TWO EXTRA GEARS...

The Laycock-de-Normanville D Type Overdive was used on many Rootes and Triumph models in the 1960s and is a typical unit of its type. Generally a reliable and robust unit, they rarely give serious trouble, though long periods of inactivity can present problems.

If an overdrive unit fails to operate then the first two things to check are the oil level in the gearbox / overdrive assembly and the electrical connections on the operating solenid. The control mechanism should also be checked for accurate adjustment.

Should it be necessary to remove an overdrive for overhaul then any remaining hydraulic pressure inside the unit should be released first. To do this turn on the car's ignition (do not start the engine) engage top gear and then flick the overdrive selector from direct drive to overdrive a dozen or so times.

With the unit removed from the car and on the workbench, stripdown and overhaul can begin.

 Start the dismantling procedure by removing the operating valve plug, plunger and spring. Then use a magnet to withdraw the valve ball followed by the valve itself. Next remove the cover plate on the operating mechanism and the two solenoid screws so that the solenoid plunger can be detached from the operating arm.

There are a total of 8 nuts around the centre flange of the housing and these should be loosened in a progressive manner so that the clutch springs gradually force the two ends of the housing apart.

The brake ring will come away with the main housing but is likely to be a tight fit. If so a little persuasion with a soft faced mallet my be necessary but do not be tempted to try and prise the two apart. At this stage the filter, sealing ring, and magnetic washer can also be extracted.

The two pistons are located in the main housing by bridging straps located by two nuts each. Once these have been removed you should be to easily withdraw the pistons.

Pump body and relief valve is secured by a locking wire and very small grub screw which locates into the groove of the pump body just below its spring.

 The cone clutch, sun wheel, planet gear and annulus all live in the intermediate housing. The first three can be withdrawn from the hosing, the annulus stays in place.

The sun wheel can be separated from the cone clutch after removing a snap ring. Carefully check the condition of the cone clutch lining. If this is worn or damaged in any way then it will need to be renewed. The lining is secured to the clutch body by a series of roll pins.

The clutch bearing is secured in place by a large circlip or spring ring. Once this has been extracted the bearing housing will pull free of the cone clutch so that the bearing can be pulled from the housing.

The freewheel assembly is located in the annulus behind a retaining plate. With the retaining plate removed the freewheel assembly simply pulls out - but be ready for the twelve rollers which will fall from their cage!

Prior to removing the rear cover from the intermediate housing it is necessary to remove the speedometer pinion assembly. This is just a case of removing a single locating bolt.

Rear cover is removed by undoing 6 nuts. Between the rear cover and intermediate housing there will be one or more shims. These prohibit fore and aft movement of the annulus / rear bearing assembly.

You will now have access to the speedometer pinion drive gear. Lever back the tabs on the washers and remove the retaining lock nut. The drive gear should then easily slide off the output shaft.

The bearing locating ring can then be removed and the bearing driven forward out of the intermediate housing. If the bearing needs to be replaced then it can be driven or pulled from the shaft.

The last part of the dismantling process is to prise the oil seal from the rear cover to access the two bushes that support the output shaft. With the unit fully dismantled it is now time to thoroughly clean and check all the components.

A basic service kit will be required before the unit is reassembled. This will consist of new O rings, bushes, seals, gaskets and also a new filter although, if cleaned thoroughly, the old filter could be re-used.

It is important that the rear bearing is installed with the recess for the retaining ring rearmost. The speedometer pinion drive gear must butt up against the rear bering or it will not engage properly with the pinion. Remember to refit / replace the tab washer as well as the locknut.

Rear cover bushes can be carefully drifted in with a soft mallet and a wooden block. They must be carefully position one at each end of the sleeve in the rear cover. You can then fit the new oil seal and replace the circlip that holds it in place.

If a new rear bearing has been used then new shims of the correct thickness must also be fitted. To establish the required shim thickness fit oversize shims initially, measure the gap with feeler gauges and remove shims equivalent to size of the gap.

Special tools are available to hold the rollers in the freewheel whilst the assembly is replaced in the annulus, but a large worm drive clip will do the job just as well. Remember to ensure that the thrust washer is in place first though.

Before refitting the planet gear check to see that the marking on each gear pinion is aligned with the markings on the planet carrier. If the pinions should be damaged they can be removed by shearing the locking pin.

When installing the new or well cleaned filter fit the sealing ring to the housing first. The magnetic washer can then be fitted directly to the filter before it is pushed home. You can now fit the pump and relief valve assembly.

When properly tightened lock the relief valve and pump body plugs in place with wire - .8mm MIG wire is ideal for this. Refit the pistons and bridging straps The brake ring can then be refitted making sure that its larger inner diameter is towards the rear of the unit. Install the clutch springs and tighten the main housing bolts progressively.

With the main unit now back in one piece all that remains is for the solenoid assembly to be refitted using a new gasket. Finally, the cover plate can be replaced to keep the unit clean though this will need to be removed again once the unit is refitted to the car so that the necessary adjustment of the engagement mechanism can be correctly set.