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Lockers, 4 Pins, Open Diffs, LSDs ATBs - An Overview


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Done for my website, but think it might be usefull to many here now and for the future :

Enjoy !

Land Rover Differentials - Technical Overview

For the vast range of Land Rover there are 2 x Types of Diffs, 'Long Nose' and 'Short Nose. With a few exceptions Most Series LRs, LR 90s, Discoveries, Defenders, and Range Rover Classics all have 'Long Nose Differentials. 'Short Nose' units are in Range Rover P38s and LR TD5 110s and 130s which have Short Nose Rears and 'Long Nose' fronts. The other Differentials are 'Salisbury Units' and are easily identified as they have a bolt on Cover on the rear of the axle pan. Long, Short, and Salisbury Differentials are NOT interchangeable. Long Nose differentials are either 4.7:1 Ratio and 10 Spline Drive (Series LRs), or 3.54:1 Ratio, for pretty much everything else which can be either 10 or 24 Spline Drive.

So, as Standard Land Rover have a '2 pin' unit fitted to pretty much all Land Rover with Long Nose differentials and it is very weak. Do NOT waste your money rebuilding. If broken or worn out either buy another decent 2nd hand one - or put the money saved towards something better. The End

The Question now maybe is what to go for ?. .......

The 'Answer' maybe below when the mist clears !

'4 Pin' Units

These are still an 'Open Diff', behaving exactly as a 2 pin, but is much much stronger. These can be either Land Rovers own 'Heavy Duty' units, which were fitted to Land Rover 90 V8 rear axles, or often are LR 'Special Vehicles' units fitted as a 'HD Upgrade', or very cleverly machined 'Short Nose 4 pin centre re-machined to fit into a 'Long Nose' Casing with a matched Long Nose CW&P set. (and no, not a DIY job !). We have made many of these, but P38 4.6 Rear Diffs (the only 4 pin short nose) have since become hard to source.

Other 4 pins can be obtained from KAM (now Allmakes) who manufacturer a nice 4 Pin centre, these units can be made / upgraded into a 'Plated LSD'. The Good points are the forged centre cross shaft, adds a lot of strength, Cons are these are not cheap units. Ashcroft Transmissions also do a 4 pin unit, with large 18mm Cross Pins around £100 less than the KAM unit, but doesn't have the KAMs Forged cross shaft.

Auto Lockers

Detroit (Eaton)

Often referred to as 'Automatic' Lockers. They work via maximising traction delivering 100% of the torque to both drive wheels. It is engineered to keep both wheels in a constant drive mode, and has the ability to automatically allow wheel speed differentiation when required. These pre date Command Lockers. One issue with these is after sales Support (or lack of) and spares, the other is when they go wrong its pretty much the end !. Made in both 10 or 24 spline versions, only the 24 spline is current production. Many peeps swear by them, others swear at them. For off road use they often end up locked, taking you where you do not really want to go because they will not open. Although strong they have a limited appeal. We do NOT undertake repair work on these units !. NOTE : Only ever fit in the rear axles do NOT fit in Front axles.

Tru Trac

Truetrac are a gear driven type torque bias design. Operating like an open differential in high-traction conditions (i.e. Road), but off road the helical side gears prevent one side from spinning while the other side remains stationary. If traction is very poor helical worm gears are pushed away from the larger helical gears and pushed into the carrier. The additional resistance on the side with less traction diverts torque to the other side of the axle. Truetracs are made in 2 'strengths' one for fronts and another heavier clamping unit for rears, its a very bad idea to fit a rear in the front !. Similarly although strong when they fail they go big time !

"Command" Lockers"

These are differentials that are normally 'Open' and can be locked and unlocked by a switch or lever that is decided by the operator, and not the differential itself. The vast majority of 'Command Lockers' are Air Operated via a on board compressor with a tank of compressed air. The Locker is operated by being switched on, the compressed air thereby locking the differential, switch off the air pressure exits and the unit(s) unlock There are some like the KAM 'DIL' Locker, that use electrics via a switch, and even some older units via cable pulls. The ARB compressor is viewed as the best compressor for ALL air activated differentials, as it has a built in safeguard not to overpressure the differentials seals and pistons. Ashlockers and ARBs alike will be damaged if excess of 85 psi is used. Now lets look at what's out there Locker wise

ARB - RD03 10 Spline only

Probably the Locker that started the Locking Differentials as the 1st on the market !. Back in '80s ARB launched the RD03, an air operated Locker for Land Rovers (and other makes) for the 10 Spline half shafts. These units are now quite rare, partly due to age and wear and partly due to the mass move to 24 spline.

