Second hand series gearboxes are about £200. Getting a gearbox reconditioned can be anything up to £1200 from Cradocks or Paddocks. A mechanic from the club may do it for less. An Lt77 can be had for £500. If you have the space and time you could always pull it out and recon it yourself, there are plenty of videos on the net showing you how.
Adapter kit to mate a LR 5 speed to a Series LR transfer case £460 from Ashcroft. This is what they say.
We often get asked about putting a 5 speed LT77 or R380 into a Series vehicle. This combination gives a stronger gearbox, synchro on all forward gears and overdrive 5th. There are a few ways of doing this depending on the axles fitted and if it"s an 88" or 109".
The series transfer case is part time 4 wheel drive and the front axle is fitted with Universal Joints (UJ"s). The Defender and Disco is fitted with the LT230 which is full time 4 wheel drive and because of this the front axle has CV joints.
If you have coil sprung axles on your Series then they will have CV joints and thus you can just fit the 5 speed with the LT230 and this will be fine.
If you have stock Leaf sprung axles you have 2 options :
1) You can fit the 5 speed to the engine then modify the LT230 to part time 4 wheel drive and fit this in conjunction with free wheeling hubs on the front. The way this kit works is you fit a few parts inside the LT230 centre diff to change the operation of the diff lock so that in the "diff locked" position the centre diff is locked and you have locked drive to the front and rear props but in the "unlocked" position there is no drive to the front, just the rear, so when used the Free Wheeling Hubs the front prop, diff etc is not spinning. We are able to supply these parts, please see here
2) The other option is to fit the LT77 or R 380 and then fit the series transfer case onto the back of it with this kit. This way you retain the stock "Series" transfer case with selectable 2/4WD. You can also fit freewheeling hubs so you are not wasting fuel spinning the prop, diff Etc. This ends up a shorter transmission and allows you to keep the stock front grill even on a 88".
The gearbox length is increased by 102mm (4ins). The preferred method to accommodate this is to move the engine forward as this entails no change to prop shafts or cross members. Please note on some vehicles you will also need to modify cross members as well. This is not sold as a complete kit to put a 5 speed into a Series as there are many other considerations as mentioned, this kit is just the parts to mate the gearbox to the Series transfer case.
