by Timm Cooper
A Brief History of the Rover Type Differential
After 35 + years of living with, working on and modifying Land Rovers - and wheeling them hard - I have observed (and participated in) the evolution of aftermarket upgrades to one of the legendary vehicle's classic problem areas....... The "Rover Type" differential.
The Timmco pinion solution (L) compared to stock Land Rover (R), same tooth count.
The people who designed this unit over 90 years ago (!!) had no idea what the unit would be used for decades later, and when you consider the original applications it was intended for, it has put up a darn good fight! When this passenger car differential was later put into service under the first series I Land Rovers, it was well suited for the task and rarely failed back in the era. Trouble started with the heavier Series II models, and during the 1960's the Land Rover really began to earn its reputation for breaking diffs and axle shafts. Rover acknowledged the issue with the introduction of the vastly stronger ENV type differential as an option, and on specific applications. Rover was also getting into cost cutting during the 1960's, so instead of standardizing on the much stronger ENV unit, they used it sparingly and later, with the introduction of the Series III models, they replaced it with the cheaper but still very strong Salisbury 8HA (British built Dana 60 variant). Through all this, the old Rover type differential unit soldiered on in production under most Land Rovers, and later, Range Rovers and Discoveries, thru the 1980's and 1990's, lasting in production to 2015 in Defender models.
Although the re- introduction of permanent 4WD on Land Rovers reduced stress on these differentials and stopped the "I broke an axle pulling away from a stop sign" dilemma that Series II and III owners faced, the later 10 and 24 spline differentials used in the coil sprung models were not any stronger than the ones in the Series vehicles. The only exception to this is the four pinion 24 spline variant used in some military 90 rear applications, along with early v8 90 models.
A Brief History of Rover Type Differential Aftermarket Upgrades
Now, the aftermarket upgrades!! Back in the 1980's, several folks got really involved in upgrading / replacing the old Rover type differential. First was Mal Story of Maxi Drive fame. He developed a 24 spline upgrade, using the 1.240" 24 spline pattern from the Salisbury rear axle, created a new 4 pinion center, and a really nice diff lock. Next, Jack Mcnamara Dfferentials developed a new third member casting, using Toyota 8" internals (actually 7.8" ring gear) with the choice of 24 spline 1.24" shafts or 30 spline 1.29" shafts. Jack Mcnamara also offered a 4 pinion 24 spline upgrade to the original Rover type unit, along with a really cool manual differential lock.
The Timmco ring gear solution (R) compared to stock Land Rover (L), same tooth count.
This is all stuff that was going on when I got into Land Rovers back in the 1980's, vastly pre-dating the "Toyota differential conversion" that was a thing for a short time in the early-mid 2000's.
In the mid-late 1990's, for a short time, Summers Brothers offered a 30 spline conversion axle set and special 30 spline ARB air locker. KAM diffs in England started making the industries very first aftermarket ring and pinions for the Rover type differential, and a nice locker, as well. Unfortunately, their 24 spline "upgrade" shafts were made of cheese! I personally began "thrust bolting" these differentials in the late 1990's.
During all his time, there was a huge argument that because the Rover differential had no hypoid offset, it's pinion was smaller than the Toyota pinion, and was much weaker. The fact that the 8" (actually 7.8") Toyota ring gear was over 1/2" smaller, and with a narrower tooth face, was conveniently ignored! Claims of 250% strength increase were tossed around. History proved otherwise..... I remember a guy with a Defender 90 breaking his rear ring and pinion on Axle Hill, Pritchett Canyon, Moab. He then did the Toyota diff conversion, and his first attempt at that obstacle with the new Toyota diff resulted in a broken ring and pinion...
During the early 2000's, tires were getting bigger and due to breakage, both chromoly 24 spline shafts and better ring and pinions were developed for the old Rover type differential. Ashcroft began "Pegging" (thrust bolting.....) I helped GBR get started on the first true heavy duty "coarse cut" big tooth ring and pinions for the Rover type differentials. 4.14 and 4.71 were my contributions. Years later, Ashcroft introduced their own versions, claiming "Ashcroft has done it again", like they innovated coarse cut ring and pinions!!! HaHa!!😁. Anyway, I will give Ashcroft credit for making the best currently available air locker for the Rover type differential. Way better than ARB, even though it uses smaller spider gears...... The billet steel case / ring gear flange is worlds stronger than ARB's nodular iron case, and the non-rotating air seal is also worlds better long term reliability. Service parts cost / availability is also far superior to ARB. That's why I chose to modify the Ascroft air lockers for 1.400" 32 spline shafts, as I had outgrown the 1.240" 24 spline stuff during the 1990's. I used to snap those things like carrots with my clumsy driving!!
A Solution For The Demands of Modern Use
Well, on to the present........ Today, everyone is running bigger tires, and vehicles are generally heavier armored, and with more "Kit". Discoveries and Range Rover classics are often built up as all-round hard core off roaders, and have pushed the limits of the original 95 year old Rover differential design over the edge, in many cases. Another thing is the proliferation of "LS swaps" (something I have been doing for decades, since way before it was cool in the Rover crowd) Even in less hard core off road applications, the major HP increase is hard on these old diffs.
The Timmco pinion solution (R) compared to stock Land Rover (L), same tooth count.
One of the things that has actually held back the strength potential of the old Rover type diff is that NO MAJOR AFTERMARKET RING AND PINION MANUFACTURERS HAVE EVER OR WILL EVER TOUCH IT!!! I do not get this....... So, we have been saddled with "OK quality" aftermarket gears, leaving potential strength on the table. After looking into getting improved gear sets manufactured to current, modern standards, I realized that I was wasting my time, and would not be able to offer a wide enough range of ratios at a price point anywhere near current offerings...........Frustrationville !!!!
Well, with the fact that I have been rebuilding and modifying differentials of many makes since the mid 1980's, combined with my experience with machining and foundry pattern making I thought that I could do better. The goal is this: offer a completely new 3rd member assembly that bolts straight into the Land Rover "banjo" housing from the earliest series I's all the way to the last Discos and Defenders. This new design uses off the shelf readily available truly modern ring and pinions in 13 different ratios. Air locker is standard, with electric or cable options also available. Steel billet locker carrier, and 35 spline 1.5" is standard (requires axle and big spindle kit). Ring gear diameter is 8.7" (220 MM). Thrust bolt with cobalt alloy (stellite) shoe is standard. Massive pinion and carrier bearings. This unit will be a true "Bolt in" solution, along with the 35 spline axle kit, and offers a really nice , straight forward solution for the rear axle assembly in ratios down to 5.38. (Yes, there is even a 4.7 to match common Rover ratio)
Using this diff in the front of coil sprung Rovers presents a challenge compared to series, due to the much smaller coiler swivel balls, but I think I have found an elegant solution to get a significantly larger CV joint in there. Series have huge swivel balls that I machine out to take huge CV joints, so they have been no issue....
This new diff and axle setup is intended to handle 35-37" tires on heavy rigs, and 40" tires on more stripped down extreme rigs. Its intent is to provide a super high quality option to bridge the gap between stock type builds and full-on linked chopped up trailer queens.
- Timm
