Well it's in.
But it is not the discovery box I was originally was going to use. On deeper inspection that box had the wear of something that had driven around the world several times and really need a rebuild.

I sourced another LT230 box, this time a low mileage 1.1.222 transfer box which the serial number prefix 41D shows it was from a TD5 Discovery2 around 2002 as it has the difflock mechanism fitted (but yet unused on the D2).  Later models had the difflock gubbins totally removed.

To make this box suitable for the use on my Tdi defender a coupel of things need to be swapped with items from the old box.
Firstly the hi/low linkage arm is different so swapping the complete selector housing is a quick and easy solution.
The front prop-shaft coupling flange is larger so that need to come off and replaced with the defenders. That involves fixing a locking bar across the flange using the prop bolt fixings that stops it rotating as a breaker bar is used to undo big nut with 30mm socket.

There is no speedo pickup on this transfer box because the D2 gained speed info from the ABS system. The case has a casting dip where the hole would be. Drilling/machining that hole to take a speedo pickup would be pointless because the rear output shaft hasn?t got the gear ring fitted to drive the pickup. When removing the rearshaft end housing it comes away complete with the shaft. This make it easy to swap with the defender?s housing (and shaft).

As for fitting, I strapped it and as I pulled it upwards a few inch at a time from inside the truck, my assistant placed gravel sacks underneath to support it. Once at a suitable height it was easy to lift and slide into place.

The motorway drive is much smoother now, low revs and quite. It is possible to talk to passenger without shouting and repeating whilst cruising at 70mph.

For my reference:

Serial : 41D906137G
LABEL SAYS 1:1.211

Input gear is 135mm long with 38 teeth with part number: FTC4850
The input gear bearing are FRC5564.

To gain a better cruise speed for lower revs which will be a benefit for my forth coming November John-O-Groats road trip I am replacing the Defenders 1:1.4 ratio LT230 transfer box for 1:1.2 LT230 box fitted to the Landrover Discovery models.

I have had a disco box here for over a year awaiting for the motivation to replace the worn out input gear and fit it to the truck. Turned out the input gear on the defender is cross drilled and has no wear what so ever in the gear splines or the mainshaft splines. Lucky enough it was also exactly the same type of gear needed for the disco box (129mm length, 26 teeth part no FTC5089), so I recycled it. Even with the smallest shim fitted the preload freeplay on the input gear was nil, but it turned nice and freely. I found another LT230 service manual that give details how to check the preload on the input gear with a spring gauge ? if between 5lb and 20lb then it?s okay. Mine was measured at 7lb so happy enough with that.

I have so far removed the xferbox from the defender, it?s a awkward heavy job to do alone I can tell you that.

I managed to bend two xferbox/gearbox fixing studs so have had to order them part number FRC5595.

Needed to replace the input oil seal on the disco box -   part no. ICV100000

Also the rubber mount on the transfer box side (driver) wasn?t looking too healthy so had to order one of them too? part no ANR3200 (anr3201 is the gearbox side mount).

So next stage is to swap over the speedo gear and X-brake, fit the new bits and then see if I can put the box on by myself which I imagine is harder than take off because gravity isn?t going to be helping.

Few of my notes for future reference?

Even though serial numbers and sticky label tell the story of what transfer ratio the box is supposed to be it always worth checking to make sure. Here?s how I checked?With the hi box selected. Take up the back lash if any. Make a mark on the input gear at the 12 o'clock position.  Then for one full rotation on of the output flanges note how many turns the input gear rotates.  For a defender 1:1.4 ratio box the input gear will rotate nearly one and a half times  (marker will end up pointing about 5 o'clock). For a 1:1.2 disco box it will rotate about one and quarter times, the marker will be pointing just before 3 o'clock.