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Robert Hughes v12 420G
Robert Hughes gives us this story of a V12 420G

Please click pictures for larger sample


I bought my 420G back in 1992 for the princely sum of £400. It had been the victim of a ‘cherished number’ transfer, and so I ended up with a vehicle bearing an age related plate and a very suspect M.O.T.

When I went to view the machine, it had no brakes, the cross-ply tyres were literally thread bear, one sill was chock-a-block full with Motorcycle News and filler, and one of the tail pipes had dropped off!
This was the car for me!  I returned a few days later with a few mechanical goodies to make the vehicle a little less dangerous to drive, and set off from Preston, over the Trans-Pennine route back to Darlington. I used the 420G as everyday transport for the next two years and it soon became known as ‘The Battlecruiser’ among my friends.
One of it’s eccentricities was that when cold, it would not go forward, but would go backwards very nicely and would need to be revved for almost 5 minutes before the auto
‘box started groaning and trying to drag the car forward ! This wasn’t very popular at 6.30 on a morning !!!  The Battlecruiser was gradually improved, especially at MOT time until one day….. disaster stuck !
Whilst ambling along at 70..ish there was a loud bang from the engine . As I glanced in the rear view mirror, I realised that I couldn’t see anything behind, apart from clouds and clouds of steam. Ooooops !
When I later opened up the engine, I discovered that it had dropped a valve, which had holed a piston and made a very nasty mess of the cylinder head as it rattled around.

Solution 1.

A replacement 4.2 engine was purchased, condition unknown, and fitted to the Battlecruiser. That turned out to be a bigger job than I first thought! As the engine had come from an XJ, the exhaust manifolds and the triple carb manifold had to be swapped – however, most of the studs seemed to be either too short, or far too long and subsequently they had to be swapped over as well. The stud swap took forever using the good old nut and locknut technique. I also swapped the starter, engine mounts, water pump, alternator, PAS pump and associated brackets and to cap it all the oil filter and pressure regulator housing were incompatible. What a carry on!
A few days later, I was mobile again, unfortunately, the Battlecruiser now left a very thick blue smoke screen in its wake everywhere it went. It was drinking oil at nearly the same rate it drank petrol. The writing was on the wall, as at the next MOT, the Battlecruiser failed its test on visible emissions! The morale of this story is don’t go buying a second hand engine without hearing and seeing it run ! A second replacement engine was now required.
At this point in the story, my relationship with the XK engine abruptly ended, and I decided that more cylinders was the key to solving my situation.

Solution 2.

Purchase V12 engine and fit into Battlecruiser. Simply fabricate engine mountings and exhaust system, sort out wiring and plumbing, and then drive to MOT station. I had generously allowed myself a couple of months to pull off this minor miracle, but it was to take me the next 7 years (off and on) to complete!  This turned out to be an initial attack of 3 or 4 months graft, a break of about 5 years, and then a major attack to finish the job.

Fitting a V12 into a 420G.

A big decision, and an early decision was to retain the B.W. Model 8 gearbox, which I had just overhauled before the original XK engine blew up, and for it to remain in its original position.  Therefore the prop shaft would not need to be altered, and the gearbox rear mount, selector cable and filler tube / dipstick could remain in their original positions.
Early V12’s used Model 12 gearboxes, so I located a Model 12 bellhousing and fixed it to the Model 8 box. This meant the forward end of the bell housing would line up exactly where it used to and so the rear / bulkhead engine mount would line up perfectly.
A few quick measurements revealed that the V12 sump would just to say miss the steering linkages, and that the water pump would hopefully just miss the radiator – All systems go !

A considerable amount of work had to be done to the ‘front of transmission tunnel’ / footwell areas to make room for the rear of the V12. The thick black lines on Photo ‘A’ show the original profile of the footwell on the nearside. Photo ‘B’ shows the same modification to the offside footwell / tunnel – ultra close to the handbrake mechanism.
Note the steering column and incredibly huge steering box that would prove to be the cause of many a sleepless night !

Photo A

Photo B
Photo ‘C’  shows my third attempt at engine mounting plates. The rusty bits ate the original 420G mountings , which are going to sit well back in comparison with the V12 mounting points. My first two attempts at mounting plates weren’t quite so tall, which resulted in the sump of the V12 see-sawing on the suspension cross member.

Photo C

An exhausting time !

