Midget Differential: Repairing the Ring and Pinion

The differential is a crucial component of any rear-wheel-drive vehicle, including the venerable Midget. It allows your wheels to rotate at different speeds when turning, preventing tyre scrub and ensuring a smoother ride. However, like any mechanical part, it can wear out or suffer damage. This guide delves into common differential issues, particularly concerning the ring and pinion gears, and offers practical advice on diagnosis and repair, supplementing the invaluable information found in the Bentley Midget workshop manual. If you're looking to source this manual, a quick search on Amazon will likely yield results.

Understanding Differential Problems

One of the most common indicators of differential trouble is excessive freeplay between the ring and pinion gears. This can manifest as a clunking noise when accelerating or decelerating, or a noticeable looseness when shifting between gears. If you experience a significant amount of freeplay, it's often attributed to either worn pinion bearings or failed thrust washers. The good news is that these components are generally replaceable, and the parts themselves are relatively inexpensive, often costing around $100. However, the labour involved, especially if you're not doing it yourself, can add up.

A more serious issue arises if the teeth of the ring and pinion gears themselves are worn. Symptoms of this include grinding noises, particularly during gear changes, and a rougher transmission of power. If the gear teeth show signs of wear, such as dull spots, chips, or scouring, it's usually more cost-effective to replace the entire ring and pinion set rather than attempting a repair. However, if the gear teeth appear to be in good condition, with no visible damage, then replacing the bearings and thrust washers might be a worthwhile endeavour.

Finding Professional Help

If you're not comfortable undertaking this repair yourself, finding a qualified mechanic is key. For differential work, especially involving the ring and pinion, it's beneficial to seek out workshops that specialise in hot rods or vehicles where drivetrain modifications are common for performance enhancement. In the UK, this might mean looking for specialists in classic car restoration or performance tuning. These shops are accustomed to working with rear-wheel-drive setups and have the expertise to handle tasks like setting up a new ring and pinion, which is a more intricate process than simply replacing bearings and washers.

If your differential only requires the replacement of bearings and thrust washers, and the ring and pinion gears are in good shape, a skilled local machinist can often perform the necessary work. It's advisable to provide them with a copy of the relevant pages from your Bentley Midget workshop manual, which details the specific torque settings and clearances required. Many of these key specifications are universal across different differentials, and any specific values not covered universally are often handwritten on the edge of the ring gear itself.

DIY Differential Rebuild: A Step-by-Step Approach

For the more adventurous and mechanically inclined owner, a DIY differential rebuild is certainly achievable with the right tools and a methodical approach. The core of the differential, often referred to as the 'pumpkin', is where the ring and pinion gears reside. Disassembling this unit requires careful attention to detail, especially during reassembly.

Disassembly Procedure (with Re-assembly Torque in Parentheses):

1. Note Bearing Cap Locations: Before removing the bearing caps, mark their original positions (e.g., RH/LH) to ensure they are returned to the correct sides during reassembly. Incorrect placement can lead to improper gear meshing and premature wear.
2. Remove Bearing Caps: Loosen and remove the bearing caps. The typical torque for these caps is around 65 ft-lb.
3. Lift Out Bearing Cage: Carefully lift the bearing cage, which houses the differential gears, out of the housing.
4. Remove Crown Wheel: Detach the crown wheel (the large gear bolted to the differential cage) from the bearing cage. The torque for these bolts is usually around 60 ft-lb.
5. Remove Differential Pin Retaining Pin: Locate and tap out the retaining pin (often item 24 in the Bentley manual) from the crown wheel side of the differential cage. An 1/8" drill bit can be used to clean out the hole if it's corroded. Crucially, when reassembling, peen over the edge of this hole after reinstalling the pin to prevent it from working loose.
6. Remove Pinion, Planet, and Sun Gears: Carefully remove the pinion gear, planet gears, sun gears, and their associated thrust washers (items 19-23 in the Bentley manual). It is paramount to keep each of these components in their original positions relative to each other for correct reassembly.
7. Remove Pinion Assembly: To access the pinion gear itself, you'll need to remove the pinion nut. This will allow you to remove the pinion gear, the crush spacer, bearings, shim, seal, and dust cover (items 29-35).

