GEARBOX - Limited Slip Diffs; necessary parts for installation.

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Terminology -

LSD   - Limited Slip Differential

CWP - Crown Wheel and Pinion

FD     - Final Drive NOTE: A 'high' or 'low' ratio gear is in reference to it's performance, not it's numerical number. To illustrate - a 'high' FD ratio will give 'higher road speed', but will have a numerically low figure. A 'low' FD ratio will give lower road speed, but have a numerically high figure. A by-product of this will be reduced acceleration capability on the 'high' ratio, increased acceleration capability on the 'low' ratio. The main gearbox gears work in the exact same way.

Fitting an LSD isn't as simple as replacing the diff cage unit. In all cases a certain degree of diff housing modifications is needed - material needing to be ground/filed away to provide clearance for larger diff housing cases and crown wheel bolts. Although it has to be said the Quaife diff is supposed to fit without these mods. I've never found that.

The design and manufacture of the LSD to facilitate any other function other than that of a standard 'open' diff assembly precludes use of standard CWPs. So one suitable for an LSD is needed. Naturally Mini Spares/Mania supply these in an extensive range of FD ratios.

The output shafts are also different. A much thicker spline type is used within the LSD assembly. Consequently a suitable pair of LSD-compatible output shafts are needed. Some folk still insist on running the archaic, power consuming Hardy-Spicer type driveshaft to diff joints - although this is the only real option for rallying unless a change in driveshaft assembly is considered (another story!), as the inboard CV simply does not have sufficient engagement length to cope with full bump and full droop suspension travel.

Maintaining the Hardy-Spicer type driveshaft coupling system will not only require compatible output shafts, but retaining washers and circlips, and diff case side plates too (standard 'S' items) if not already in use. This has perplexed and vexed many who want to use the Hardy-Spicer type couplings and output shafts in rod change gearbox cases. The problem being the gear lever selector detent in the right-hand side diff casing. No problem - Mini Spares/Mania simply made a special side plate of 'S' dimensions but with the detent lug cast on.

Utilising the inboard CV joints where excessive suspension travel won't be experienced is the simplest, cheapest option, as only a pair of output stub shafts is required. The standard diff case side plates being retained and inboard CVs are a dime a dozen. For all road racers, where regulations permit, this is the better option by far as the power consumption of the joint is far less than that of the Hardy-Spicer UJ coupling type. They are certainly more than strong enough for the job. The only way to better the inboard CV as far as reducing power consumption is concerned to go to a better drive-shaft assembly.

FD options available -

3.46 - 52 x 15 teeth - P/No. C-BTA1250

3.76 - 64 x 17 teeth - P/No. C-BTA1248

3.93 - 55 x 14 teeth - P/No. C-BTA1252

4.07 - 53 x 13 teeth - P/No. C-BTA1246

4.23 - 55 x 13 teeth - P/No. C-BTA1251

4.31 - 56 x 13 teeth - P/No.C-BTA1249

4.67 - 56 x 12 teeth - P/No. C-BTA1253

Required retaining bolts CWP to Diff case - P/No. BTA370 (6 off)

NOTE; as the FD becomes lower, the pinion gear becomes smaller, so is less strong. Where possible it is advised the required gear ratio should be achieved with the strongest pinion size (highest FD ratio) allied to the required ratio drop gear set. Also note although some pinions have the same tooth count, they are not interchangeable because of pitch correction to achieve required mesh with crown wheel cut.

Output shaft and diff side plate parts -

'S'/Hardy-Spicer type coupling output shaft              - P/No. C- BTA1262

Relevant thrust washers - one each side needed      - P/No. C-BTA1243

Relevant retaining circlips - one each side needed   - P/No. CCN122

'S' diff case side plate                                                     - P/No. 22G419

As above but for rod change gearbox case               - P/No. 22G420

Seals for above side plates                                           - AHU1082

Inboard CV joint type output shaft                             - P/No. C-BTA1263

Diff case side plates for above                                     - CHM85

Seals for above side plates                                           - ADU5738

Article Date: Oct 16, 2000
Car Accociations: MINI,MORRIS,SPRITE,
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