Tried fitting my Sierra drive shafts to a differential I picked up about a year ago (new old stock) to find the drive shafts are immeasurably too big to go into the holes in the differential.
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The Drive Shafts are from my Donor Car, an F Registration 2.0 Carb model which according to the manual has a 3.62 Ratio Diff.
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The Differential is for an unknown Sierra but has 3.62 marked on its tag.
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I erroneously assumed that all sierra drive shafts from drum brake style cars are interchangeable, can any body shed any light on what model this diff is from and hence which drive shafts I need to acquire.
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Diff Tag has the following numbers:-
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83 BG 4006-CD
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TS O C21N3.62
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Thanks Gareth

I suspect the diff from your donor car wasn't a 3.62. I know the drive shafts from a 3.38 won't fit a 3.62, and vice versa.
Ford are fairly notorious for not sticking too strictly to dff 'a' in car type 'b'.
if you still have the chassis no. plate donor you could try looking up box '10' the axel type. Even that's not fool proof - my donor was wrong in this regard!

From the Haynes 4 cylinder petrol manual.

Final Drive ratio
1.3 models....3.77:1

1.6 models except economy:
Saloon & Hatch....3.62:1
Estate.......3.38:1

1.6 Economy.....3.38:1

1.8 models
4 speed and autos.....3.62:1
5 speed.....3.92:1

2.0 carb models except P100
manual....3.62:1
Auto....3.38:1

2.0 fuel injected
Manual...3.92:1
Auto.....3.62:1
P100 (Live beam axle remember)....4.625:1

Doesn't look like theres any such thing as a standard Sierra diff.

1.3litre 3.77:1
1.6 econ 3.38:1
1.6 3.62:1
1.6 est 3.92:1
1.8 4 speed 3.62:1
1.8 5 speed 3.92:1
2.0 carb 3.62:1
2.0 inj 3.92:1

Box 10 - Axle
M=3.14
A=3.36
U=3.38
V=3.62
3=3.64
D=3.92

One other thing, the drive shafts are handed (I think I remember - not too sure), so check you have the right one in the right side.

The only other thing I can think off is you might have a 3.62 LSD? Have a look in some of the other folks photo archives (most of the BEC MK builders use the 3.62 LSD) and see does your diff look like that.

Cheers for the reply.
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The chassis plate does read as it being 3.62 Ratio but accept this means very little on a old ford.
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Gareth