Sierra 3.9:1 Front Differential

It’s been a few weeks since my last blog and for good reason. My project has a new home. I had to knock down walls and do some pretty scary electrics, but the work is coming to an end. The next job is a heavy duty chassis table.

More good news…

I’ve finally unearthed a Sierra 3.92:1 Front Differential. They were fitted to the Sierra DOHC 2.0i GLS/X 4×4 01/15/1990 – 15/02/1993 (non-cosworth). I’ve bought 2 previous diffs that claimed to be 3.92 but this one really is!!

It’s the real thing! I was seriously beginning to believe that they were an internet myth. I’d even been to the local Ford dealer and searched dozens of microfiche over several hours.

As you can see by the drawings top left, I intend to replace the Cosworths’ sump assembly with a custom built unit. The assembly will also include a strengthening plate.

In this build, the diff will sit in front of the engine and not alongside.

The Sierra 3.92:1 tag reads:

V90BB	AA
C9P5A	CG02

I will confirm the precise ratio when I rebuild it.

The FINIS number for the 3.92:1 is 1661121

1	1634747	Axle Assy – Front Diff	to 15/1/1990 \| 3.62 from 15/1/1990 to 15/2/1
1	1661121	Axle Assy – Front Diff	from 15/1/1990 to 15/2/1993 \| DOHC 2.0 3.92
1	6478336	Axle Assy – Front Diff	from 3/1/1990 to 15/2/1993 \| Cosworth 2.0 3.62
2	1637137	Shaft – Intermediate	to 15/2/1993 \|
3	1632648	Tube – Intermediate Shaft	to 15/2/1993 \|
4	1627953	Gasket	to 15/2/1993 \|
5	1627954	Gasket	to 15/2/1993 \|
6	1634748	Bearing Assy – Ball	to 15/2/1993 \|
7	1637272	Gear – Speedometer	to 15/2/1993 \|
8	1634753	Circlip	to 15/2/1993 \|
9	1542801	Circlip	to 15/2/1993 \|	40 x 1.85MM
10	6802339	Circlip	to 18/1/1993 \| Cosworth 2.0	25 x 1.6MM
10A	6008347	Circlip	to 15/2/1993 \| Cologne V6to 15/2/1993 \| D	38 x 1.8MM
10A	6586453	Circlip	to 15/2/1993 \| Cosworth 2.0
11	1643701	Cover – Intermediate Shaft	to 15/2/1993 \|
12	6152992	Bearing – Speedo Driven Gear	to 5/1/1993 \|
13	1630829	Pin – Roll	to 15/2/1993 \|	4 x 20MM
14	6028927	“O” Ring	to 5/1/1993 \|
15	1629528	Gear – Speedometer Driven	to 15/2/1993 \|
16	1634970	Shaft – Front Axle	to 15/1/1990 \|from 3/1/1990 to 15/2/1993 \|
16A	6478335	Shaft – Front Axle	from 3/1/1990 to 15/2/1993 \| Cosworth 2.0	RH
16A	6672254	Shaft – Front Axle	from 3/1/1990 to 15/2/1993 \| Cosworth 2.0	LH
17	1627944	Joint And Stub Shaft Assy	to 15/1/1990 \|from 15/1/1990 to 15/2/1993 \|
17A	6672253	Joint And Stub Shaft Assy	from 3/1/1990 to 15/2/1993 \| Cosworth 2.0
18	1632763	Shaft Assy – Drive	to 15/2/1987 \| Cologne V6 2.8
18	1659240	Shaft Assy – Drive	to 15/2/1993 \| Cologne V6 2.9 EFIfrom 3/1/1
19	1627950	Kit – Drive Shaft Boot Repair	to 15/9/1991 \|from 15/9/1991 to 15/2/1993 \|
19	6672258	Kit – Drive Shaft Boot Repair	from 15/9/1991 to 15/2/1993 \| Cosworth 2.0
19A	1638320	Kit – Drive Shaft Boot Repair	to 15/9/1991 \|from 15/9/1991 to 15/2/1993 \|
19A	6672257	Kit – Drive Shaft Boot Repair	from 15/9/1991 to 15/2/1993 \| Cosworth 2.0
20	1436268	Washer – Flat	to 15/2/1993 \|	10.5 x 20 x 2.5MM
21	6036438	Washer – Flat	to 15/2/1993 \|	20.2 x 40.3 x 5MM
22	6104837	Bolt – Hex.Head	to 15/2/1993 \|	M8 x 20MM
23	1613366	Bolt – Hex.Head	to 15/2/1993 \|	M10 x 55MM
24	6152647	Screw – Socket Head Cap	to 15/6/1988 \|	M8 x 16MM
24	1650759	Screw – Socket Head Cap	from 15/6/1988 \|	M8 x 16MM
25	6036437	Nut – Special Hex.	to 15/2/1993 \|	M20 x 1.5MM
26	1660443	Bracket	to 15/2/1993 \| Cosworth 2.0
27	1660444	Bracket	to 15/2/1993 \| Cosworth 2.0

Here it is… All bright and shiny……..

I have two rear differentials. One is a 3.92:1 tripod diff (push in shafts) and the other is a 3.62:1 Lobro LSD unit (bolt on shafts) from an Escort Cosworth. I have read many articles on rebuilding these diffs, but I think I will leave the job of swapping the crown and pinion gears between these two, to the professionals. If a front and rear rebuild kits turns up cheap on Fleebay then I’ll hand them over at the same time. I’ve searched long and hard for these now rare and increasingly expensive diffs and I’m not about to ruin them!

For more info on the Sierra front diff click here