Two pinions can both have twenty teeth and still be incompatible with the same spur gear. Tooth count describes how many teeth the gear has. Pitch or module describes their size and spacing in a particular measurement system.
Read the whole gear description
An RC parts listing may use an imperial diametral-pitch designation, such as 48 pitch, or a metric module designation. Those are not tooth counts. The KHK gear fundamentals guide explains these tooth-size systems: a larger diametral-pitch number describes finer teeth, while a larger module describes larger teeth. Mixing the systems without checking the actual specification invites mistakes.
Start with the vehicle manual and the existing spur gear’s approved specification. Then check the replacement pinion’s pitch or module, tooth count, bore size, width and mounting arrangement. A photograph of a gear that looks similar is not a dimensional specification.
Use tooth count only after compatibility
As an illustrative purchase check, imagine a list offering two 20-tooth pinions: one for a 48-pitch system and another in module 1. Their matching tooth counts do not make them substitutes. Choosing between them requires the drivetrain’s specification first.
Even a matching pitch is not the whole compatibility decision. The shaft fit, available clearance and manufacturer’s permitted gearing range still matter. Follow the model’s guidance rather than assuming that any physically installable combination is suitable for its motor and intended use.
Keep the packaging information
Label spare gears with their full specification and part number before they become an anonymous group in a toolbox. If the marking is missing, ask the manufacturer or supplier to identify the part rather than estimating compatibility from tooth count alone.
Disconnect the battery before inspection and refit guards according to the vehicle’s instructions. This check concerns selecting the correct tooth geometry; adjusting the spacing between two incompatible gears will not turn them into a correctly matched pair.