These suffer from a range of issues. A BIG one is the lack of spares, many parts are NLA. However Xcess 4x4 have remade many of the NLA parts that fail, even improving on them. However, the biggest issue (with all ARBs) is air leaks. This can be the internal Piston itself, the 2 x air seals, both can be repaired by us. However wear on the Air Journal Flange or end flange, (both NLA) up to now has rung the death bell for these Diffs. the good news is we stock of these NLA parts made by us, and using far stronger modern steels. The Plastic Cage centre is also prone to failure from long term use, which whilst cheap entails a 100% strip down to change. But, if you need a 10 spline unit these are great little lockers if in decent condition, often many that come to us have been horribly abused and past any sensible reconditioning, but we do get them from time to time.

ARB - RD56 24 Spline only

These are as the RD03 above but with some minor changes. Be aware that many RD56s parts are NOT interchangeable with RD03s, and RD56s suffer pretty much the same failure issues. End Flanges breaking, CW&P holes cracking and Air Journal Flange wear and Air leaks as above. We regularly repair these units, often using the parts we make to replace damaged parts NLA. The plastic cages are the same as the RD03 and suffer similarly !. However, these are a great 'Bread & Butter' Locker, rarely when they break can they not be repaired. with our end flanges on each side and a new cage and thrusts and maybe pegged as well, these are a really good locker for someone on a budget. The Centre Tube bolts are very weak grades, and the locking clips can drop off ad actually blow the differentials up (RD03 and RD56s. We upgrade these). Warning : Many you may see for sale on eBay etc will be desperate for a big rebuild, so beware, parts prices are not cheap, and some parts are NLA

ARB - RD128 24 Spline only

The current latest ARB offering. some will say better - I am not so sure !. The centre tube now has a end plate as part of the unit - these shear off, pegging the diff does help strengthen these. The air pipe air coupler swaps sides and the soft easily damaged copper pipe now ends up going over and close to the Crown Wheel - probably the daftest idea ever. The plastic cage is now replaced by a more conventional all steel unit with bigger massive 20mm pins etc, but, should these fail (which they do) the results can often be catastrophic as everything smashes into itself. We do an upgrade kit for the daft 'Over the Crown Wheel' copper pipe to help protect it. Issues again are air leaks, internal piston and external seals, and the air flange wears and then leaks - which like the replacement centre tube is very expensive.

Ashcroft Transmissions - Ashlocker 24 Spline only

Currently, and has been for some time our No 1 favourite. Yes we are approved Ashcroft dealers, but we chose Ashcrofts as our No 1 choice because of service back up and fairness. EVERY single part for an Ashlocker is available and from experience in 24 hours to us should we need a part we don't have. The BIG difference is in the way the air system works. On an ARB the seals rotate with the wheels, irrespective as to if locked or unlocked. The Ashlockers Seal / Piston does NOT rotate, and hence doesn't wear it moves ONLY to lock and then back when unlocking. Pretty much trouble free. You have to destroy a bearing to get the air ring off, more annoying than anything else, and you have to be super careful undoing the very small countersunk Allen bolts as they can round off it a second or less !. Cages are steel but can wear in heavy use / abuse, and the air ring can damage seals should you get foreign bodies and swarf in the oil which can embed in the bronze air ring and make the seal leak. Very tricky to dismantle the long 3mm cotter pins. But keep the oil clean and they seem to just go and go and can take a LOT of abuse....

McNamara Locker - 24 Spline only

Rare. Made yonks back using very high grade metals even by todays standards. Air operated by an external piston and a sliding fork assembly. Parts next to non existent, but never seen a broken one, just normally needing bearings and a clean up. If you find one it will not be cheap - in any condition, prob the holy grail of lockers. Super Super strong and were near 2 x an ARB price when new - hence rarity !. The modern ones are not a patch on the originals !

KAM - KAM Quill Locker +LSD Packs - 24 Spline

No longer in production. Electrically operated by switch and huge powerful solenoid via a 'locking dog' which engages into the side of the end flange. Also had a one piece solid forged cross shaft to support the 4 x sun gears. Unit itself is massively strong !. Parts - we prob have the largest world stock on these, so ring. Lovely unit, downside was often seen as the need to cut the diff pan off the axle tube and weld the mounting ring on (which we have stock of) but once done the locker when fitted was ultra strong and reliable, shame it was discontinued IMHO

KAM (aka Allmakes) -KAM DIL - 24 spline

When Allmakes bought KAM they discontinued the old style locker above -it was expensive and complex to build up, and decided to go with the current DIL (Drop in Locker). Named as such as it fits straight in without the need to cut axles and weld rings on etc. Much is made of the fact it doesn't need a compressor (true) and is thus easier to fit (also prob true). But pricing wise its weird. You can buy 2 x Ashlockers AND a ARB Compressor for less than 2 x DILs, and the delicate springs on the brushes holder and the brushes themselves have proved to be more than a little delicate. We have had 2 x for repairs with the same issues on both. Dirty oil is a killer on these. Also very oddly the T&Cs say " 2 year warranty (excluding competition use, max tyre diameter 35”)" which is a bit worrying ?

