The plastic gear in the first-generation Bosch Pedelec Motor, also known as Bosch Classic, has gained notoriety for its tendency to break. Having owned the bike since 2011, I experienced a gear failure in 2022.

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Constructed from POM, a durable plastic commonly used for gears, the material posed a challenge for repair due to its resistance to adhesives. Furthermore gluing would not be strong enough in general. Consequently, replacement became imperative.

Disassembling proved relatively straightforward using a gear puller.

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The gear itself comprises an aluminum hub press-fitted onto a bearing and an outer plastic component. Fortunately, the aluminum hub remained undamaged.

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Two variations of the aluminum hub exist, and mine belonged to the first generation of the gear. The main difference is that the tooth interface area is a bit smaller.

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I discovered that someone had already designed a 3d printable replacement gear.

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Further research revealed that a 3D-printed gear might not withstand the gearbox’s substantial forces. None of the materials will be strong enough when printed with a FDM printer. However, an alternative emerged on a pedelec forum—a user manufacturing replacement gears from fiber-reinforced nylon.

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The gear appeared to be milled using a CNC router, leaving some residual glass fibers that required removal.

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Remounting the gear proved challenging due to its tight fit on the bearing, necessitating considerable force. A common technique involves using heat to expand the aluminum hub and freezing spray to contract the bearing. I placed the gear on my 3D printer’s heated bed set to 120°C, allowing it to reach nearly 100°C.

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While the gear was heating, I cleaned the motor and removed old grease. As a crucial pre-assembly step, I applied Liqui Molly 1080 Schmierfix to lubricate the one-way clutch. Following the assembly, the gear teeth were generously coated with grease to ensure optimal performance. Liqui Molly 1080 Schmierfix is renowned for its adhesive properties and compatibility with plastic gears.

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