3.12.1 - Fitting a 4.2 Engine in a MK2 or S-type ( Terry Handley,
November 25, 2003
"Fitting a 4.2 engine to a Mk2 and or an S Type makes some sense,
firstly 4.2 engines cost less to overhaul and because of the vast amounts of 4.2
engines made there is a lot in wreckers, breakers and even in Jag Lovers back
yards to choose from, some model 4.2's are better then others and the
differences between different models are vast, at the end of the day they are all
good enough for all but the fastest race cars, then there is some things in
favour of just a couple of engine blocks & it's not the short stud
Whichever 4.2 engine is chosen to fit to a Mk2 there is only a few
differences that can make fitting to a MK2 from very easy to a little
tricky, the only real snags are the Generator Mount, alignment of the
Pulleys for w/pump, harmonic balancer, always use the 4.2 Harmonic
Balancer and figure out the alignment problems, the 3.8 balancer is not good
enough even for a 3.8.
If one takes a 4.2 Harmonic Balancer & Split Cone to say Ross Balancers
in Sydney they should be able to quickly mod a Balancer from another
engine, the Australian Designed and made Cyco Industries marketed as a B&M
works fine but is now too expensive as it goes from OZ to the US and comes
back here too expensive, there maybe some stocks in OZ but generally too
greedy stockists bung to days prices onto it even though it has been sitting
around for a few years.
The one thing to keep in mind is the early 4.2 as fitted to the 420
sedan & Mk10 4.2,ser 1 E Type has the larger casting for the water pump but
retains the same size w/pump impeller, the advantage is the larger dia intake.
I have built many 4.2 engines with the 3.8 pump and timing cover, 420
timing covers will fit, same pump impeller size as a 3.8 , see my cooling
The B type 3.8 cylinder head can be fitted to any 4.2 engine, when
fitting to the later 4.2 engine blocks the 2 water ways at the back of the
engine block need to be tapped out and plugs made from threaded bar or bolts
screwed in by a slot hack sawed ontop, use a loctite product, if ever
theyneed to be removed a small amount of heat in that area softens the
A B Type head with the ports taken out and metal removed from under
the valve seats works very well, but good B Type Heads are getting hard to
find, don't discount a 2.4 head it is usually in better condition then a 3.8
and there are many sitting around on 2.4 engines digarded many years ago,
only the ports need to be taken out or run it as is for torque like a Mack
A S/Port Cylinder head from the many jag model sources can be fitted to
any 4.2 engine.
For most uses the 420 or XJ6 engine inlet manifold is easiest ,a 240,
XJ6 Ser 1, 420 , 2 x 2" SU's work well enough for all but the Cars that
owners are looking for the mostest, a good 2 x 2" SU set up can beat an
ordinary 3 x 2" SU set up.
Proper porting of the 2 X SU Manifold can flow more then enough.
One little snag that's found out often at the last hour of the
conversion is the Water Outlet from any S/Port Inlet Manifold does not match up to
the MK2 radiator,when one looks at the problem it can be sorted out relatively
The down side of fitting the 3 x SU's to a RHD MK2 is many, one has
to overcome the fact that the Clutch & Brake M/Cylinders are right in way
of the Inlet Air Cleaner, that is a Filter system needs to be used and the
rear most carby is so close to the Clutch M/cylinder.
The other major problem is the inner guard needs to be modified to get
the needed clearance to fit a proper effective air cleaning system.
The beating cutting of the inner guard area causes warping of the
mudguard unless real care, expertise is used. the gap between the bonnet and
the guard becomes greater as it's pulled down once the inner area is
played with, it's usually bent down bad enough from years of mechanics leaning
over top of the mudguard.
Use the 420 Air Filter assembly it works well enough, there is nothing
to be gained but noise using anything else.
One can duct cold air through the N/S inner guard and hook it upto the
420 air filter, early Mk2's had the hole there for the fitting of a Tube to
the under N/S Air Filter that was used on some export models, many MK1's
had the said, make sure you continue the pick up to a position up front rather
then just picking up air from under the mudguard.
You can see the proud area of the inner guard where the connection was
made, it was blocked by Jaguar on some early model MK2's & then later
pressings, the hole was not included.
Obviously I have only touched on the surface of fitting a 4.2 engine
and the possibilities are numerous many listers have done and are doing this
I have found that a well prepared, slightly modded 4.2 engine with 2 x
2" will still to day surprise many a modern car driver, recently I had
the occasion to let loose a Slightly modded 4.2 MK2, it was only the aged
tyres that made me back off & let an XR8 Ford Falcon go buy.
Make no mistake a modded MK2 4.2 is still today a very fast car that
can embarrass even GM & Fords supposed Quick Road Cars, you can see this
when Normie turns up at Oran Park with Holden.
A 3.8 engine needs to be modded a lot to perform as well as even a
standard 4.2 engine performs in a MK2, the extra torque of the 4.2 means a lot
more grunt without the down changes, quicker acceleration more power up the
hills, near forget 1st gear.
A 4.2 engine with a safe low compression of 8-1 goes harder then a 3.8
with 9-1 + Compression and it will run nicely on lower grade fuels.
A lightened Steel Flywheel & a 9.5" Clutch on a 4.2 livens the engine
considerably without affecting tractability.
The later XJ6 Engines are heavier then the early 3.8 engines and the
extra weight can be felt at kerb speed, however the standard MK2 Springs are
really too light for even a standard 3.8 engine so an update to at least
1000lb Front springs is needed for a MK2 that does other then sit around.
In a Racing MK2 you need 1600 lb front springs, I like 1200 lb front
springs for my road Mk2's and use a Front Anti say Bar from a 420, a heavier
bar gives too much understeer for a road car, that is unless some other
thingsare not done to make the MK steer & handle better.
A MK2 with heavier springs suprisingly does not ride too hard, most
new sporty cars with the low profile tyres ride more harshly then a modded
Obviously there are many aspects of a 4.2 engine conversion that can
be talked about on the List but simplicity for most is the key to a good
reliable fast MK2, often lot's of money can be saved by buying
scrapped 4.2 engines and pulling them down and building one nice engine.
Mod the head & inlet manifold, fit the exhaust headers that allow the
fitting of 2" od primary pipes some Headers are not large enough inside
to realise the benefit of the said primary pipes.
Avoid fitting Tube Type Headers with larger then 13/4" OD Primary pipes
you will gain nothing but heat and lose plenty of power, biggest is not
One can use the standard MK2 exhaust and a 4.2 engine , still gives
more useable power then the 3.8. Don't believe the myths about a 3.8
engines being faster, those early 3.8 E Types when compared to later
Ser 1 4.2 E Types had gearing advantages and lighter flywheels the
4.2 engines usually had the heavier flywheel.
Bring the primary pipes into one 2.5" pipe from about central position
of the g/box through to a Walker or Lukey Free Flowing 2.5 in & out
muffler and on the outlet siamese to 2 x 17/8" Tail Pipes, this set up
gives a nice note and flows enough & better then a standard MK2
twin system and it's not too noisy.