The Dos and Don'ts of Diffs


by Iain Woolley

As the miles add up, there will come a time when the differential in your Spitfire will wear out. That time has recently come for my 1978 Spitfire 1500, at a grand total of 130,000 miles. The symptoms were a loud clunk from the rear end when taking up drive, and a high pitched whine that was loudest at 50-60mph and disappeared at 70mph. With the rear end of the car jacked up I could twist the prop shaft through about 45 degrees without either of the half shafts rotating. So I made the decision to change the differential for a reconditioned unit.

Removal

The general technique for removing the differential is as follows, 1. Remove the entire exhaust system. 2. Undo the prop shaft from the differential. 3. Remove the half shafts. 4. Remove the road spring. 5. Remove the differential from the car.

However there are a number of tips that make this process a lot easier,

Get the car up on two axle stands, positioned almost at the rear corners of the chassis. Then chock both front wheels.

By removing the entire exhaust system (from the manifold to the back box), you can lay the prop shaft on the ground out of the way of the differential.

Undo the two lugs at the front of the diff first, and let the front of the diff swing down. Support the diff on a wheeled jack, then undo the bolt through the back of the diff. With the diff supported on the jack, move the jack forward so that the back of the diff clears the chassis. The jack can then be lowered and pulled out from under the car with the diff balanced on it.

Replacement

If it is possible, make sure that you get your own differential reconditioned. Most spares retailers will want to sell you an exchange item, so go to a specialist mechanic and get them to recondition the diff you bring them. There is a sound reason for doing this. Spitfire chassis were hand welded at the factory, and consequently each Spitfire chassis is slightly different to the next one. Because of these differences some of the components bolted to the chassis were designed to be adaptable. The differential is one example. The front plate with the two "ears" is designed so that it can be bent to fit the particular chassis it is being fitted to.

When the differentials were fitted at the factory they were put in before the bodies were attached to the cars. This made it easier to appy the force necessary to bend the front plate. Take my word on this one, trying to bend the front plate whilst under the car is frustrating and usually fruitless work. So it makes sense to get your own differential reconditioned as it is guaranteed to go back into the car. If you get an exchange item then it will be pure luck that it fits back into your car.

In true Haynes fashion, the refitting procedure is reasonably straightforward, 1. Refit the differential. 2. Replace the road spring. 3. Replace the half shafts. 4. Reattach the prop shaft. 5. Replace the exhaust system.

Again the following may be helpful,

If your handbrake cable is looking a little tired, change it now before you refit the diff. After the diff is back in changing the handbrake cable is many times more difficult.

Before refitting the diff fill it with the correct oil. Then tilt it forward to make sure that oil is getting to the front of the diff, and rotate the input flange several times. This works oil into all the right places at the front of the diff.

Balance the diff on a wheeled jack, then offer the diff up to the underside of the car with the jack. Attach the back of the diff first, then swing the front of the diff up to the two bolts.

Use fresh Nylok nuts for the diff mounting bolts, prop shaft flange bolts, half shaft flange bolts, road spring studs, and vertical link bolts.

If you refit the diff without any oil, put oil into it before reattaching the road spring or half shafts. This is the most access you will have to the filling hole.

If you refit the diff with oil in it, it is normal for some oil to leak from the front of the diff whilst the car is up on axle stands. The seals were designed to work with the car horizontal, and don't cope very well with the car on a slope. This may also be relevant if you park on a slope, try to park with the car facing up the slope.

baddogracing@yahoo.com


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