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…
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.”
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.
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.
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.
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.
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…
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