V12 engine rebuild


Click to view engine rebuild photos


My venerable Jaguar V12 engine was performing well and had no noticeable problems, but as it was out of the car for the engine bay restoration it was a good opportunity to bring it up to scratch.  It did have a slight noise when it first started cold, but that quickly disappeared as the oil pressure built up.  Later inspection confirmed that the big end bearings were the cause of this noise.  My original intent was to tidy up the outside of the engine and just remove the sump and replace the big end bearings.  Unfortunately it was not to be that simple.......


When I removed the sump I found pieces of the timing chain tensioner in it!  Further investigation and research indicated that the timing chain tensioner had to be replaced and that required the removal of the heads and timing cover etc.  Of course while it was completely dismantled it made sense to have the whole engine “refreshed”.  So like many other aspects of this restoration the engine grew into a much more involved and expensive exercise than I originally planned.


To me, the internals of the V12 are rather complex around the overhead cams, timing chain and heads etc.  Too many complications, special tools and specialist knowledge required for my liking, therefore a home rebuild was out of the question and the engine went to a Jaguar V12 expert for a rebuild while I concentrated on the rest of the car.


The removal of the heads was the first obstacle encountered and I was pleased that I had taken the engine to an “expert”.  As is common with the all alloy Jaguar V12, the heads were “frozen” in place due to chemical reaction between the steel head studs and the aluminium head castings.  Obviously previous owners had not always kept corrosion inhibitors in the cooling system.  I would never have been able to remove the heads myself.  It was a very slow process (over a week) using a custom made jig, lots of penetrating oil and patience to finally ease the heads off without damage.


Once it was fully dismantled the extent of the rebuild became obvious. As expected the timing chain tensioner was broken and the timing chain stretched, but also the big end bearings were badly worn (no. 3 very bad) and the main bearings were also showing wear.  In fact it was fortunate that I had decided to restore the car at this stage because the engine would have begun to give serious trouble in not too many more miles.


The engine bearing problems can be attributed to the difficulty of starting the V12 when it's cold.  It will not start without a generous amount of choke being applied to the Stromberg carburettors and when the engine fires it immediately revs and settles into a fast idle. All this occurs on a cold engine with no oil pressure and if the car has been sitting unused for some time the oil will have drained in important places.  So, every time the engine is started from cold it's very hard on the bearings. When the car was back on the road I developed the habit of leaving the choke off when the engine was cold and turning it over on the starter for a while to circulate some oil, and then applying the choke to start the engine.  Later V12's with fuel injection did not suffer from this issue.


The engine was totally original and generally in good condition for its thirty years and 77,000 miles. The V12 has a reputation for being virtually indestructible as long it’s regularly maintained and the oil and water (with corrosion inhibitor) are kept in the right places.  The timing chain and big end bearings are known areas for concern and this held true for my engine.  The crankshaft, conrods, pistons, bores, valves,  heads, cams and other major components were all in excellent condition and so no major reconditioning was required.  The rebuild included:


  • Replacement of big end bearings, main bearings, piston rings, timing chain & tensioner, welsh plugs, head studs, crankshaft seals and valve guides
  • The valves were ground and the heads surfaced
  • The intake manifolds and cam covers were chemically stripped and powder coated in satin black
  • The exhaust headers mating surfaces were machined to provide a flat surface to match the heads
  • The alternator was rebuilt
  • The carburettors were water blasted and rebuilt
  • All engine brackets and fittings were painted and plated
  • A new high torque starter motor fitted
  • All water hoses and clamps replaced
  • All bolts, nuts and fasteners replated or replaced
  • External mild steel water pipes were replaced with stainless steel
  • The oil pressure sender unit and oil pressure warning light sender were moved from the LH rear of the block to the rear of the centre valley using Jaguar parts from a later model V12


Automatic transmission


The auto did not have any real problems, but it was a bit slow to change at times and a slight whine could be heard in first gear.  Not enough reason to specially pull it out of the car but like many other aspects of the restoration I decided to rebuild it while it was out of the car. The Borg Warner automatic fitted to the series III E-Type Jaguar is the same as used in the Ford Falcon XY GT and other high performance Fords of the era like big block Mustangs, so it was not difficult to find someone to work on it.  It’s a very robust and smooth changing auto with a very heavy cast iron casing and aluminium bellhousing.


A local automatic transmission specialist took on the task and after he dismantled it I picked up the housing and painted it.  He then completely reconditioned it including the torque converter.


High-Torque starter motor


The original factory fitted starter motor was a behemoth of an antique and after serving its purpose for over thirty years I felt it was time to replace it with its modern equivalent.  It would be difficult to replace the original starter with the engine in place and now was the time to change it to a modern High-Torque unit that is much more power efficient and about half the weight and size of the original.  High-Torque starter motors are readily available but the catch is that the flex plate/ring gear has to be changed to match the teeth on the new starter motor.  The E-Type flex plate/ring gear was replaced with one from a later model XJS V12.  Once the correct item was sourced it was a simple bolt on change.


Cooling System


I had noticed a tendency for the engine to run a bit hot before the restoration began so I gave the cooling system a very thorough going over during the restoration.  In the end the main reason for the hot running turned out to be that the temperature gauge was reading too high and had to be recalibrated.  The top and bottom radiator tanks were removed and the core thoroughly cleaned.  Lucky I did, as despite recently having a new heavy duty core fitted the radiator was 25% blocked. It was clogged with rust particles from the decaying mild steel joining pipes in the cooling system and the old header tank which had rusted away and been replaced. Obviously the previous owner had replaced the rusted header tank and recored the radiator but had not thoroughly flushed the system to remove all the contamination.  Another lesson to be learnt!


The electric radiator fans were rebuilt and the radiator shroud painted and new rubber seals fitted.  A new “otter” switch was installed to ensure that the fans came on at the correct temperature.  The cooling system hoses were all replaced with new Kevlar reinforced hoses and all clamps were replaced with stainless steel units.


The external mild steel water pipes were badly corroded and were replaced with custom made stainless steel pipes.


Putting it all back together again


Despite its obvious size and complexity the V12 is not that difficult to remove and refit.  The secret is that it’s built in “layers” and you just have to keep removing things until you get to the part you want to work on and the reverse happens when you put it back together.  Sounds simple and it is.


I found the V12 to be no more difficult to remove and refit than a large V8 in a compact sedan.  So once the engine was back in it was a simple matter of bolting everything back together in the right sequence until it was a complete unit once again.


If you are going to work on the V12 E-Type just be prepared to remove lots of parts before you can work on the bit you want to.  Not hard just time consuming, and if you completely remove the bonnet the job is so much easier.   The Jaguar V12 is a beautiful piece of engineering, overwhelming at first sight but once you get to understand it you appreciate what a wonderful engine it really is.