To build your own snowblower gearbox, you must first identify which brand of snowblower you are using. Certain parts, including the shear pin, are specific to particular models. Manufacturers such as Honda, Toro, Craftsman and Husqvarna sell shear pins and other replacement parts online.
First obtain a gearbox diagram. You can request diagrams for snowblower transmissions from the manufacturer. You need a number of parts, including two case gears, nuts and bolts, worm gears and bearings.
Turn off your snowblower, siphon out the gas, and make sure the engine is cool before performing any maintenance. Assemble the gears inside the case. Use a key to secure the worm gear to one end of the auger. Connect the worm gear to a gasket using a washer. Secure a case gear to the gasket with a sleeve bearing. On the other end of the augur, use a washer to attach a sleeve bearing and another case gear. Mount the oil seal to the outside of this case gear. To complete the shaft assembly, connect the ring quad to the snowblower flange bearing. Use a washer to secure this assembly to the flat roller bearing.
Keep your snowblower transmission lubricated and free of rust to ensure longevity. Check the transmission oil level regularly. Routinely lubricate the gearbox shaft, drive shaft and auger shaft as part of regular snowblower maintenance. If you are using your snowblower in wet or high-speed conditions, or if you are operating the machine in temperatures lower than 20 degrees Fahrenheit, make sure that your lubricant can withstand those conditions.