Recently changed my rear diff oil and on draining the old fluid noticed a build up of sludge on the drain plug magnet. This, combined with a worsening (but not 'serious) whine from, I believe, the rear differential has me perusing Ashcroft Transmissions in preparation for a possible replacement diff.

First thing are Ashcroft the 'go to' people for a P38 diff?

Secondly, I believe the diff on my 2001 4.6 Vogue is a 4 pin diff and 3.54 pinion/ring - is this correct (it hasn't been changed from new)?

Thirdly, Ashcoft offer a pegged diff - is there a significant advantage to this?

Finally, I've just been looking at RAVE - is replacing a rear diff on a P38 really that easy?

Yes, I've got an Ashcroft on the rear and another from a different rebuilder on the front (a place up north who could do one for me in a couple of days whereas Ashcroft gave me a lead time of a month). The front has leaked slightly from the pinion seal from day one.

Correct, later 4.6 has a 4 pin as standard, earlier ones and diesel and 4.0 litre have a 2 pin. Diff ratio is 3.54 on all P38 versions.

Only if you spend more time crawling over rocks than driving on the road. You've got traction control that does the job on all but extreme surfaces.

Yes it is. Drop off the rear propshaft at the diff and tie it up out of the way, pull both hubs and halfshafts out by a couple of inches (you can even leave the brake callipers in place), undo the nuts holding the diff in place, whack it with a mallet to break the RTV seal and keep your hands out of the way as it drops as it is heavy. Hardest part of the whole job is lifting the new one into place..

Under heavy shock loading (think low range, big knobbly tyres, offroading) the rover diffs suffer from flex in/around the pinion support bearings. This causes the pinion to move away from the crown wheel under load which increases wear, and in the worst case, it can move far enough that it jumps a tooth which is likely to break things.

The pegged diff adds a "bump stop" essentially behind the pinion, to limit how much it can move under extreme loading, making them much stronger for extreme use.

If its a road car on standard tyres, its unlikely you'll need a pegged diff.

Thanks Richard. Ashcroft it is then and no need for pegged diff - it looked like another couple of openings in the diff housing to leak anyway!

Have changed many diffs in my time but all have required thrusts/preloads, backlash and pinion to crown wheel measurements/adjustments et al to be made. Usually meaning the damn thing has to go in and out at least three times! This looks to be pretty simple in comparison . . . except for the weight issue. I do have various jacks to help on that score though.

Final question - any recommendation on brand of RTV sealant for this job?

That is the advantage with a diff from Ashcrofts, all the pre-load and backlash is set up by them so it is just a case of fitting it. The main problem with the weight is that it isn't in one place. The end that you can't put a jack under is the heavy end and there's no really flat surface to sit it on.

This is what I use anywhere that calls for an RTV silicone https://www.autodoc.co.uk/reinz/982547. My local factors (Millfield Autoparts) stocks it. Not cheap but the Victor Reinz stuff is recommended by Porsche amongst others and it is very easy to use being in the aerosol type dispenser. The only problem is once opened the nozzle gets clogged so you may end up throwing half of it away.

I will probably employ the two jack / two stud method. Two lengths of studding in the diff casing. Two jacks under the diff itself. Lift both jacks in sync until diff lines up with studding, push jacks forward until diff is located on studding. Lower and remove jack closest to axle. Push diff on second jack along studding as far in as will allow. Lower and remove second jack. Push diff home. Locate with two bolts, remove studding and locate all othe bolts. Torque to spec. This also ensures that the sealant is not disturbed in all the ‘machinations, of trying to locate the diff.

Removal of existing diff will be by reversal of this method. I just hope the prop shaft can be moved sufficiently out of the way. It may be possible to do this with just one jack, we’ll see!

Victor Reinz sealant on order.

Garvin wrote:

Have changed many diffs in my time but all have required thrusts/preloads, backlash and pinion to crown wheel measurements/adjustments et al to be made. Usually meaning the damn thing has to go in and out at least three times! This looks to be pretty simple in comparison . . . except for the weight issue. I do have various jacks to help on that score though.

Thats a function of the design. Theres essentially two common ways to build a diff.

On some, (Ford Atlas or Landrover Salisbury for instance) the cast part the holds the pinion and centre bearings is part of the axle case. On those, you have a removable rear cover on the axle case, and then need to load a bare diff into the axle case, and thus need to setup all the preloads and pinion engagement etc.

On others, like the P38 (and other Landrover Axles), or the Ford English or 9", the cast housing holding all the bearings is a seperate piece which bolts into the front of the axle case. With those, you can thus have the cast "pumpkin" all setup ready to go so it just needs to be installed into the case.

I guess you've just been unlucky to have always had the former style.

Ofcourse, to fit the actual diff into the cast part still requires the same work. The advantage being you can easily ship the bolt-in cast housing and thus have someone else do the work for you, rather than having to ship the entire axle.

Nearly all the diffs I’ve changed have been on old cars and quite a few years ago now. However, I changed the diff (to change the ratio) many moons ago on an Austin mini where it’s integral with the gearbox slung under the engine - that’s a third type!

Why change the ratios? It was my mate’s car and I swapped out the 850cc ‘A’ series engine in it for a full race prepared 1340cc ‘A’ series with twin Dellorto side draught carbytooters (that looked bigger than the engine). That car was rapid but 1st gear became redundant with only the remaining 3 gears remaining useable and could that thing post a 0-60 time! Trouble is it ran out of revs somewhere around 80mph so new diff gearing required so a) 1st gear could be sensibly used again; and b) a suitable top speed, appropriate to such a modified car, could be achieved.