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Diff question.......


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Often hear it said that the diffs are weak. I know that on the early cars there were smaller output shafts, but I am not talking about that.

On the later diffs with the round flange and large shafts, what is it that actually breaks ? The diff pin or gears ? pinion shaft ? Or is it still the output shafts ?

I gots to know........

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Failures I have come across:

- Output shaft breakage (yup, big ones too)
- Cross pin supporting the planet wheels can break
- The curved thrust washers behind the planet wheels may break up or dissolve if the wrong oil is used.  Some had fibre washers which fall apart eventually anyway.
- The actual diff carrier can break - usually where the cross pin goes through
- Pinion bearing wear
- Broken or damaged teeth on crown wheel or pinion.  3.27 is apparently more prone to crown wheel damage due to smaller size - but maybe I'm passing on popular myth here - no direct experience.  Lost teeth can be a underwear damaging experience as the loose tooth may jam the diff and lock your back wheels........
- Probably the biggest killer is oil leaks leading to low or no oil in the diff.  Worth keeping it topped up as they don't have that much to loose.

The diffs are actually quite strong when in good condition and should be perfectly adequate for all but highly tuned 4 cylinder cars driven hard.  6 cylinder cars, especially 2.5 are getting a bit marginal!  Full power starts at your own risk!


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I can vouch that the 3.27 is indeed weak, having well and truly mangled 2 (with my 2.5 vitesse, yes, being a bit heavy footed). I have also seen a 3.63 equally mangled by a 2.5 spitfire, all losing teeth.
But to be honest, they are normally fine unless you do indeed do those flashy standing starts. All is fine while the wheels spin, it's when they suddenly grip it all goes horribly wrong, and you look a complete ****!
The diffs normally just get noisy and whiny, but usually go on for ages as long as they have oil. Nick is right, well worth keeping it topped up!

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  • 3 weeks later...

So really then, whats wanted is a quaife diff ?

I have been musing today.........and yesterday, and the day before, and...........
which ratio to use. 3.89 or 3.63 ?

My Herald has a 1500 engine with a stage 4 head and a Kent TH5 cam, amongst other things, and a hope to get a genuine 100hp out of it. I dont know what the mods will do for its willingness to rev, as they do not like to as standard, but I dont want to run out of revs too soon, hence the attraction of using the 3.63 diff, but I know the 3.89 will enable it to accelerate faster, PROVIDED, it is willing to spin up quicker than the standard engine.

The weight of the Spit 1500 and the Herald are similar, although obviously the Spit is more aerodynamic.................

I am getting a headache !

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