Spline Size: 7 Steps That Work

Table of Contents

• Why Servo Spline Size Matters for Racing Performance
• How to Count Servo Spline Teeth Step by Step
  • Step 1: Count the teeth accurately
  • Step 2: Measure the spline dimensions with calipers
  • Step 3: Cross-reference with the servo model and manufacturer documentation
  • Step 4: Separate spline size from case size and screw size
• RC Servo Spline Size Chart: Common Standards and Tooth Counts
  • How to use the chart
• Identifying Spline Type From the Servo Model
• Servo Horn Compatibility: Matching the Arm to Your Output Spline
• When You Need a Servo Spline Adapter (and When You Don't)
• Choosing Your Servo Arm or Horn Before You Order
• Frequently Asked Questions

Last Updated: October 28, 2026

Choosing the wrong servo spline is one of the most common ways a racing build goes sideways before it ever hits the track. A servo arm that looks right can still slip, strip, or refuse to seat on the output shaft. This guide explains how to match your servo and arm.

Here is the core idea: spline size is a count of teeth, not a measurement of how big the servo is. Two servos can look nearly identical from the outside and still need different arms. The good news is that you can identify the right spline with a few simple checks and a clear process.

Why Servo Spline Size Matters for Racing Performance

A servo spline is the set of teeth on the output shaft where the servo arm or horn locks on. When the tooth count doesn't match the arm, the arm can't seat properly.

How to Count Servo Spline Teeth Step by Step

Counting teeth is the most reliable way to confirm a spline, but tooth count alone is only one part of the identification puzzle. Use this three-part workflow, count, measure, and cross-reference, to avoid ordering the wrong arm.

Step 1: Count the teeth accurately

1. Remove the servo horn or arm and its retaining screw. Work over a tray so small screws and washers don't disappear.
2. Look straight down at the output shaft under good light. A magnifier or phone camera zoom helps on small micro servos.
3. Pick a starting tooth and count every raised tooth around the full circle once. Do not count the gaps.
4. Rotate the shaft slowly as you count so you don't double-count or skip teeth at the back.
5. Write the number down and repeat the count a second time. If the two counts differ, count a third time.
6. Compare your count to the arm's listed spline (for example, 23T or 25T).

A common mistake is counting only the visible half of the shaft. Rotate the shaft slowly and count the whole ring so you don't double up or miss teeth. Another mistake is counting the valleys between teeth instead of the teeth themselves, pick one method and stay consistent.

Step 2: Measure the spline dimensions with calipers

Tooth count alone doesn't tell the whole story. Two splines with the same count can still differ in diameter or tooth profile. If you have calipers, measure the outer diameter of the spline across the teeth (major diameter) and note it next to your tooth count. For a more complete picture, also measure the minor diameter across the valleys if your calipers can reach them.

Measure in millimeters and keep the reading consistent. Take the measurement at the same point on the shaft each time. Mixing units or measuring at different points is a frequent source of wrong-fit orders. Record the tooth count, major diameter, and servo model number together, those three data points are what you'll use to confirm compatibility.

Step 3: Cross-reference with the servo model and manufacturer documentation

The servo model number is a fast shortcut. Look up the model on the manufacturer's site or in the product manual and check its listed spline. Then confirm that number with your physical count and measurement. Use the model as your starting point, not your final answer, manufacturers occasionally revise output shafts without changing the model name, and clone or rebranded servos may not match the original spec.

If you can't find the model online, trust your count and measurements over a guess based on the brand or box. When the manufacturer's documentation is unavailable, post the tooth count and diameter in a reputable RC forum or ask the retailer's support team before ordering.

Step 4: Separate spline size from case size and screw size

One of the most common sources of wrong orders is confusing three different dimensions:

• Spline size is the tooth count and diameter of the output shaft where the arm mounts.
• Case size is the physical footprint of the servo body (for example, standard, mini, or micro). Case size does not determine spline size.
• Screw size is the thread of the retaining screw that holds the arm on the shaft. A matching spline with the wrong screw thread will not clamp securely.

Check all three before you buy. A servo that looks identical to your old one from the outside can still have a different spline, case, or screw thread.

RC Servo Spline Size Chart: Common Standards and Tooth Counts

Many racing servos use a few common spline standards. The chart below summarizes the tooth counts, approximate spline diameters, and typical horn compatibility you are most likely to run into. Always confirm against your specific servo, because manufacturers don't always label splines the same way and dimensions can vary slightly between production runs.

Schumacher Aerox AX062 7075-T6 alloy clamping servo arm - 25t Futaba →

If your count falls outside these common values, don't force a fit. Look up the exact servo model and confirm the spline with the manufacturer's documentation before buying. For uncommon tooth counts, a spline adapter or a replacement output gear may be the only practical route, check that the adapter is listed for your exact spline pair and that it does not introduce excess play.

How to use the chart

1. Find your servo model in the manufacturer's documentation and note the listed spline standard.
2. Count the teeth on your actual output shaft and compare to the chart.
3. Measure the major diameter with calipers and compare to the approximate range.
4. Match the standard name and tooth count to the arm or horn listing before ordering.

When the chart and your measurements disagree, trust your measurements and the manufacturer's documentation over the chart. The chart is a starting reference, not a substitute for confirming the specific servo you own.

Identifying Spline Type From the Servo Model

The servo model number is a fast shortcut. Look up the model and check its listed spline, then confirm that number with a physical count. Use the model as your starting point, not your final answer.

If you can't find the model online, count the teeth directly. Trust your count over a guess based on the brand or box.

Servo Horn Compatibility: Matching the Arm to Your Output Spline

Once you know your tooth count, pick an arm that lists the same spline. Pay attention to the arm's hole positions and offset too, because those affect steering geometry as much as the fit does.

For a 25-tooth Futaba-style servo, the Schumacher Aerox AX062 7075-T6 alloy clamping servo arm - 25t Futaba is listed as a 25T Futaba spline arm built from 7075-T6 alloy. For a 23-tooth layout, the Schumacher AX010 23t Spline Servo Arm - LD/2/3 is listed as a 23T spline arm. Confirm your count against the listing before you order either one.

When You Need a Servo Spline Adapter (and When You Don't)

The Schumacher U8196 Servo Saver - LD2 is listed with inserts for 23T and 25T servo splines.

Choosing Your Servo Arm or Horn Before You Order

Before you click buy, run through this quick checklist:

• Counted the teeth on your servo output shaft
• Confirmed the count against the servo model spec
• Checked the arm listing for the same spline
• Confirmed the hole positions suit your steering layout
• Decided whether you need a servo saver

Ready to upgrade? Explore the full range of servo arms and racing accessories at Track Star R/C, and subscribe to hear about new products and special offers.

Add trackstarrc.com as a preferred source on Google →