21 February 2023
Trevor Cuthbert uses a neat, low-cost kit to adapt a Discovery gearbox for his Defender
Need to know
Time: 2 hours
Difficulty: 2 out of 5
Models: Defender 90, 110, 130.
Tools needed: Spanners, socket wrenches, screwdrivers, pliers.
Parts & costs: R380 Discovery to Defender Adapter Kit, £72.50 from Synchro Gearboxes.
• Wear protective gloves and safety boots when handling heavy objects, such as the gearbox.
• Wear eye protection when releasing and fitting clips and springs, and when using compressed air lines.
Contacts: Synchro Gearboxes, onlinegearboxparts.com
See also Trevor's previous Project Defender Td5, parts 1-7, in our How-to section.
Ever since the first Discovery came on the market with its superb 200Tdi engine, Land Rover owners have been transplanting the engine to their utility coil-sprung vehicles as an upgrade from their lower-powered 12J diesel engines and 19J turbodiesels. In the early days, the engines were taken from accident-damaged Discoverys but, as values dropped because of age and corrosion, the practice only became more popular. This is also true for the 300Tdi-engined Discovery and even the Td5 version, where a good engine can be taken from a rusty vehicle to keep a Defender with an expired engine on the road. This is exactly what is being done for this 110 HCPU project.
Often, these Discovery donor vehicles have excellent transmissions, too, and the transfer gearbox from a Discovery is a popular upgrade to a Defender. However, the R380 five-speed gearbox is not a straight swap because in both the 300Tdi and Td5 Discovery, the gear shifter is located differently from that of a Defender. The gearboxes can be adapted by significant internal modification, but this is beyond the scope of many owners and is really best left to a professional gearbox specialist – with the associated cost.
Consequently, good Discovery gearboxes are of lower value than good Defender gearboxes, as they are of little use to Defender owners. However, a very clever adapter kit is available from Synchro Gearboxes that is easy to fit and within the scope of any owner. This piece of kit has allowed me to make use of a very good R380 gearbox, from the same Discovery that the engine came from – replacing the Defender’s original high-mileage R380.
Removing parts needed from Defender gearbox
The use of the Synchro Gearboxes R380 gearbox adapter kit requires the gear shifter housing from a Defender, as well some other associated parts. For example, the high-low/differential lock shifter is absent in a Td5 Discovery, as it uses a cable-operated high-low shifter. However, most owners wishing to utilise a Discovery R380 ’box will be replacing their old Defender gearbox and therefore have all the necessary parts available.
Some of the Defender parts required may be tired and worn, so careful inspection is recommended as you harvest the parts from your old gearbox. For example, the slots in the gear lever turret can become enlarged with wear and age; a replacement at around £60 will ensure smoother and more precise gear shifting.
The donor: A number of parts are needed from the old Defender R380 gearbox in order to convert the Discovery gearbox for use in the Defender 110.
Transfer ’box linkages: Linkages for high/low selection and to operate Centre Differential Lock (CDL) already disconnected from transfer ’box when it was detached from the car.
Crack them off: There are four bolts to be removed, which hold the gear shifter’s housing to the gearbox. These are loosened with a 10mm socket wrench.
Undo the nut: Before the gear shifter’s housing can be tapped free, the CDL linkage is detached by removing the bolt, using a 7mm and 8mm spanner.
Completely free: The CDL linkage is released from the pivot post on the side of the gearbox by removing the retaining clip and wiggling the link off the post.
Off it comes: Now, with a tap of the mallet to break it free, the gear shifter housing can be lifted clear of the gearbox and onto the bench to be cleaned up.
Important component: The lock nut that secures the pivot post on the right side of the gearbox is loosened using a 13mm spanner, and screwed out by a couple of turns.
Also frees breather pipes: The pivot post is now unthreaded from the side of the gearbox and will be used on the Discovery gearbox to operate the CDL.
Breather option: If needed, the gearbox breather pipe can be removed from the Defender gearbox, although there is no reason why the Discovery breather cannot be retained, if intact.
Peculiar to Td5 Defenders: This bracket is needed to secure the two big wiring harnesses (body loom and engine loom) that pass along the gearbox and under the driver’s seat.
Preparing the Discovery gearbox
Now that we have all the parts required from the old Defender gearbox, work can commence on converting the Discovery R380 gearbox. This can be done with the gearbox in situ, so our Discovery gearbox has already been fitted to the back of the Td5 engine and installed in the 110 chassis.
Likewise, the transfer gearbox is also already in place behind the R380, which is necessary when it comes to fitting the Centre Differential Lock (CDL) and high/low linkage, which was taken from the Defender transmission.
No small gear shifter: The Discovery R380 not only has a main gear shifter that is incompatible with the Defender layout but also lacks a high/low and CDL shifter.
Remove the unwanted: The Discovery gear shifter housing is held by four M8 bolts in exactly the same location as the four bolts that were removed from the Defender gearbox.
Give it a smack: With the housing bolts loosened the mallet is again used to un-stick the gear shifter housing from the gearbox with a gentle shock.
Off it comes: Without the CDL or high/low shifter linkages to worry about, it is a very simple job to get the housing off the Discovery gearbox and set aside.
Check for these: There are two locating dowels between the gearbox and the selector housing. It is important to make sure that both are in place in the finished job.
Filled in: This grub screw was initially hidden under a coating of gearbox oil, but a quick blast with the air-line got the head of the screw revealed.
Needs a snug fit: The grub screw holding the Discovery selector quadrant is loosened using a 5mm Allen key, in this case operated by a 3/8in socket wrench.
Completing the conversion
Don’t drop it in the R380: The grub screw needs to be removed from the selector quadrant completely, and then cleaned up for re-use in the new selector quadrant.
Out with the old: The Discovery selector quadrant is now slipped off the selector shaft
and removed completely form the Discovery gearbox.
Almost a shame to fit it: The new Defender adaptor quadrant is a beautifully engineered component and clearly a lot of research and effort was carried out to get it just right.
Get the positioning right: The new quadrant is fitted over the selector shaft, and the grub screw hole is aligned with the threaded hole in the selector shaft.
This is too much: After cleaning the grub screw, a drop (not this much) of thread lock is
applied to it so that it cannot work loose within the gearbox.
Job done: The grub screw is tightened up and, after wiping off excess fluid, the Defender adaptor quadrant is now ready for the Defender selector housing to be fitted.
RTV sealant also works: After cleaning the mating surfaces, a new gasket is placed over one of the locating dowels. The other dowel is in the Defender selector housing.
On it goes: The Defender selector housing is now fitted on the R380 gearbox, aligning the selectors with the quadrant, and the gasket with the locating dowels and bolt holes.
Caution – don’t over-tighten: The four bolts that held the gear selector housing to the Defender gearbox are used to fix it to the Discovery gearbox and tightened to 25Nm.
Factory fit: The bracket that was transferred with the Defender selector housing allows the Td5 wiring harnesses to be correctly attached along the side of the gearbox.
Convenient electrics: The reverse switch lead on the Disco R380 gearbox plugs directly into the Defender wiring loom in this example, so no need for re-wiring. The conversion is complete.
LIKE TO READ MORE? Try our Budget Digital Subscription. You'll get access to over 7 years of Land Rover Monthly – that’s more than 100 issues plus the latest digital issue. The issues are fully searchable so you can easily find what you are looking for and what’s more it’s less than 10p a day to subscribe. Click here to find out more details and start enjoying all the benefits now.