Photo ‘D’  shows the home-made exhaust manifolds and down pipes for the nearside. The ‘design criteria’ was to get the pipes back far enough to clear the engine mounting, whilst hugging the oil filter dangerously close so as not to fowl on the ‘chassis rail’ running through the engine bay. I decided to tackle this side of the engine first, to get my confidence up, as I knew that the offside of the engine would be an absolute nightmare.

Photo D

Photo E
Photo ‘E’  shows the manifolds and down pipes for the offside under construction.  This was one of the hardest challenges of the whole job. The ‘design criteria’ was to hug the engine as close as possible to give clearance to the steering column (remember the earlier photo ? ) and come down between the engine mounting and the starter motor with melting either!
I didn’t realise at this point that if I had planned the down pipes  less than a half inch further forward, they would have fowled the steering tie rod which runs across the vehicle just in front of the sump – oh – and I nearly forgot , I had to be able to get to the sump plug !!
Have a close look at the offside of the engine.  You might be able to see a very bright and shiny area just underneath the front of the engine mounting plate. If you haven’t already guessed, this is where I attacked the engine with an angle grinder to provide clearance for the incredibly huge steering box – remember the earlier photo ? The challenge was to remove just enough metal so that as the engine moved around, it wouldn’t constantly thump against the steering box, and to leave enough metal so that the strength of the engine wouldn’t be compromised around such an important area – very nerve racking !

A Massive Setback.

After the completion of the exhaust manifolds and down pipes, and the hammering of the nearside inner wing to make way for the carbs, I decided that it was time for the V12 to be finally lowered into position and bolted in, but I thought it would be a sound move to start the engine before fitting it, as it was an unknown quantity, and I still remembered the saga of the XK oil burner !  As the engine had stood for goodness knows how long, the very least I could do was check, clean and re-gap the spark plugs. The first couple of plugs came out with no trouble at all, then…..oh no…a tight one! From previous experiences with the XK engine and the Rover V8, the technique of just to say moving the plug, then tightening, then slackening a few degrees more, then tightening again and repeating this procedure, very gradually increasing a few more degrees every time had worked wonders but not so in this case. The spark plug sheared off and left all of its threads in the cylinder head. On removing the rest of the plugs, the same thing happened again, but on the other cylinder head!  After trying fruitlessly to remove the sheared threads, by repeatedly heating with a No.15 nozzle in the welding torch followed by immediate chilling with freezer spray to break the bond then subtle use of an ‘easy out’, I decided that it was easier to purchase a second V12 engine than mess on removing both cylinder heads etc, etc, etc.
Was the Battlecruiser going to languish in my garage forever, covered in dust and cardboard boxes ?  ( Photo F ) No !   Act quickly and get another engine!  I can hear all of you saying “oh no, not another   engine !!!!!

Photo F

A few phone calls located a PRE-HE engine just south of Birmingham for £150 with spark plugs removed !  When I got the engine back home, I changed it over to carbs, attached the home made exhaust down pipes, connected the oil cooler, poured two gallons of oil down it’s throat, and with it not only suspended from the hook but also lashed down to the feet of the engine crane, we prepared to start the engine.

Blast Off !!

Picture the scene.….. a V12 on down pipes, lashed to an engine crane, Ray and Colin on the nearside of the engine, Chris and me on the offside.  Ray’s job was to pour petrol into a big funnel to feed the four Strombergs, Colin was in charge of the choke on the nearside, I worked the choke on my side and all of the throttles, and Chris had the honour of twisting wires together to operate the ignition and starter motor.

Contact !

Ray poured petrol into the funnel for what seemed like an eternity whilst the carbs filled up, then Colin and I set the chokes to full.  Chris switched on the ignition and engaged the starter.  The engine turned over for ages, and then began chuffing and popping out of one of its down pipes.  Moments later there was a loud bang, a tube of flame shot out of one of the carbs and then the engine roared into life !  The temporary oil pressure light went out, and as I revved the engine, it seemed to levitate – straining all of the ropes.  As we were all beginning to choke on exhaust fumes, it was time to let go of the controls and as the engine slowed, spluttered and stopped, I realised that I had created a monstrous hybrid – a V12 folding crane!

Time to fit the engine.

