Sierra Differential

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.

Sierra Rear Diff Video (LSD)

Sierra Rear Differential Image Gallery

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.

A=3.36(crown wheel 44 teeth, pinion 13 teeth, 7.5" diff casing)	B=3.77	D=3.92(crown wheel 51 teeth,pinion 13 teeth, 7" diff casing )
F=3.842	J=4.056	K=3.31
M=3.14(crown wheel 47 teeth, pinion 15 teeth, 7"/7.5" diff casing)	N=4.29	P=3.58
U=3.62(7" diff casing)	V=3.33(crown wheel 47 teeth, pinion 13 teeth, 7" diff casing)	Z=3.59
2=4.27	3=3.62 / 3.64?	4=3.82
5=3.26	6=3.12	8=3.56
9=3.91(7.5" diff casing)

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 4x4.

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)

Sierra Diff

Engine	Model		Transmission		Ratio	Number of Teeth
	Sedan Axle	Estate Axle	Man	Auto		Ring Gear	Pinion
1.6 OHC 2V	6.5" 7"	7"	X	-	Sedan 3.62 Estate 3.92	47 47	13 12
1.8 OHC 2V	6.5" 7"	7"	X	X	Sedan 3.62(1) Estate 3.62(1) Sedan 3.92(2) Sedan 3.38(3)	47 47 47 44	13 13 12 13
2.0 OHC 2V	7"	7"	X	X	3.62 3.38(4)	47 44	13 13
2.0 OHC EFI	7"	7"	X X	X	3.92 3.62 3.62(5)	47 47 47	12 13 13
2.0 OHC EFI MC	7"	7"	X	X	3.62	47	13
2.8 V6 HC FI 4x4	7"	7"	X	X	3.62	47	13
2.8 V6 HC EFI	7.5"	7.5"	X	X	3.36(5) 3.36(5)	47 47	14 15
2.8 V6 HC EFI 4x4	7"	7"	X	-	3.62(5)	47	13
2.3 Diesel	7"	7"	X	-	3.38	44	13

⁽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)

 

