Your Cars : P6B ZF gearbox transplant – Part One

AROnline reader, Warren Loveridge, from Christchurch, New Zealand owns a unique 1972 Rover 3500 named Brown Rover. However, when the original Borg Warner 35 transmission blew up, Warren opted to replace that with a later, modified ZF unit. Here, in the first part of a revised version of an article which was originally published in the October, 2014 issue of Driving Force, the Rover P6 Club’s magazine, Warren tells the tale of how he undertook the conversion…

Warren Loveridge's Brown Rover.1
Christchurch, New Zealand-based AROnline reader Warren Loveridge’s much-loved Rover P6B, Brown Rover

What do you do when you’re 50 kilometres from home and your beloved 1972 Rover 3500 blows its transmission and coasts to a halt at the side of the road? Well, you call the cavalry and wait for them to arrive with a tow rope and endless jokes about old Rovers. Upon reaching home you scull a stiff orange juice, lament your ruined weekend, and thank your lucky stars that you weren’t closer to your intended destination, a further 300 kilometres away.

It was a bitter blow. Needing much work, Brown Rover had been temporarily retired years before whilst I accumulated all the encumbrances (wife, mortgage, offspring) that stand between a man and his true calling (messing about with old cars). Eventually time, money and motivation coincided, and a further three years on Brown Rover was ready for its first decent out-of-town trip in a dozen years. Or so I thought!

In fact, impending Borg Warner doom had been evident for a while, but I had wilfully ignored it in my eagerness to get back out Rovering. Now I was right back to square one. A disgraced Brown Rover sat in the garage beneath old blankets whilst I grieved. Two months later, the dark cloud’s silver lining began to gleam. Perhaps this was all predestined…

You see, the Rover P6 is a wonderful car, a high water mark for the British motor industry in general, and Rover in particular. To this day, a P6 remains the everyday and only choice of vehicle for many devoted owners. In V8 form particularly, it is a fast, reliable and comfortable carriage that remains astonishingly capable.

However, to me, the principal weakness of the standard 3500 is its automatic transmission. The Borg Warner 35 was adequate in its day, but by modern standards it is slushy, stubborn and frustratingly under-geared. A torquey V8 does not need to rev hard. A busy 3000-3200rpm at New Zealand’s statutory open road speed limit of 100kph takes the shine off an otherwise very refined car. In years past I had considered various alternative transmissions, but these consisted almost entirely of manual boxes, and a manual conversion did not interest me. A lazy V8 and an auto go hand-in-glove for me, and I wanted to improve my P6B, not alter its essential character. What could I do?

Eventually, the ZF 4HP22 came to my attention. A four-speed overdrive transmission with a lock-up torque converter, it was well-regarded and, crucially, found in many Land Rover products behind the Rover V8. Moreover, these Land Rovers were by now beginning to die of old age, their cheap bones available for picking over. Ideas formed – could the ZF be used in the P6? Various people agreed such a conversion might be possible, but a Land Rover box would require modification for two-wheel-drive. Would the gearbox and the V8 bellhousing physically fit in the P6 tunnel? What about the propshaft? The selector? The speedometer? The kick-down cable? Cooling? Two gearboxes would be needed for the conversion – a Land Rover version to provide all the Rover V8 bits and something else to provide the 2WD rear end. I decided that an early hydraulic control version would be preferable, needing only Bowden cables and thus avoiding any electronic complexity. Significantly, a custom cross-member would be necessary. Who could make this for me. How would it attach to the car? And so on, and so on.

When I joined the Classic Rover Forum in 2009 I discovered that the idea had been discussed at some length, but no completed conversions were confirmed, and the idea seemed to have progressed no further than theory. Nevertheless, I was sufficiently enthused to pounce on a cheap ex-BMW ZF that came my way, speculating that it could furnish the 2WD back end of my Frankenbox. A year or two later, I unearthed a cheap ex-Discovery transmission, which joined the BMW unit in a dark corner of the garage awaiting its fate.

Fast forward to May 2013. I thought of the two ZF gearboxes hiding behind stacked timber and unloved furniture. A portentous voice echoed in my head… “It is time.”

Assuming battle stations...
Assuming battle stations…

I faced numerous difficulties. Finances dictated that I must carry out as much work myself as possible. A bush mechanic with only the vaguest idea of how an automatic gearbox worked, I was very unsure of my ability to see this project through successfully, so I decided on a simple line of little resistance. The BMW ‘box was theoretically the better of the two, so I took a leap of faith in its internal condition and decided to simply graft the necessary Land Rover V8 bits onto its front, use the Land Rover valve body and governor (for V8-friendly shift points), and sort out the other details as I went.

However, first I had to relieve Brown Rover of its failed gearbox. This was accomplished in my garage with a couple of trolley jacks and a lot of swearing. A copious spill of transmission fluid and several bleeding knuckles later, the task was complete.

