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MG Midget and Sprite Technical - Diff Evaluation

Yes!! I finally got a reasonable 3.9 diff to put in my Sprite. You might recall that the current diff in the Sprite howls like a banshee. Well I put up with it all summer and I'm ready to try another used diff. The 'new-used' unit (image)is supposed to be from a low mileage midget. Can you check these on the bench before installation or is the only way to test them for noise to put them in the car? I noticed that the factory etched the backlash figure in the crown gear. Has anyone checked this figure on a used unit with a dial gauge? How much movement is acceptable in the diff cage?

0.007" is easily visible on the crown gear.


B C Hamper

Hey BC,

what do the teeth look like, all those white specks...is that just photo residue, or is it metal shavings??? If that is metal partical dust, Id be worried.

try doing a google search on differential backlash measuement...I know there are some articals on measuring the backlash, but I cant remember where they are?

prop
Prop

Howling occurs when the position of the pigeon and crown has been or is not right. This make the teeth wear, you can see that they are not flat. The number etched in the pigeon is the most important to prevent howling. See photo how backlash can be measured. The Sprite original handbook is very clear about restoring diffs and all the numbers etched in crown wheel and pigeon. You should measure the backlash in at the 4 quarters of the crown wheel.

Flip



Flip Brühl 948 frog 59

I raised a similar query a couple of months ago and I received this very helpful reply from Paul Walbram.

Quote: "Paul Walbran, New Zealand
There are a nummber of useful checks you can do.

Backlash occurs in two main places: Between the crown wheel & pinion, and between teh gears in the differential cage.

For the CWP, the original backlash is amrked on the back of the crownwheel. If you can borrow a dial indicator you can see if it has changed significantly.

For the differential gears (the most common cause of backlash, particlarly in cars with no LSD and observing the time-honoured tradition of 6000 in first) the usual source is wear in the thrusts behind the gears. Usually as a result of wheel spin :-)

If there is any lash there it would be worth replacing the thrusts before fitting it, as once the are worn through the gears start to grind into the cage.

If you can get some bearing blue, smear some on some of the crownwheel teeth and rotate the drive flange by hand to turn the crownwheel a few times, in both directions. The blue will show up a print of where the pinion is contacting with the crownwheel. It should be a patch along the centre of the tooth, slightly nearer the inside edge of the crownwheel than the outside. Bad mesh can occur in two ways - either too high or too deep on the teeth, or too near either the outside or inside edge of the crownwheel. Misalignment of the teeth from poor meshing is what causes the whine, so have a good look at this.

The misalignment can also be brought about by poor bearings, sometimes in conjuction with a lack of pinion pre-load, which allows deflection of either crwonwheel or pinion under load with the result that they drop out of perfect mesh. There are some simple checks of the bearings you can do yourself:
Feel for any sign of roughness as the diff is turned over by hand.
If you remove the crownwheel & cage assembly, you can better check the feel of the pinion bearings. They should be slightly stiff to turn, definitely not loose or rough. (The original drag on the pinion required a torque of 11-13 lb-in to overcome it)

While the cage is out, you can better feel the diff cage bearings for roughness. " End Quote

Hope it helps.
Graham P 1330 Frogeye

This thread was discussed between 01/10/2008 and 08/10/2008

MG Midget and Sprite Technical index

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