LSDs (limited Slip Differential) / ATBs (Auto Torque Bias)

Quaffie LSD 10 / 24 ATB

Quality Quality Quality. The finest machining we have ever seen and everything is just quality. Gear driven LSD (ATB) 10 or 24 spline available, and brace your self on prices. Quality is never cheap. The Ashlocker ATB is a Quaffie copy, and outstanding value for money, but a Ashcroft ATB is Not a Quaffie in terms of build quality and materials, but then it is less than half the price. Quaffie is for those who want the very best LSD / ATB period.

Ashcroft ATB

Basically a Quaffie copy. Stunning Value for money. Gear driven and mighty strong and price is really amazing. No its not a Quaffie, but your not paying Quaffie money. If you want a LSD and if money is no object (or it has to be 10 spine) then Quaffie, then Ashcroft ATB then KAM Plated LSD

KAM LSD - 24 spline

The KAM unit is a 'Plated' Type LSD with the KAM Lockers Forged Cross shaft (making it strong) for the centre, and it is basically the 4 Pin unit with some internals changed to make it a LSD. A VERY strong unit, but, due to the plate design the preload often will drop. What is good is that you can set preloads for differing amounts front and rear, but they will need constant resetting, which requires the diff to be removed to screw in the preload screws to regain the correct clamping. Vastly cheaper than the Quaffie Unit - which is a LSD with gears so the forces of clamp remain come what may, and the Ashcroft ATB a Quaffie copy. The plated design is 'Old School' some prefer the flexibility over geared LSD some prefer the fit and forgetness of the geared LSD. you pays your money ....

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Nice writeup :)

Is the TruTrac an ATB/LSD type diff or something different?

Description of it makes it sound like an ATB, but they're seperated out in a different section in the writeup!

Also might be useful discussing when/why you'd select one or the other type? IE what situations you might select a LSD over a command locker?

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What about the options for the Salisbury? I must admit, historically I've always liked the Salisbury diffs (other people may disagree) having been brought up on them in various road-cars (Mk.2/S-type/E-type/XJ6/12 Jaguars, Marcoses, Aston-Martins, Reliant Scimitars, a 300BHP-at-the-wheels nitrous-oxide-injected Volvo 164E etc) and BDA-powered Ford Escort rally-cars too.

The "Pow-R-Lok" LSD is a tough and useful fitment to a Salisbury diff; it'll handle maximally-sticky competition tyres on tarmac circuits without any problems at all.

Just make sure you use the right oil! [EP90 will kill them in short order].

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Nice writeup :)

Is the TruTrac an ATB/LSD type diff or something different?

Description of it makes it sound like an ATB, but they're seperated out in a different section in the writeup!

Also might be useful discussing when/why you'd select one or the other type? IE what situations you might select a LSD over a command locker?

Yes the Truetrac is an automatic torque biasing diff, and very well made, how it stacks against the Ashcroft version is unknown though :)

Their design is inherently strong, my Audi has one in the middle diff, and although tiny, people don't upgrade them until they start reaching north of 800BHP.

Same with the rear diff from the Audi V8s, take massive strain :)

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Great write up Nige.

But one comment, I believe though happy to be wrong, Torque sensing gear driven ATBs don't work if one side has no traction. They are great for rally cars where both wheels will have some grip but off road where it is common to have no grip at all on one wheel it will act as an open diff! The way around it is to apply braking to both sides to allow the gears to work. If you have the axle off the floor and turn one wheel the other can be held stationary the same as an open diff.

I am happy to be wrong as I have one in my new rear axle though mine is a Torsen which although possibly the original designer as you say the Quafe is stronger as possibly is Ashcroft's version not sure.


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Still struggling to find the £1100 to upgrade each diff to locked and pegged spec :(

I saw exactly as you described with a 24 spline ARB locker I missed out on, it was bought by a friend who sent it away to be built up into a spare diff, rebuilder said it was completely wore up and had two hairline cracks in the plate the crown wheel bolts to, possibly you had a call? Thing is, every time I see them for sale they're so very close to 3/4 the price of a new one, my friend Luc has had the misfortune of finding his bargain was expensive scrap and no doubt others have had the same misfortune.

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Good information. What was the Detroit with the castellated plates that are locked and pop out to allow slip on grippy surfaces? It doesn't seem to fit in the categories above.

I quite fancied the idea of Ashcroft ATBs until you said the quality was lower than the Quaifes (albeit at a good standard and good value). If I had the cash, I'd consider them in my late RRC as part of its restoration - it only has ETC on the rear, so ATBs in both axles would be a nice mod, and I like the automatic application, simplicity of installation and the lack of need of cabin controls. I would like something similar in the 109 too, but the Salisbury rear is a sticking point. Still, I have more urgent needs, like sorting out the body shell and a respray on the RRC, its transmission and axles, and it'd be good to diagnose and cure the oil burning on the 109's Tdi...

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Good stuff, would be good to hear the reasons for not fitting various ATB diffs in different ends of the truck, and as has been said how some types don't actuate if there's no traction (wheel in the air).

Have a spare photo or two for illustration purposes:



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