The Critical Role of the Crush Spacer

One of the most delicate aspects of pinion installation is correctly setting the bearing preload using the crush spacer. Crush spacers are designed to be compressed to a specific point, thereby setting the correct tension on the pinion bearings. Modern replacement crush spacers can sometimes be manufactured to be stronger than the originals, meaning they compress more easily.

If you install a new crush spacer and torque the pinion nut to the specified 140 ft-lbs without checking the bearing preload, and you have one of these stronger spacers, you risk over-compressing it. This can lead to excessive bearing load, which will rapidly wear out the pinion bearings. It's essential to monitor the bearing preload as you tighten the pinion nut. The target bearing preload is typically measured by the rotational torque required to turn the pinion shaft, usually in the range of 11-13 in-lb. If you overshoot the correct compression, the crush spacer is ruined, and you'll need a new one to try again.

Essential Tools for the Job

While the differential rebuild can be done with basic machine shop tools, precision is key. Here's a list of essential equipment:

• Dial Indicator: Crucial for measuring run-out and ensuring components are seated correctly.
• Hydraulic Press: Necessary for pressing bearings on and off the pinion shaft and for seating new components. Ensure you have a range of suitably sized arbors.
• Bearing Puller: Specifically for removing the inner pinion bearing from the pinion shaft.
• Seal Puller: A seal puller is ideal, but a sturdy screwdriver or a small crowbar can suffice in a pinch for removing the old pinion seal.
• in-lb Torque Wrench: This is perhaps the most critical tool. Standard ft-lb torque wrenches often don't have a low enough range for the precise in-lb measurements required for bearing preload. You'll need a torque wrench specifically calibrated for low in-lb readings, ideally in the 6-15 in-lb range. Precision Instruments is a reputable manufacturer of such tools.
• Blueing Ink: Used to check the contact pattern and alignment of the ring and pinion gear teeth.
• Clean and Orderly Workspace: This cannot be stressed enough. Keeping all parts organised and in their original positions is vital. This includes the relative 'clock' position of the ring gear to the differential cage, the arrangement of planet and sun gears, and ensuring the correct left and right-side bearing caps are reinstalled.

Alternative Torque Measurement Method

If a dedicated low-range in-lb torque wrench is unavailable, a DIY method can be employed. This involves using a known weight and a lever arm. A common approach is to use a 2-foot breaker bar with a calibrated weight hanging from it. With the breaker bar horizontal, a 1lb weight placed 12 inches from the centre of the pinion nut will exert a torque of 12 inch-pounds (Torque = Force x Distance). It's important to measure this force when the pinion is already rotating to overcome static friction or 'stiction', which can give a false reading. By moving the weight along the bar, you can achieve the desired torque reading. This method requires careful calibration and a steady hand but can be an effective substitute.

Parts You'll Likely Need:

When undertaking a differential rebuild, be prepared to replace several wear items. These typically include:

• Pinion bearings (both inner and outer)
• Bearing cups
• Thrust washers
• Pinion seal
• Crush spacer
• Crown wheel lock tabs (usually three)
• Differential pin
• Spring pin that retains the differential cross pin in the cage

Important Considerations:

• Gear Tooth Contact Pattern: After reassembly and before final torqueing, use blueing ink on the ring and pinion teeth. Rotate the assembly to check the contact pattern. An ideal pattern shows a smooth, even transfer of ink across the tooth face. Adjustments may be needed by shimming the pinion or ring gear, as detailed in the Bentley manual.
• Backlash: This is the small amount of rotational play between the ring and pinion gears when they are meshed. It's measured with the dial indicator and is critical for smooth operation and longevity. The Bentley manual will specify the correct backlash value.
• Pinion Depth: The depth at which the pinion gear sits relative to the ring gear is set by shims. Incorrect pinion depth is a primary cause of premature gear wear.

Rebuilding a Midget differential is a rewarding task that can significantly improve the driving experience and longevity of your classic car. By understanding the components, using the correct tools, and following precise procedures, you can ensure your Midget's drivetrain remains in excellent condition for years to come.