I quickly found that the engine would not go in with all of its down pipes on, and so was forced to remove the nearside pipes. In fact , things were so tight that it practically had to be dropped in vertically, and then waggled around the steering box, before arriving on its mountings. There was no chance of attaching the bellhousing at this point, however, with just the torque converter on, the engine was shoe-horned into position. I had previously twigged that when the engine was in position, it would be impossible to install or remove  the starter motor and so just before the engine went into the hole, the thoroughly overhauled starter was loosely held in position with a few bolts and its wires were connected. Once the engine was in position, the bell housing was fitted and I could see that the rear upper engine mount had lined up perfectly as per the original plan. This was a milestone. On a job like this, with so many engineering nightmares to overcome, it was very reassuring to find things finally coming together.

By now, those of you who are reading this article ‘wide eyed’ in disbelief, will be gripped with panic as you realise that I now have to fit the incredibly heavy cast iron Model 8 gearbox from underneath. In fact some of you will probably scream when I report that if wasn’t going to go in! Underneath a 420G, the vast expanse of floor pan is mind blowing. You can’t even see the prop shaft forward of the centre bearing as it really does run in a ‘tunnel’ , totally enclosed by the floor. The gearbox fits up into the structure of the vehicle extremely snugly, the result being that to attempt to fit the gearbox onto the bellhousing safely and accurately, without damaging the torque converter, requires dropping the rear of the engine by an angle of nearly 40 degrees. Guess what?
The ultra tight fit of the V12 allowed little more than 5 degree drop. To say that I had overlooked this part of the plan was the understatement of the year.
It looked like I might have to assemble the engine and box as a unit, raise the front of the vehicle, drop the front suspension right out of the way, insert engine and box from above, somehow reattach front suspension and finally lower vehicle back to floor. No way !
At an all time low point like this, it is normally a good idea to walk away from the job, consume gallon’s of beer and wait for a brain wave, which I’m glad to say did not let me down.

Bands, clutches, pipes and oil !

The brain wave was a master stroke of pure madness.  As Edmund Blackadder would have said ‘The plan was so cunning, you could brush your teeth with it!’ The weight and overall length of the gearbox were the major problems, so the plan was to take the gearbox apart and then in the tradition of all good Haynes manuals which say that  reassembly is the reversal of removal, simply put it back together attached to the engine – laid on my back, working upside down – a daunting prospect, but the only way out .
Photo ‘G’  shows the box with its sump removed. The lump on the back of the box with the prop shaft flange is called The Governor Housing, and when removed greatly reduces the overall length. It cannot be removed until the hydraulic valve block has been removed as there is a large diameter feed pipe running at an angle from the valve block, through the rear carcass of the box up into the governor .

Photo G

Photo H
Photo ‘H’  although it is out of focus, shows the valve block removed, and governor housing separated. The next step was to remove to front and rear servos, and then withdraw the gear train and clutch assemblies through the back of the gearbox. That just left an empty carcass with only the front pump on. This was then lifted up and bolted into position. Even with all it’s internals removed, the carcass is still a very heavy item when you are on your back with it hovering just above your nose! The sequence of events was then to insert the clutches and gear train through the back of the box, fit servos and engage with brake bands (easier said than done upside down!), fit governor housing (another heavy assembly - best tackled when fresh! ), fit valve block, associated pipes, filter unit, sump and filler tube, refit prop shaft (which I’d undone at the diff and centre bearing so that the prop could be withdrawn down the tunnel to gain a vital inch or two when fitting the Governor housing), and finally fit gearbox mounting assembly.

Whilst working on the auto box, I had ton’s of clean newspapers to lay parts on, plenty of clean rags, and never allowed my hands to become filthy.  Cleanliness is important with automatics!  Taking automatic gearboxes apart can be a bit tricky, so if in doubt, go and see a professional.

Once the gearbox was in position it was time to make the rest of the exhaust system.  I fabricated “Y” pieces for the down pipes once they had reached the sump.  Photo ‘I’ shows some of the exhaust pipes coming together, only one silencer for each side, arched sections for getting over the drive shafts and through the suspension cradle, and one of the long bits for going down the length of the vehicle.

Photo I

Photo J
Photo ‘J’’ shows the engine coming together, carbs fitted, petrol and breather pipes taking shape, the oil cooler connected.  Brackets and supports had to be constructed for the PAS pump and alternator.  V12 owners will notice that these are on the opposite sides to usual.  I decided that it was easier to make some brackets rather than have a carry on re-routing wires and hydraulic hoses.  Incidentally, because of the tight fit between the carbs and the inner wings, I slotted the bottom mounting holes on the carbs and used bolts to secure the top holes.  The carbs then simply dropped vertically into position and were tightened up without requiring lots of room to manoeuvre.

