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Morris Minor Differentials

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A Plain‑English guide to what the rear axle does, which ratios exist, how to identify yours, the right oil to use, and how to cure the classic whines, clonks and leaks.

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Quick Facts

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What it is: The differential lives inside the rear axle. It lets the back wheels turn at different speeds in a corner while multiplying engine torque.

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Factory ratios: 5.375 (early/803cc cars), 4.55:1 (948cc), 4.22:1 ( 1098cc).

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Oil: Use EP90 GL‑5 gear oil (some people prefer to use GL4)

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How To Identify Your Diff Ratio

 

Check the tooth numbers on the crownwheel (often stamped on the rim). Divide the crownwheel teeth by the pinion teeth: 41/9 = 4.55, 38/9 = 4.22.

 

Road test method: Mark the propshaft and tyre. Jack one rear wheel, handbrake off, gearbox in neutral. Turn the wheel exactly two full turns while counting propshaft turns (~4.2 ≈ 4.22, ~4.5 ≈ 4.55).

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Engine/axle mix clues: 948 cars typically 4.55; 1098 cars usually 4.22; early 803 cars 5.375. Many cars have been modified over the decades, so test to be sure.

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Choosing a ratio

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3.7

Very long legged, Best for strong heavily modified Morris's with 1275cc+, Would feel flat with stock engines

5.375

Very nippy and revvy, best for Hilly areas and 803 engines. Suits the 803 but would be noisy at high speed

3.9

Long legged, good for cruising, best for upgraded engines such as the 1275cc. Smooth and quiet, best pared with brand new 1098 or 1275 engine as found in the Spridget

4.22

Balanced and calmer when cruising, not so brisk in town but still adequate. Best for 1098 engines and cruising. Popular all rounder, suits most driving conditions

4.55

Brisk Town Driving. Best for 948 engines and town and city driving. Good pull but not so good for motorway cruising

Maintenance & oil​

Oil type: EP90 GL-5

Capacity:

Fill until oil weeps from the side filler plug (with the car on level ground).

 

Servicing and maintenance:

Check the level every 6,000 miles or annually; change the oil every 20,000–30,000 miles, or after water ingress.​

Seals & breathers: A blocked axle breather or tired pinion/hub seals cause leaks. Clean/replace as needed.​

 

How to change the diff oil (quick guide):

Warm the car with a short drive. Park level. Crack the filler plug first. Remove the drain plug; allow to drain fully. Refit the drain plug; fill with EP90 GL‑5 until it just creeps out of the filler hole. Refit filler.​

 

Troubleshooting: whines, clonks & leaks

​Whine that varies with road speed:-

On drive only: Pinion depth/backlash wear.-

On overrun: Crownwheel wear.

All the time: Worn bearings.

Clonk on take‑up:- Excess propshaft U‑joint play, loose half‑shaft splines or diff side‑gear wear

Oil leaks:-

At drums: Rear hub oil seal.

At nose: Pinion oil seal.

All over: Blocked axle breather.

Breakages:-

Snapped half‑shaft: For overseas trips carry a spare halfshaft seal and halfshaft

Chipped teeth: Requires gear set replacement.​

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FAQs:

  • What’s the correct oil again? EP90 GL‑5 (some people prefer GL4)

  • My car buzzes at 60 mph. Will a 3.9 help? This depends on the engine size and driving conditions — expect lower revs and less noise. Acceleration in top will soften.

  • Can I just bolt another ratio in? Yes — but proper setup is critical to avoid noise and wear.

  • Where can I purchase a replacement Diff?

ESM sell reconditioned differentials, as do Heathrow Transmissions.

Minor Parts of Oxford may have second hand ones for sale.

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