The ubiquitous Ford Sierra has donated its trusty differential unit to a large number of specialist sports cars in recent years, not least countless Caterham Sevens. From the time it was first adopted by various sports car manufactureres however, it has been all too apparent that the standard Ford final drive ratios were less than ideal for sports motoring. At 3.62:1 and 3.9:1, they were clearly designed for economical cruising, and not lively acceleration. That, one suspects, was the primary motivation behind Caterham Car's decision to commission, at considerable expense, their own six-speed gearbox for the Seven.

Inevitably perhaps, Caterham's six-speeder is an expensive - albeit desirable - solution to the problem of an unsuitable final drive ratio. Raceline decided on a different approach, and commissioned a replacement crown-wheel-and -pinion for the Sierra differential. For those using a Sierra differential with a more conventional four or five-speed gearbox therefore, our crown-wheel-and-pinion units are available and offer alternate final drive ratios of 4.4:1, 4.44:1 and 4.7:1. The 'higher' (numerically lower) ratios are ideal for fast road use, perhaps including the occasional track day outing: while the 'lower' (numerically higher) ratios provide maximum acceleration, and are primarily intended for those intended for those involved in sprint and hillclimb competition. For those requiring a limited slip differential, we can offer a variety of options.

We also offer a differential rebuild service, specialising exclusively in the Sierra diff. You can therefore, buy a Sierra-type differential unit in a fully refurbished casting with your choice of final drive ratio and your your preferred type of LSD, and which looks to all intents and purposes brand new. We can also supply alternate final drive ratios for the 71/2 in 'Cossie diff', and for the 'English' axle.