When it came to balancing the throttles, it became apparent that my air flow balancing meter would not fit between the carb mouths and the inner wings, and so a different approach would be necessary. I removed the damper assembly from all the carbs, so that the bottom of the butterflies were visible. Then I partially opened the throttles by pressing the accelerator pedal slightly and then clamping it in position with mole grips. By bending a gas welding rod into an ‘L’ shape and inserting it through the damper opening so that it located in the gap between the bottom of the butterfly and carb body, the welding rod could be used as a slip gauge to ensure that each butterfly is set to the same position, starting with the rear carbs and finishing with the front carbs. Following this procedure ensures that any errors due to worn or slack linkages and bearing play on the throttle pedestal are eliminated as there is tension in the whole system during adjustment.

Photo ‘K’ shows just how tight the offside of the engine was.  You can see how the down pipes follow the line of the engine to provide clearance for the steering column (just visible), the starter motor nestling at the back (now unremovable !) and you might be able to see the steering box nudging its way into the area of the engine that I ground away!

Photo K

Photo L
Photo ‘L’ shows everything really rocketing ahead.  Control cables in place, wiring underway, cooling system plumbed in (apart from the heater unit).  I had my original radiator re-cored as it was ideal for the job having a top inlet on both sides.  I did however have a baffle fitted about one third of the way down on the near side header tank.  Without this the water leaving the near side cylinder head would have gone straight down to the water pump connection  at the bottom of that header tank, with disastrous results.

With cooling firmly on the agenda it was crisis time again.  There appeared to be no room left to fit an engine driven cooling fan.  In fact, the fit of the V12 proved to be so tight that I cannot slide my hand in between the water pump pulley and the radiator .
Photo ‘M’ shows the solution.  After spending hours hunting around in scrap yards, I discovered the humble Nissan Cherry.  The Cherry’s cooling fan was exactly the right dimensions for my radiator. It has got a marvellously slim profile thanks to the use of a  ‘pancake motor’. The frame is made from metal so that it can be welded and have brackets easily attached, and the big bonus was that two of them would fit in, side by side.  The only difficulty was that on the Cherry, the fan lives in the engine bay and sucks air through the radiator.  My fans were on the opposite side and would have to blow air through the radiator.  In this situation, reversing the power supply to the fans does not solve the problem – fair enough, the fans run backwards but they hardly move any air.

The solution for each assembly was to pull the fan off the motor shaft and refit it backwards, then get the direction of rotation correct.  Next time you are tucking into a nice bowl of vegetable soup, try turning your spoon upside down and see how long it takes you to finish the soup – that’s the nature of Nissan Cherry fans!

Photo M

Photo N
A side effect of fitting electric fans on the front side of the radiator was that the torsion bar springs attached to the bonnet hinges (visible at the bottom of photo J) had to be removed due to lack of space. So now I have a hi-tech bonnet stay in the form of a broom shank !
Photo ‘N’ shows the horror I saw when I removed the heater box – no wonder it was never very effective.

Well, that’s my story.  Needless to say there were lots more bits and pieces to sort out, that if detailed, would turn this narrative into an epic.

Once the V12 job was finished a lot of money and time was lavished on the Battlecruiser to ready it for MOT e.g. front and rear sandwich mounts, radius arm bushes, caliper repairs, brake pipes, flexi pipes, front discs, various bearings and areas of welding.  I am happy to report that, as of April 2000 the Battlecruiser is MOT’d and road legal.

At the time of writing I have just addressed problems of petrol leaking from the Strombergs and incorrect gear changes due to kick down cable adjustment, and have yet to solve the relentless surging of the engine at tickover and small throttle openings, but I’ve had a succulent taste of the combination of V12 and 420G,and I’m addicted. It is silky smooth to drive and seems to perform reasonably well, though on a spirited get away it seems to suffer from a bit of petrol starvation. The petrol gauge appears to have developed an athletic nature – I can’t remember it moving as quick as it does now when compared to how it used to be.
Perhaps the V12 might be a good candidate for an LPG conversion.

The Battlecruiser badly needs a repaint and rechrome but that will have to wait as I’m now nearly bankrupt but very happy.

Photo O.  The Battlecruiser lives!  Does anybody know what happened to the few original V12 420G’s that Jaguar converted as a test bed for the engine ?

Photo O

Robert Hughes

Last Update 06 June 2000
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