Sierra Differential

Fed up with reading dodgy information on all those forum sites?

Have you ever needed Ford Sierra Diff Ratios?
Or needed Sierra Part Numbers?
Find Exploded diagrams and FINIS part numbers.
This is the most accurate Sierra Differential information available on the web.

Ford Sierra 86 Diff Microfiché

Ford Sierra 07/92 Diff Microfiché

Ford Scorpio Diff Ratios, link to make and model,
complete with Scorpio Part Numbers.

All diagrams are listed with FINIS information making this the most accurate Scorpio Differential information available.

Ford Scorpio 95 Diff Microfiché

Ford Granada Diff Ratios, Granada Part Numbers. and Granada Diff Overhaul and Rebuild information
Accurate Granada Differential information direct from Ford. 

Ford Granada 92 Diff Microfiché

CAD drawing of Ford Sierra Diff, 7″ push in shaft type (Tripod).

Despite its’ age, the Ford Sierra Diff still dominates as the No. 1 choice of diff for kit car builders. It’s been well documented that they can cope with 300bhp without too much hassle and even very high mileage units show very little signs of wear. Various differentials were looked at for this design. For instance the modern Jaguars have strong differentials but their final drive ratios are far too low, the same also applies to many Nissan and BMW diffs.

http://www.youtube.com/watch?v=llLvCEz4AhE
The other advantage of these differentials is that they can still be found in reasonable supply on auction websites. The price is slowly increasing and maybe one day soon the BMW unit will take over, but for now it is here to stay.
There are a vast array of drive ratios, some more common than others. Casings are available in 7″ or 7.5″ formats with push in or bolt on shaft fitments. The push in versions are also available with a different number of splines. Some are available with LSD others not. The choice is bewildering.

Identifying your Ford Sierra differential.

If the donor car is available you can read the VIN plate, for the “Final Drive Code” or “AXLE”. The final drive ratio is describe by a letter.

If the VIN plate is not visible, there maybe a small aluminium tag attached to the top left hand side of the rear cover. There are a vast number of codes and it maybe that a visit to your friendly Ford dealer is in order. There are many web sites and magazine articles, quoting codes and ways of deciphering the codes and often the information is conflicting or simply incomplete. The only real way to identify a diff is to physically test it. For instance, if you turn the front flange and both drive flanges rotate in the same direction, it’s an LSD.

As a rough guide…..

Identification on Tag.

1. Casing style
2. Axle Ratio E.g. 3.36:1
3. Assembly Plant
4. Build Date
   • 4 = Build Year (1984)
   • L = Build Month
     • A = Jan, B = Feb, C = Mar, D = Apr, E = May, F = Jun, G = Jul, H = Aug, J = Sep, K = Oct, L = Nov. M = Dec
   • 11 = Build Day (11th)

On some Sierra diff ID plates the numbers will be in a different format. There will be some numbers / letters (look at the top row)
EG. 85GG 4006 DC
The first set (85GG) is the case type (7″ or 7.5″) and the easiest way to identify a 7″ and 7.5″ is to look at the top mounting. The 7″ (left) has one long bolt/nut and the 7.5″ (right) has 2 bolts.

NB. The unit on the left is a Tripod unit with push in shafts and the right hand unit is the more desirable Lobro unit with external bolt flanges for the drive shafts.
In this view you can clearly see the size difference (7.5″ on left):

NB. The internals of the 7″ are almost identical to the 7.5″ (apart from the reduction gear ratios), so there are no differences in strength.
The other non−numeric code (middle or end), in this code its ‘DC’ which is an ‘OPEN’ type diff (NON-LSD). Generally, any diff with 2 or more “A” in the middle code will be an LSD e.g. “AA” or “ABA”. This doesn’t mean that if it hasn’t, it’s not an LSD, it’s just easy to remember. There are several exceptions to this rule.
Some codes are in the format 85BB 362 AA.
In this case, the first 4 digits (85BB) are still the casing code and the AA is the diff style (In this case LSD). The middle 3 digits represent the ratio (in this case 3.62 :1)

Another way to identify what ratio you have is to undo the T50 Torx bolts securing the rear cover and inspect the writing and stamps on the internal gearing.
If you are lucky there will be some handwriting to the large Crown gear. This is easily rubbed off.
The 7″ differential below is from a Escort Cosworth 4×4.