Meanwhile, ZF chatter continued on the Classic Rover Forum. “Geordie Jim” had started a conversion and got as far as test-fitting a ZF bellhousing, confirming that with some minor fin-trimming it would go in. Following Jim’s instructions, I prepared my own bellhousing and bolted it to the BMW gearbox for a trial fit. I now discovered that only minor trimming of the existing tunnel brackets was required, and a new cross-member could be devised to bolt to them. Thankfully, no cutting or welding of the car’s structure would be required.

I noticed that the trial box was canted over at about 10° from the vertical. This is a feature of the Discovery installation, and since the twist is built into the bellhousing, the new custom cross-member would need to take it into account. I would also have to consider the consequences for the sump and pickup, because the Discovery’s were too deep for use in the low-slung P6.

Trying a ZF for size - the first trial fit
Trying a ZF for size – the first trial fit

I also found that the early Discovery flex plate and ring gear were an exact match for the P6B, meaning there was no need to touch the original starter motor. Happy days!

Now my friend, John Weir, a talented engineer and fabricator, who owns Canary Automotive Limited here in Christchurch, came by and made his initial sketches for the new cross-member. With his confirmation that nothing was insurmountable, I felt confident enough to leap into the next phase of the operation, which involved preparing my hybrid gearbox for installation.

The initial rough sketch for the new crossmember
The initial rough sketch for the new cross-member

My largely accidental use of a BMW box as the rear end donor was now justified. The ZF 4HP22 is fitted with various application-specific tail housings. An early contender had been the LDV/Sherpa version. Mated to the Rover V8 and sporting a 2WD rear end with a mechanical speedometer drive it seemed ideal, but this box is next to impossible to find in New Zealand, and it is also about 4 inches longer than the BW35. The Land Rover/BMW combination happened to be almost exactly the length of the BW35, meaning that the P6 prop shaft swung into place quite neatly and I needed only a means of attaching its 4-hole flange to the 3-hole BMW output flange. This too was soon solved. Someone on the Forum knew that the output flange from a Jaguar XJ40 ZF fitted the Rover P6 prop shaft, thanks to decades of Hardy Spicer product consistency. It also fitted the BMW ZF output shaft. One was quickly obtained from a local wrecker.

The BMW box lacked a speedometer drive, but I already had a solution in mind. Dakota Digital Inc. manufacture a kit that comprises a magnet and pick-up kit for the prop shaft, and a black box that installs under the dash or in the engine bay, providing a mechanical drive for the existing speedometer. This was duly ordered.

The photograph below shows the gearbox in place, with the P6 prop shaft fitted to a Jaguar flange sitting in the BMW tail housing. The points of contact where the tunnel brackets needed trimming can also be seen.

The second trial fit...
The second trial fit…

My initial ideas were continually changing – if you have no precedent to follow, everything is trial and error. For example, I discovered that the Land Rover’s inhibit/reversing light switch occupied the equivalent position on the ZF to the Borg Warner’s. It would end up in exactly the right spot in the car and it could be wired directly into the existing P6 loom. The BMW box had no such switch, these functions being part of the selector, so the Land Rover case with its switch and mounting hole was now the obvious choice, but the BMW box was the one that had the 2WD output and was supposedly in better condition!

I was still terrified of dismantling internals. Could the BMW case be machined to take the Land Rover switch? Yes, but I’d have to empty it first, so I might as well just use the Land Rover case filled with the supposedly better BMW internals. But that would mean… disassembly! I was getting painted into a corner.

Enter Classic Rover Forum member “Eightofthem”, known to his mum, Mrs Morgan, as Andy. He had never fitted a ZF to a P6, but he is a 4HP22 guru, and he became the hero who guided my project to a successful conclusion. Over the course of innumerable emails and forum posts, he guided me through the task of taking two ZFs entirely to pieces and assembling a hybrid. My confidence grew in leaps and bounds, and Andy earned my everlasting gratitude. I literally could not have done it without him.

ZF innards all set to be mixed and matched
ZF innards all set to be mixed and matched

I now discovered that, despite its low mileage, the BMW box was in worse shape than the Land Rover. For the record, an un-abused ZF 4HP22 will practically last forever, and neither box showed any appreciable wear of the metal componentry. The difference lay in the amount of sludge and friction material in the sump. Andy also pointed out minor differences between the two, such as intermediate gear ratios, and the Land Rover box soon replaced the BMW as the principal donor.

Now in for more than my original penny, I coughed up the entire pound and purchased a transmission overhaul kit. With the help of exploded diagrams, a Discovery workshop manual, Google and Andy, I gave the box a thorough overhaul. My thanks must now go to Nigel Whitaker from Transgear Limited here in Christchurch. As well as supplying me with all my bits and pieces, Nigel recommended that the Land Rover torque converter be rebuilt, and he was proved correct when, upon being sliced open, it turned out to be in poor shape.

