Technical Library

Technical

For those requiring something different after an engine change or ?tune up? the following will help explain the effect of ratio changes. When deciding, remember R & P changes will change H & L ratio, T case change will only change H ratio (all have the same low ratio). This can be used to advantages; e.g. in a LR90 change R & P from 3.54 ? 4.1 giving a 13.65% drop, now change the T case from a 1.4 ? 1.22 giving a 15.38% raise. Effect is to lower low range only leaving high range approximately as it was. We have included a simple calculation for those wishing to be precise about their speeds. A word of caution ~ over gearing often causes loss of economy as the engine needs to work above its optimum performance.

Torque Settings

Pinion flange bolt 100 Nm
Crown wheel bolts 60 Nm
Journal end cap bolts 90 Nm

R & P Comparisons

3.54 > 4.1 = 13.65% Drop
3.54 > 4.7 = 24.70% Drop

4.1 > 3.54 = 15.81% Raise
4.1 > 4.7 = 12.76% Drop

4.7 > 3.54 = 32.76% Raise
4.7 > 4.1 = 14.63% Raise

Installation

Setting the pinion height is critical to the correct operation of the ring and pinion, if this height is not correct the contact bearing surface will not be in the centre of the tooth and this will cause noise and less than optimum strength. The pinion head is marked with the nominal height above bearing shoulder.

Ideal height is 4.375ins we recommend setting this distance using a depth micrometer, place a bar of known thickness across two journal faces and measure down to the bearing face. If the pinion is marked other than 4.375ins, e.g. 4.370ins it means the pinion should be set at this height or .005ins higher than nominal. Backlash is also important in the noise/strength equation, this is different for each ratio but each ring is marked with the design backlash, e.g. for the 4.11 this is .006/.008ins

The final check should be made using blue and should show a contact bearing surface half way between root and tip and towards the inner end of the tooth on the crown wheel (ring). Under load the contact bearing surface will move along the tooth towards the centre but stay half way between root and tip.

Remember "only the blue is true" anything else is fiction.

Ideal surface contact.

High contact, pinion set too low will cause noise and excessive tip loading on the crown wheel.

Low contact, pinion set too high will cause noise and excessive tip loading on the pinion.

Torque Settings

10 x Crown Wheel Bolts - 60Nm

4 x Bearing Cap Bolts - Rover 90Nm, P38 180Nm

1 x Pinion Bolt - 100Nm

1 x Rover Pinion Nut - 129Nm

1 x P38 Nut - 135Nm

Please see our installation guide