In this case the yellow writing states ‘2E 603 362’. The ‘362’ indicates this is a 3.62:1.
If this writing has been lost then also stamped into this Crown gear is the Ford part number.
The number reads ‘Ford 90BG-4210AA 3,62’.
By counting teeth of the Crown and Pinion gears you can calculate the ratio.
⁴⁷/₁₃ ≈ 3.6154’ (3.62:1)

⁽¹⁾Not for 5 speed right hand drive sedan (& LHD Italy + Spain)

⁽²⁾Only for 5 speed right hand drive sedan
⁽³⁾Only for France
⁽⁴⁾Not for Sweden
⁽⁵⁾Only for Sweden
The 7.5″ rear Granada / Scorpio differential has a different shaped housing.
(FINIS CODE 6141389)
The following table is a collation of what I have found :
(unfortunately, I cannot verify this data)

85GG 4006 FB,
8500 4006 FB

The following cars were fitted with LSD’s but it also always important to double check.

• All 4×4 Sierra’s and Granada’s (2.8/9 and 2.0DOHC and Cosworth)
• Most but not all 24v Cosworth Granada/Scorpio (up to approx 1994 are LSD, after that they are ‘open’ type because the Scorpio 24v had traction control instead)
• All RWD Cosworth Sierra (3dr, Sapphire, RS500)

Diff Mountings

As mentioned above there are two sizes of diff casing (7″ and 7.5″).
The 7.5″ is available with two different rear covers (top mount and ‘Koala’ rear mount). Both covers are interchangeable. The ‘Koala’ rear mount diffs are from the late model ‘face-lift’ Granada’s, known as Scorpio or Ultima.
The top mount diffs have 4x13mm bolts (early) or 4x15mm (later).

The Sierra diff has 2 types of drive-shafts: Lobro (Bolt-on – left picture) and Tripod (Splined – push in – right picture).
Diffs with Tripod type shafts are designed to be used with Drum Brake equipped hubs, However, it is possible with the use of adaptor plates or hybrid shaft / hub combos, to use disc brakes. Bolt−On units are favoured as ultimately they are stronger, but for less extreme applications the Lobro units are fine.

Final Drive Ratios

The gear ratios of the MT75-4×4 gearbox are :

• 1st Gear – 3.89
• 2nd Gear – 2.08
• 3rd Gear – 1.34
• 4th Gear – 1
• 5th Gear – 0.82

I’ve found a very useful gear calculator here :
Ms Excel Gear Ratio Calculator
The plan for this project is to use 265x30x19 tyres, coupled to a 3.62 diff. Ideally, the diff ratio should be closer to 4.27:1 but unfortunately this front diff ratio is not available. With this combination the theoretical top speed would be close to 155mph. Having been in a Super 7 at 120mph, I know 150mph would be more dumb than challenging.

The Sierra Diff 4×4 route

With the aim being to go 4wd, the choice drops drastically. The Sierra front diff comes in 2 ratios, 3.62:1 & 3.92:1 (now very rare).
The target donors would be :

• 2.0lt DOHC 4×4 Sierra (1982-1993) – 3.92:1 (rare!)
• 2.9V6 4×4 Sierra (xr4x4i) -3.62:1
• Cosworth Sierra 4×4 -3.62:1
• 1.8 Turbo Diesel Sierra (with ABS) -3.62:1
• Cosworth Escort 4×4 (1992-1996) -3.62:1
• Granada Scorpio (1985-1991) -3.62:1
• Granada 4×4 -3.62:1

If you want LSD the choice drops even further – see above list
Going the 4×4 route with the MT75 gearbox and Sierra diffs, there will be a permanent 34% / 66% split on front / rear power. Therefore, most of the power is going to the rear, hence why only the rear differential was fitted with Limited Slip.