Welcome to the next instalment of our DIY Mini Restoration series, where we revisit the Bogus 2 Mini build. This guide covers the gearbox and differential build, providing detailed instructions and practical tips to help you achieve optimal performance for your Mini. If you need technical advice or assistance, don't hesitate to contact us. Let's dive in!
Gearbox and Differential Build
Straight-Cut, Close-Ratio, and Four-Pin for Bogus 2
Choosing the Gear Set
We selected a straight-cut close-ratio gear set kit and used the original casing from the donor vehicle. This gear set is inspired by the original BMC Abingdon Special Tuning ratios but features modern heat treatment and materials for enhanced durability. For detailed information on the ratios, visit our website (Part No: MS4014).
Building the Gearbox
Here's the gearbox built up. We chose a 4.1:1 differential ratio to excel in hill climbs and sprints. A central oil pick-up has also been installed, along with an in-line magnetic oil trap, to reduce the risk of oil starvation during hard cornering. Useful websites can help you select the best ratios for your car. Discover our range of built gearboxes >
Selecting the Differential
We opted for a 'four-pin' differential to handle the power and torque. Various differential options are available, including limited slip (LSD) and automatic torque biasing LSDs, ideal for high-torque applications like turbocharged engines. This four-pin diff provides the same response as a standard diff but can handle more power. Mini Sport offers this kit pre-assembled (Part No: MSLMS0516).
Additional Gearbox Features
Magnetic Oil Trap
The gearbox includes a magnetic oil trap, positioned just above the diff casing on the back of the gearbox. This trap is directly in the oil flow from the central oil pick-up pipe and should be checked and cleaned each time you clean the drain plug magnet during oil changes (Part No: C-AHT54TRAP). Shop Now >
Gear Selector Rod Support
An aluminium collar, shown here without its O-ring, supports the gear selector rod. This non-OE part should be fitted in addition to the black oil seal. While not a replacement for the standard oil seal, it provides extra support to prevent significant oil leaks, especially when hammering home roll pins.
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Important Checks and Tips
Inspecting a Secondhand Gearbox
When buying a secondhand gearbox, thorough inspection is crucial. This image shows the damage caused when standard transfer (drop) gears are used in a powerful engine, resulting in a badly worn outer casing.
Detecting Cracks
Some cracks may not be as obvious as this one in the third motion shaft bearing support, part of the central web in the gearbox. Kent Auto Developments offers a pinion bearing support housing kit that reinforces this central rib by supporting the end of the shaft, which is essential under hard acceleration.
Need Help with Gear Box Parts?
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Damage Indicators
If you find a smashed second gear stripped in the gearbox, it indicates significant damage. Since the engine shares the same oil supply as the gearbox, ignoring gearbox issues can lead to engine failure. Discover our range of Mini Gearboxes and parts >
Want to read Keeping Your Mini Alive 6th Edition Magazine? Download the PDF here
Don't miss out on the rest of the Bogus 2 Series!
This blog series is your go to for a guide to taking your Mini from a bare shell to a road-worthy classic. Check out the rest of the series for tips and advice from our Mini Experts for every stage of your Mini restoration!
This guide is meant to provide general guidance and a step-by-step approach to build your Mini's Gearbox. It is always recommended to contact a trusted professional if you are unsure about any part of the process or lack the necessary tools and experience.