Nigel also reassembled the overhauled clutch packs when my modest home facilities proved not up to the task, and assisted me in measuring and adjusting clutch tolerances. Finally, he came to my rescue when I had a problem when in the midst of installation I discovered the torque converter jamming and refusing to spin.

Very soon I had a reconditioned ex-Land Rover ZF 4HP22 fitted with a BMW output shaft and tail-housing, a Jaguar flange and the Land Rover valve body and accompanying governor. It was ready to install…

What has Warren got himself into?
What has Warren got himself into?


Clive Goldthorp


  1. The P6 was a big car in its day, but is narrower and lower than a Peugeot 307!
    Interesting that Warren has SD1 alloys on his (top shot) – my brother had them on his 3500S, but found them to be porous, so replaced them.
    Look forward to part 2…

    • Interesting comment about the porosity of the alloys, Ken. Brown Rover acquired them shortly after the gearbox transplant was completed, allowing me to fit the car with 215-section tyres. The result was a huge improvement to the handling and steering. The alloys were thoroughly blasted and then powdercoated, and Brown Rover leads a sheltered existence, so I hope that porosity isn’t a problem.

    • I don’t know about it being a big car in its day. It was once known as “the only four door two-seater” on account of the very cramped rear legroom. As we know, the spare wheel was often relocated to the boot lid to make more space available in the boot itself. The single carb 2000 was underpowered and the petrol consumption was hefty, particularly with auto transmission which many of these had.

      Having said this, it was a ground-breaking design for the late 60s early 70s and Rover had a lot of good ideas which this car embodied.

      • Should be ‘early sixties’?? It was launched in 1963. It was getting on for 10 years old by the early seventies!

  2. As an owner of a P6B auto (and member of the RP6C), this sort of job is truly inspirational and is typical of the quality articles present in the club mag. Looking forward to part 2 myself.

  3. I had a 1975 P6 V8 auto in the exact same colour – Tobacco (Leaf?) Brown it was called. Gorgeous car wish I had never got rid of it. Well done to this guy!

  4. Very interesting article so far and appropriate accolades should be sent to the author. I’m a fan of modern autoboxes with their lockup clutches and wider choice of ratios than 70s boxes. So much better than faffing around with a clutch pedal and gear lever which really ought to be consigned to the first half of the 20th century where they belong. There was a similar conversion on the blue forum where a stag owner replaced their autobox with a ZF unit (probably a 4HP22 as well) from a BMW.

  5. ” A bush mechanic with only the vaguest idea of how an automatic gearbox worked, I was very unsure of my ability to see this project through successfully ” – a truly amazing undertaking then !!!

  6. the early xj40s also used the mechanical ZFHP22 gearbox, and might be worth looking into especially with the rear gearbox mount. be aware though that the later ones ZFHP2x are fully electronic. good luck bro, alex (in Taranaki)

    • IIRC, the XJ40 tailcone is longer than the BMW’s. The beauty of the BMW tailcone is that it is the perfect length for the P6. The P6 propshaft swings right back into place. But you use the XJ40 output flange – it mates to the P6 flange, the BMW’s doesn’t.

  7. Just as an aside, ive just completed the fitment of a Discovery 1 3.9v8+zfhp22 into my 1986 Defender 110 ( took out the RV83.5 + LT85). I used the complete discovery engine including the 14cux fuel injection system. I used a jaguar xj40 gearshifter. I modified the shifter so I can get first gear, and had to make a special crank for the gearbox end of the cable, but that was relatively easy. I can relate to the plumbing, that was a little problematic, and not fully resolved, but the Def110 is useable and drives fairly nicely with an autobox. the speedo cable thing, the land rover transfer case can take a mechanical cable or a sensor that can replace it. the early discos used a mechanical cable that went to a speed transmitter, then another mechanical cable goes from the speed transmitter to the speedo. the speed transducer sends a pulse to the 14cux. easy!(on a land rover). alex.

  8. Err the ZF 4HP22 as fitted to a BMW will NOT last forever! there is a design fault on these boxes that causes the first gear clutch pack to wear out, eventually destroying the linings. This was very well known even in the 80s, so when Jaguar chose it to use in the XJ40 they investigated it and found a very minor mod to relieve excess pressure on the clutch pack eliminated the issue, and so all XJ40 boxes had this mod (and failure of them in the jag is almost unheard of) Landrover did not apply the mod till 1990. BMW never did

  9. Fascinating article. I’ve just acquired an early Range Rover and am planning to put the (3.9 EFi) engine into an MGB. I’d like to make it an automatic but, like Warren, I don’t really want to use the Borg Warner box. I’ve also been wondering whether I have any chance of using the ZF 4HP-22 that came with it. Separating the gearbox from the transfer box looks easy enough but then there is the question of how to make it suit a 2WD vehicle. Lots of BMW boxes on eBay, so that sounds like the way to go. Just need to check whether there’s enough space for the ZF within the MGB tunnel without too much cutting and carving!

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