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Published 2026-04-21
This guide explains exactly what “25T” and “5mm” mean on a microservo, why matching these numbers is critical, and how to avoid common fitting mistakes. Whether you are building a robot arm, a RC model, or any mechanism using a smallservo, correctly identifying the output spline and shaft size ensures yourservohorn will not slip, crack, or fail under load.
When you see aMicro Servolabeled with “25T” and “5mm”, these two specifications refer to different physical parts of the servo’s output shaft:
25T= The number of teeth (splines) on the output gear. “T” stands for teeth. A 25T spline has 25 ridges and 25 grooves around the circumference. This is one of the most common spline standards forMicro Servos in the hobby and light‑duty robotics markets.
5mm= The outer diameter of the output shaft (the round metal or plastic cylinder that rotates). This is the dimension that fits inside the servo horn’s central hole.
Both numbers must match the servo horn you intend to use. A mismatch will either prevent the horn from mounting at all or cause a loose fit that strips the spline or breaks the horn under torque.
In the micro servo industry, the 25‑tooth spline has become a de facto standard because it offers a good balance between fine positioning (more teeth = finer adjustment) and mechanical strength. The 5mm shaft diameter provides enough material to handle typical loads from micro servos (torque ratings around 1.5 to 3.5 kg·cm at 5V) without bending or shearing.
Real‑world example:You buy a standard micro servo from an online store. The product page says “25T 5mm”. You also buy a pack of “universal” servo horns. When you try to install a round horn, it feels too tight or won’t go all the way down. Looking closely, you realize the horn’s spline has only 23 teeth. This is a common case – many “universal” sets include horns for 23T, 24T, and 25T, but you grabbed the wrong one by mistake. The solution is to always check the horn’s label or count the teeth before pressing it on.
Follow these steps to confirm the specifications before buying a servo horn or replacement gear set.
Clean the output shaft with a soft brush or compressed air.
Use a magnifying glass or your phone’s macro camera.
Mark the first tooth with a fine marker or a tiny dot of paint.
Rotate the shaft slowly and count each tooth until you return to the mark.
If you count 25 teeth, the servo is a 25T type. If you count 23 or 24 teeth, it is a different standard and will not accept 25T horns.
Use a digital caliper (accuracy to 0.1mm is sufficient).
Measure the outer diameter of the metal output shaft, not including the spline teeth. The measurement should be taken across the round section just below the spline or at the very tip if the spline does not extend to the end.
A 5mm shaft typically reads between 4.95mm and 5.05mm due to manufacturing tolerances.
Common case:A user measures 4.98mm and worries it is not 5mm. In practice, horns designed for 5mm shafts have a slight interference fit (0.02‑0.05mm undersize) to grip securely. Your measurement is perfectly normal.
If you cannot measure or count easily, compare the servo’s appearance with documented standards:
25T spline – teeth are relatively fine and equally spaced. The shaft diameter is ~5mm.
23T spline (often found on older or cheaper servos) – teeth are coarser, shaft diameter ~4.8mm.
24T spline (some Korean or specialty servos) – in between.
When in doubt, always rely on physical counting and measuring rather than memory or product photos.
Once you have confirmed your servo is 25T with a 5mm shaft, choose a horn that explicitly states these exact numbers.
Plastic (nylon or POM)– Suitable for most micro servo applications (robotic arms, steering, light levers). They absorb shock and prevent gear damage during accidental overload. Cost is low.
Aluminum (CNC machined)– Required when torque exceeds 2.5 kg·cm or when the horn mounts a heavy load (e.g., a 200g link). Aluminum will not flex or crack but transfers stress directly to the servo’s internal gears.
Horns labeled “fits 23T/24T/25T”– These often have a universal spline that does not fully engage any of them. They will strip after a few cycles.
Horns without a retaining screw hole– For 5mm shafts, a center screw (usually M2 x 5mm) is essential to keep the horn from sliding off axially. Never rely on friction alone.
3D‑printed horns– Unless printed from reinforced nylon with 100% infill and post‑processed (e.g., drilled to exactly 5mm), they will fail unpredictably. Use only for prototypes.
Follow this procedure to ensure a secure, long‑lasting fit.
1. Power off the servo.Unplug it from the controller or receiver.
2. Manually rotate the servo to its center position(usually marked by a line on the case or by counting pulses).
3. Place the horn over the shaft.Align the splines by gently twisting the horn left and right until it drops fully onto the shaft. Do not force it – a proper 25T fit requires only light finger pressure.
4. Check the bottom of the horn– it should sit flush against the servo’s top case (or within 0.2mm). If there is a visible gap >0.5mm, the spline is mismatched or there is debris in the teeth.
5. Insert and tighten the retaining screw.Use a screwdriver that fits perfectly (usually a PH0 or PH00). Tighten until snug, then add a 1/8 turn. Over‑tightening can strip the screw or crack the horn.
6. Test by hand– with the servo still off, try to rock the horn. It should have zero play. If it moves at all, stop and verify the spline and shaft diameter again.
Common case:After installation, you notice the horn wobbles when the servo rotates. The most frequent cause is a cracked horn near the screw hole – often from using a screw that is too long (e.g., M2.5 instead of M2). The long screw bottoms out and pushes the horn upward, creating a gap. Always use the screw length recommended by the servo manufacturer (typically 4‑6mm for 5mm shafts).
No. A 5mm horn’s central hole is designed for 5mm ±0.05mm. Forcing it onto a 4.8mm shaft will leave a radial gap. Under load, the horn will wobble and quickly strip the spline. You must use a horn that matches both the spline count AND the shaft diameter.
This is rare but happens with cheap or counterfeit products. A 5.2mm shaft will stretch or crack a standard 5mm horn. Solution: measure before purchasing. If you already own such a servo,you may need to ream the horn’s hole to 5.2mm – but this weakens the horn. Better to replace the servo with a genuine 25T/5mm unit.
Apply a drop of isopropyl alcohol around the shaft to dissolve any oil or adhesive. Use a small gear puller or two flat screwdrivers as levers on opposite sides. Never pull by twisting – that will damage the internal gears. If the horn is plastic, you can carefully split it with a flush cutter, then replace the horn.
No. Standard (large) servos typically use a 23T or 25T spline but with a 6mm shaft diameter. A standard servo horn will not fit a micro servo’s 5mm shaft. Always check both specifications.
To summarize and ensure success every time you work with a micro servo labeled “25T 5mm”:
1. Verify before you mount– Count the spline teeth (must be 25) and measure the shaft diameter (must be 5.0 ±0.1mm). Do not trust printed labels alone – counterfeit parts exist.
2. Match both numbers exactly– Buy horns that explicitly state “25T / 5mm”. Avoid “universal” or “multi‑fit” horns unless they are from a reputable source and you have physically tested them.
3. Always use the retaining screw– For a 5mm shaft, the screw is not optional. Use the correct length (4‑6mm) and torque just past snug. Replace any horn that shows cracks near the screw hole.
Final recommendation:Keep a small digital caliper and a magnifying glass in your servo toolbox. When you receive a new micro servo, take 60 seconds to count the teeth and measure the shaft. This simple habit will eliminate 99% of fitment problems and save you from broken horns, stripped splines, and failed mechanisms. If you ever encounter a servo that does not match 25T and 5mm, return it or clearly label it so you never mistakenly pair it with a 25T/5mm horn. Your projects will run smoother, last longer, and perform exactly as designed.
Update Time：2026-04-21
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