Information from Peter Walker of DJ Sportscars
Firstly, let me say that it is not surprising that there is sometimes some confusion about the types of Sierra and Granada differentials which are available. After all, even Ford doesn't seem to have very complete records of what diffs were fitted to which car.
Fortunately, the situation is not quite as complicated as it might seem. Essentially, there are only three basic types ... and we supply fixing kits for fitting any of them to a Rush.
Firstly, there was only one type of diff fitted to the drum-braked cars. This has simple plug-in splined drive shafts, unlike the disc-braked cars which have flange-bolted driveshafts.
There were, however, two types of diff fitted to the disc-braked cars - the seven inch diff and the 7.5 inch diff. You can tell the difference between them by measuring the distance across the two uppermost mounts. (All Ford diffs should have a stamped aluminium tag on the back of the cage giving the part number and diff ratio, but this is often missing.)
For Cosworth and Rover V8 powered Rushes we recommend using the stronger 7.5 inch diff.
In addition, all Cosworth and 4 x 4 Sierras/Granada Scorpios were fitted with limited slip (viscous coupled) diffs. LSDs on other Ford variants were largely only available as options.
The two wheel drive Cosworths, however, used a unique version 05 7.5 inch diff with heavier duty UJs and drive flanges which are a quarter of an inch larger in diameter (4.25 inches rather than 4 inches) and arguably over the top for a car as light as a Rush.
As far as ratios are concerned, the most common 7.5 inch diff is the 3.6 to 1, which was the standard ratio for Cosworths.
For Rover V8 powered cars we recommend using the lowest possible 7.5 inch diff ratio, which is 3.36 to 1. This diff was used in most of the 2.9 V6 Granadas. I should sound a word of warning here, though. Don't assume that a diff taken from a car in a scrap yard will necessarily be the one which was originally fitted by Ford.
I hope the above brief run down of available and recommended diffs proves helpful, but don't hestitate to ring me on +44 (0)1279 442661 if you have any questions or concerns.
Chief Designer, DJ Sportscars International