Ford Sierra Differential Ratios

Ratio	Vehicle	Shaft	Diff	Teeth	FINIS	Part No.
3.14	Sierra[TL16](−Mar83)(D), Sierra[TL16](−Apr83)(UK), Sierra[TV23,PD23], Sierra[TV28INJ−AUTO], Sierra[TL20H](non−ABS), Sierra[DL20H](non−ABS), Sierra[PD23](−Jan90)(non−ABS)	Tripod	Non−LSD		6107624	83BG 4006 KC
3.14	Sierra[TV28INJ](−Jan90)	Lobro	Non−LSD		1644899	85GG 4006 GB
3.14	Sierra[DL20](ABS), Sierra[PD23](ABS)(−Jan90), Granada[DL20](Feb92−)	Lobro	Non−LSD		6152100	87BG 4006 BA
3.14	Granada[DL20](Feb92−)	Lobro	LSD		6193181	88GG 4006 AEA
3.14	Sierra[TL16,TV23,PD23]	Tripod	LSD		1619200	V83BG 4006 AA
3.14	Sierra	Lobro	LSD		1648395	V87BG 4006 DAA
3.36	Sierra[TV28INJ](5Spd−Type9)	Tripod	Non−LSD		6131195	83BG 4006 BA
3.36	Sierra[TV29](Jan90−May91)	Lobro	LSD		6142222	85GG 4006 AAA 8500 4006 AAA
3.36	Sierra[LL23INJ](XR4)(USA/CDN), Sierra[TV28](−Jan90), Sierra[TV29](−May91)	Lobro	Non−LSD		6124451	85GG 4006 FB, 8500 4006 FB
3.36	Sierra[TV29](2WD)(May90−), Granada[TV29](2WD)(Feb92−), Granada[PD25T](Feb92−)	Lobro	LSD		6663905	91GG 4006 AAA
3.36	Sierra[TV29](May90−), Granada[TV29](2WD)(Feb92−), Granada[PD25T](Feb92−)	Lobro	Non−LSD		6663908	91GG 4006 FA
3.36	Granada[CP29]	Lobro	LSD		6831237	93GG 4006 AAA
3.36	Granada[VM25](Aug93−), Scorpio[VM25](Oct94−Jan96)	Lobro	Non−LSD		6803399	94GG 4006 AA
3.36	Scorpio[VM25](Jan96−)	Lobro	LSD		1026116	96GG 4006 BDA
3.36	Scorpio[VM25](Jan96−Sep96)	Lobro	Non-LSD		1015630	96GG 4006 FA
3.36	Scorpio[VM25](Sep96−)	Lobro	Non-LSD		1035424	97GG 4006 AA
3.38	Sierra[HL18](Jan91−), Sierra[D18T](Jan90−), Sierra[TL16,TL18,TL20,TL20H, TV20,TV23,PD23]	Tripod	Non−LSD		6107625	83BG 4006 LC
3.38	Sierra[TL18,TL20,PD23] Sierra[HL18](Aug89−),	Lobro	Non−LSD		1630004	85GG 4006 AC, 8500 4006 AC
3.38	Sierra[TL16,TL20,TV]	Tripod	LSD?		1619201	V83BG 4006 BA
3.38	Sierra	Lobro	LSD		1648394	V87BG 4006 CAA
3.62	Sierra[TL16,TL18](Jun85−),	Tripod	Non−LSD		6107623	83BG 4006 CC
3.62	Sierra[TL16,TL18](Jun85−),	Tripod	Non−LSD	23	6157715	83BG 4006 CD
3.62	Sierra[TL18](Man)(Jun85−),	Tripod	Non−LSD	26	6157715	83BG 4006 CD
3.62	Sierra[TL20H,TV,TL18,HL18], Sierra[TL20INJ](GL,GHIA,2.0iS], Sierra[TL20,DL20](−Jan90), Sierra[TL20,DL20,D18T](Jan90−)	Tripod	Non−LSD		6107626	83BG 4006 MC
3.62	Sierra[TL16,TL18](Jun85−), Sierra[TL16]	Tripod	Non−LSD	26	6107626	83BG 4006 MC
3.62	Sierra[TL18](Man)	Tripod	Non−LSD	28	6107626	83BG 4006 MC
3.62	Sierra[TV28INJ](XR4)	Tripod	Non−LSD		6124295	83BG 4006 RC
3.62	Sierra[TL16,TL18,TL20,DL20]	Lobro	Non−LSD		6146560	85GG 4006 BC, 8500 4006 BC
3.62	Sierra(TV28INJ)	Tripod	LSD		1619202	V83BG 4006 CA
3.62	Sierra[TV28INJ,DL20,TV29], Sierra[CH20](Feb90−), Granada[CP29](4wd)(Feb92−)	Lobro	LSD		1627946	V85BB 4006 AA
3.62	Sierra[DL20](Jan90−), Sierra[D18T]	Lobro	LSD		1648393	V87BG 4006 BAA
3.64	Granada(85−92)	Lobro	LSD			85GG 4006 ACA
3.64	Sierra[LL23INJ](XR4)(USA,CDN)	Lobro	Non−LSD		6137510	85GG 4006 HB
3.64	Sierra(Cosworth)	Lobro	LSD			86BB 4006 AAA
3.64	Granada(2WD)(Feb92−)	Lobro	LSD		6663907	91GG 4006 ACA
3.64	Scorpio(Cosworth)	Lobro	LSD			91GG 4006 ADA
3.64	Scorpio	Lobro	LSD			95GG 4006 ACA
3.64	Granada(2WD)(Feb92−)	Lobro	Non−LSD		6663910	91GG 4006 HA
3.64	Scorpio[CP29](Oct94−Jan96), Scorpio[TV29](Oct94−Jan96)	Lobro	Non−LSD		7035888	95GG 4006 JA
3.64	Scorpio[TV29](Jan96−Aug96)	Lobro	LSD		1026114	96GG 4006 BCA
3.64	Scorpio[CP29](Jan96−), Scorpio[TV29](Jan96−Aug96)	Lobro	Non−LSD		1015634	96GG 4006 EA
3.77	Sierra[TL13]	Tripod	Non−LSD		6099509	83BG 4006 AC
3.77	Sierra[TL13]	Tripod	Non−LSD		1637523	83BG 4006 AD
3.91	Sierra[DL20](Aug90−)	Lobro	LSD		6490065	89GG 4006 AA
3.91	Sierra[DL20](Jan90−May91)	Lobro	LSD		6490066	89GG 4006 AAA
3.91	Sierra[DL20](May91−), Granada(Feb92−)	Lobro	LSD		6663906	91GG 4006 ABA
3.91	Sierra[DL20](May91−), Granada(Feb92−)	Lobro	Non−LSD		6663909	91GG 4006 GA
3.91	Scorpio[DH23](May96−)	Lobro	Non−LSD		1017523	96GG 4006 BA
3.92	Sierra[TL16](−Oct84)	Tripod	Non−LSD		6099514	83BG 4006 JC
3.92	Sierra[TL16](Oct84−), Sierra[TL20INJ](2.0iS), Sierra[TL16,HL18,TL20]	Tripod	Non−LSD		1627164	83BG 4006 JD
3.92	Sierra[TL16,TL18,HL18,TL20INJ], Granada(Feb92−)	Lobro	Non−LSD		6146561	85GG 4006 DC, 8500 4006 DC
3.92	Sierra[DL20](Jan90−), Granada(Feb92−)	Lobro	LSD		6171465	86GG 4006 AAA, 8600 4006 AAA
3.92	Sierra[DL20](−Oct90)	Tripod	Non−LSD		6490064	89BG 4006 AA
3.92	Sierra[DL20](Oct90−May91)	Tripod	LSD		6490064	89BG 4006 AA
3.92	Sierra[DL20](May91−)	Tripod	LSD		6663904	91BG 4006 AA
3.92	Sierra[TL16]	Tripod	LSD		1619204	V83BG 4006 EA
3.92	Sierra[TL16], Sierra[TL20INJ](2.0iS)	Tripod	LSD		1630675	V83BG 4006 EB
4.09	Scorpio[DL20](Jun95−Jan96)	Lobro	LSD		1013747	95GG 4006 AAA
	Scorpio[DL20](Oct94−Jan96)	Lobro	Non−LSD		7035886	95GG 4006 AA
4.09	Scorpio[DL20](Jan96−)	Lobro	LSD		1015626	96GG 4006 BAA
4.09	Scorpio[DL20](Jan96−)	Lobro	Non−LSD		1015621	96GG 4006 CA
4.27	Scorpio[DH20](Oct94-Jan96)	Lobro	Non−LSD		7091521	95GG 4006 LA
4.27	Scorpio[DH20](Jan96−May96)	Lobro	Non−LSD		1015627	96GG 4006 DA

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

• All 4x4 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 MT75-4x4 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 4x4 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 4x4 Sierra (1982-1993) - 3.92:1 (rare!)
• 2.9V6 4x4 Sierra (xr4x4i)  -3.62:1
• Cosworth Sierra 4x4 -3.62:1
• 1.8 Turbo Diesel Sierra (with ABS) -3.62:1
• Cosworth Escort 4x4 (1992-1996) -3.62:1
• Granada Scorpio (1985-1991) -3.62:1
• Granada 4x4 -3.62:1

If you want LSD the choice drops even further - see above list

Going the 4x4 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.