Diff identity

Clutch, Gearbox, Overdrive, Propshaft, Differential, Drive Shafts, Hubs.
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Binny
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Diff identity

#1 Post by Binny » Thu Apr 15, 2021 10:18 am

Morning all,

Is it possible to identify what ratio a diff is from any external markings or will I have to strip it and count the teeth?

About twenty years ago I transplanted the complete rear subframe/suspension/diff assembly from a very low mileage Mk2 2.5 PI (which think was an automatic) into my manual Mk2 2.5PI.

At the time I didn't give any thought to the fact that the diff ratio was different but now I'd like to establish whether it's a 3.7:1 auto or 3.45:1 manual diff.

johnnydog
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Re: Diff identity

#2 Post by johnnydog » Thu Apr 15, 2021 12:36 pm

Theoretically, assuming it was the correct diff in the auto PI when you did the original swap, it should be a 3.45 ratio. As long as it hasn't been stripped and messed about with previously, the easiest way is to check the identification letters stamped into the casing which should be MG for a 3.45 diff (a 4.1 is stamped ME, and a 3.7 is MEC). Also there should be a splash of red paint on the large nut of the driveshaft output put flanges. If the paint is yellow or white, then its is a 2000 diff (either 3.7 or 4.1 - can't remember which is which).
As you are no doubt aware, 3.45 diffs were fitted to both autos and manual 2.5 / 2500 cars, so it is irrelevant which transmission the donor car had. The rear cover and mounting plate can be swapped over depending on whether saloon or estate.
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1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red

Binny
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Re: Diff identity

#3 Post by Binny » Thu Apr 15, 2021 1:01 pm

Thanks very much for that information Johnnydog - I'll inspect the diff tonight to see what I can find.

For some reason I thought 2.5 autos all had a 3.7:1 ratio!

Clifford Pope
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Re: Diff identity

#4 Post by Clifford Pope » Thu Apr 15, 2021 5:24 pm

Whatever the markings, whatever has actually been done, the ultimate test is to jack up one wheel and count the turns of the prop v turns of the raised wheel, and double (? or is it halve ? :( ) the ratio.

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