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Finding a small-module spur gear that fits a specific bore diameter without custom machining is a recurring challenge for RC hobbyists, robotics builders, and small-machine designers. This 0.5 mod pinion gear in black steel delivers a ready-to-install 25-tooth spur gear with a stepped hub design, available in three bore diameters to match the most common motor shaft sizes used in compact drive systems.
Each gear is machined from black steel — a carbon steel with a black oxide surface treatment that provides moderate corrosion resistance and a clean, low-reflectance finish. The module is 0.5, giving a pitch diameter of 12.5 mm (approx. 0.492 in) and an outer (tip) diameter of approximately 13.5 mm (approx. 0.531 in). The stepped hub extends the gear's axial engagement length beyond the face width, improving shaft grip and set-screw retention. Bore options are 4 mm, 5 mm, and 6 mm to suit common motor and shaft standards.
Typical applications include RC car and truck drivetrain pinion gears where a 0.5-module mesh is required, stepper motor output gears in compact robotics and automation rigs, and small gearbox stages in DIY CNC pen plotters, camera sliders, and motorized mechanisms where a low-profile 25-tooth spur gear keeps the gear train tight and efficient.
| Module (Mod) | 0.5 |
|---|---|
| Number of Teeth | 25 T |
| Pitch Diameter | 12.5 mm (0.492 in) |
| Outer (Tip) Diameter | Approx. 13.5 mm (0.531 in) |
| Bore Diameter | 4 mm / 5 mm / 6 mm (see variant options) |
| Hub Style | Stepped hub |
| Material | Carbon steel |
| Surface Finish | Black oxide |
| Gear Type | Spur gear (straight tooth) |
| Pressure Angle | See variant options |
| Face Width | See variant options |
| Pack Quantity | 1 / 2 / 3 / 5 / 10 / 20 pcs (select above) |
Measure the outer diameter of your motor or drive shaft with a caliper. Select the bore option that matches: 4 mm for micro gear motors and small RC motor shafts, 5 mm for NEMA 17 stepper motors and standard brushless RC motors, or 6 mm for larger hobby motors and shaft couplings. The bore should be a close sliding fit on the shaft — not loose — so the set screw can clamp it securely.
Module (Mod) is the metric standard for gear tooth size, defined as pitch diameter divided by number of teeth. A 0.5 module gear has a tooth pitch of approximately 1.571 mm (π × 0.5 mm). This gear will mesh correctly with any other 0.5-module spur gear or rack, regardless of tooth count, as long as the pressure angle also matches (typically 20° for standard metric gears). It will NOT mesh correctly with gears of a different module.
The center distance between two meshing spur gears equals half the sum of their pitch diameters: C = (D1 + D2) / 2. For this 25-tooth, 0.5-module gear, the pitch diameter is 12.5 mm. If it meshes with a 50-tooth, 0.5-module gear (pitch diameter 25 mm), the center distance would be (12.5 + 25) / 2 = 18.75 mm. Use this formula to lay out your gear train accurately.
Black oxide is a conversion coating that provides mild corrosion resistance suitable for indoor, low-humidity environments. It is not equivalent to zinc plating or stainless steel in wet or outdoor conditions. For applications exposed to moisture, splashing, or outdoor use, apply a light coat of machine oil or grease to the gear teeth and bore after installation to extend service life.
The stepped hub is a raised cylindrical boss on one or both faces of the gear that extends the bore length beyond the gear's face width. This longer bore engagement gives the set screw more clamping surface on the shaft, reducing the chance of the gear spinning or shifting axially under torque. It also makes it easier to position the gear at a precise axial location on the shaft by using the step as a reference shoulder.
Yes — this 25-tooth pinion is commonly used as the driving (input) gear in a reduction stage. The gear ratio equals the number of teeth on the driven gear divided by 25. For example, pairing with a 50-tooth gear gives a 2:1 reduction; a 75-tooth gear gives 3:1. All mating gears must be 0.5 module. This makes the gear well-suited for stepper motor drives, RC car differentials, and small gearbox stages where torque multiplication is needed.
The gear is designed for set-screw retention. Check whether a set screw is pre-installed in the hub; if not, a standard M2 or M3 grub screw (depending on hub size) fits the threaded hole in the hub. For shafts without a flat, apply medium-strength thread-locking compound to the set screw before tightening to prevent loosening under vibration. For shafts with a D-flat or keyway, align the set screw with the flat for maximum retention.
