Sierra Cosworth 4×4 Front Differential

I cut up a sump from a Ford Sierra XR4x4 (It might have been a Cosworth??!?) to make a custom differential shaft housing. I added some strenghtening hollow tubes and two thick aluminium plates. The remants of the sump, were then TIG welded to the new material to form a very strong but light, bolt on assembly. The new set-up is actually lighter than the Sierra sump, yet I am positive that mine would be stronger.

Ford Sierra 4x4 Front Differential

Normally, the differential is bolted to the side of the sump and is free to move as the engine rocks around. Although the central drive shaft passes right underneath the crank and it’s housing is part of the sump, engine oil does pass inside. With my layout, the differential is in front of the engine, therefore a replacement central housing is needed.

I incorporated a sump reinforcing plate into the central assembly. This should hopefully strengthen the notoriously weak standard main differential housing. The whole assembly, including the sump plate, is now solidly mounted to the chassis in several places.

Now the differential is in, I fell much better about adding the missing triangulation from the front of the chassis. The differential assembly and the extra framework will work together to make this part of the chassis incredibly rigid. Before mounting the diff, when you hit the chassis with a hammer, it rang like a bell. Now it just makes a ‘tink’ sound like hitting a solid block. Extra tubes will also form mounts for the rocker arms for the inboard shock absorbers.

Because of the 19″ wheels I need to get the optimum differential ratio. The standard diff would give me a top speed of nearly 160mph. The Standard Cosworth and XR4x4 ratio is 3.62:1 but I have a 3.9:1 differential from a 2.0 DOHC GLS 4×4. This gives me a top speed around 140mph. The tag reads:

V90BB AA
C9P5A CG02

Haynes Roadster 4x4 fron differential

OK, so not much in terms of actual work, but when there is ice on the workshop floor I’m quite pleased the progress. The only thing to go wrong was the discovery that the water pump outlet is a little too close to the prop-shaft UJ for comfort. The rubber hose would miss the UJ by around 7mm. Therefore, the water-pump will be the next thing to be cut up and modified. Not by much, in fact, I’ll cut a 5mm V of material out, bend the outlet up and re-weld the join. This should give me a much healthier 15-18mm clearance. I’ve also considered fitting a stabilizer bar to the top of the engine, to prevent it from twisting around too much.

You might ask why I went to all the effort of making a central section for the front diff. Other cars have been built with the nearside drive-shaft being a single long shaft attached directly to the differential. The central section is omitted.
If I did this then the extra space the long drive-shaft would need as it moves up and down would, cause it to foul the suspension push rods.
Some pick-ups and 4×4’s have axles that are potential candidates but are simply the wrong ratio and widths.
Another option would be to use two rear differentials, as they rotate correctly when reverse 360° (I checked). A special central shaft assembly would be needed but this is potentially the strongest setup.

The next jobs are to add the extra triangulation tubes and to mount the steering rack. I’ve always been a little worried about the rack as UJs are needed for the steering shaft to wrap its’ way around the diff. One UJ is giving me particular concerns. I won’t know just how tight it is going to be until the rack is solidly mounted.

Steering shaft and new UJ

I had to make my own Universal Joint to fit the Citroen BX rack. I needed to make a steering shaft and new UJ's. It's all getting a little tight and I still have some bracing to add. Because this Locost chassis is 4 wheel drive, the rack was initially mounted ...

4x4 Haynes Roadster

I've started to mock up the front 4x4 system. In CAD everything looked extremely tight. I spent ages moving components back and forth to get everything to fit. So far, in the mock up. There looks like bags of room. When I say 'bags' I mean a good 6-7mm! ...

Sierra Front Differential Part II

Sorry, but there is not much to show again this week. I ran out of welding wire and the reel I ordered on Fleabay still hasn't arrived. The same applies to the aluminium plate, aluminium tubes, gasket set and bolts. I'm rubbish at excuses when it cold........... What remains of ...

Ford Sierra Front Differential

Apologies for the delay in writing a new blog, but things have actually been happening behind the scenes. The MT75 gearbox is away with the brother in law, getting the new bell-housing welded on. I've been stripping and cleaning loads parts etc etc etc. 4x4 Kit Car To state the ...

Kit Car 4wd Front Differential

Fitting the front diff has been one of the hardest parts of this build so far. The Rover V8 is not a huge engine but still finding room has been tricky. Changing the front timing chain cover has proved very successful. The differential has been angled forward by 5°. The Rover ...

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 ...

Sierra 4x4 Front Driveshaft Cover

This Sierra 4x4 Front Driveshaft Cover forms part of the Cosworth front axle assembly normally bolts to the nearside of the sump. In this design however, it will be mounted to a custom axle assembly. The cover would normally bolt to the right hand side of the sump / shaft assembly ...

Ford Sierra Front Differential Cover

This project will use a Ford Sierra Front Differential 4x4, 3.92:1 front differential (from a non-Cosworth Sierra 2.0s 4x4). Normally this diff would bolt to the alloy sump with integral drive shaft. However, for this application the diff will be mounted in front of the engine, with a custom drive-shaft assembly. ...

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