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Swapping an Underdrive


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From Ashcofts site:

The unit will only be supplied in the 26 tooth, suffix C onwards input gear format, if you have a 28 or 38 tooth input gear fitted you will also need to change the intermediate and the high range output gear at an additional cost of £150. To find out if you currently have a 26/28/38 tooth input gear please refer to our fitting instructions.


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Fitting instuctions


The serial numbers of the transfer cases that have the 26 tooth input gear and will accept the underdrive without further modifications are:

Prefix Suffix

20D C, D, E, F, G 22D C, D, E, F, G 28D C, D, up to 308626E 32D up to 309416D 34D C, D, E, F 43D G, H 47D G 57D G, H

Add £ 150 for the additional gears if you have a transfer case with the serial number:

Prefix Suffix

28D from 309143E, F 32D from 309417E 36D G 40D G 41D G 42D G 59D G 61D G 62D G 68D G 69D G 70D G 71D G

Always check main shaft wear on installation

In addition to the main unit you should receive :

6 x 10mm studs, nuts and washers 1 x stone guard 1 x LT230 input gear "bearing cone" 1x PTO gasket

Split the underdrive, (note the main support plate to underdrive body has a gasket fitted be careful not to damage it) by removing the outer ring of 9 bolts remove the rear support plate and gearset. Note two holes are fitted with a short set screw, these are "jacking points" use two 45 mm long set screws, if necessary, to split the joint. Put the rear plate and gear set on one side for now.

You now have the output shaft and gear and the main casing to work with. Remove the PTO cover and bearing support plate from the LT230 and clean the joint face. Check the condition of the main shaft spline, a worn spline will cause premature wear on the UD input shaft. Fit the 10mm studs, used in place of the bolts. Remove the handbrake drum. Replace the inner input gear bearing cone using the new one supplied.

The first step is to check the preload on the underdrive output shaft; it must be the same as the original LT230 input gear. Put the main box and transfer box in neutral and offer the shaft into the transfer case.

Offer the underdrive main case to the transfer case, using the gasket supplied, and after fitting the washers and nuts tighten down evenly. Occasionally rotate the shaft, which can be gripped via the open end of the underdrive main case, to settle the bearings. If the shaft locks up stop tightening!!. With the main gearbox and transfer box still in neutral rotate the underdrive output shaft, and check for end float and or excess pre load, the preload was set on a test box with a gasket fitted but may vary from box to box. If it is necessary to reset the preload change the shim behind the bearing cone. It was preset with gasket fitted and 2.5mm shim FTC 748 (thinner shim is FTC 746, thicker is FTC 750)

Reassemble the rear support plate and gear set to the main housing, using the gasket supplied plugging the �jacking� holes with the short (25mm long) set screws.

With the main assembly in place but before re-fitting the handbrake drum it is advisable to grind/machine two flats, at 180deg, on the ridge at the front of the brake drum. (the second flat is to balance the drum). This will allow removal and refitting of the handbrake drum without disturbing the underdrive. Do not forget to fit the stone guard, essential to prevent small rocks falling into the natural trap between the drum and main casing.

The selector is not supplied, we can advise you need a movement of 27mm of the selector rod from one range to the other. Many people use the high/low lever from a classic Range Rover �Borg Warner� transfer case, attaching the lever to the seat box front.

Fill the underdrive adding the "Lubeguard" and top up with same oil as the LT230 via the filler level plug. There is a small �well� on the top lip of the main case where it enters the LT230, this catches oil from the LT230 and feeds it into the underdrive. be very careful not to block the feed with sealant. Oil will find its way out via the splined main shaft keeping the splines lubricated, or the max oil level hole. Once filled the underdrive will keep its own level, approx half full, feeding from the LT230. Check the level after approx 100km to be sure the oil feed is working correctly.

When shifting ranges allow the vehicle to roll, slowly, to enable the drive dogs to engage, do not use excessive force on the lever to engage the gears.

Please call us if you have any queries during installation, we will be happy to answer any queries,

Get ready for a phone call Dave ;)

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