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Classic XK Forum

Which Engine for an XK - how important is it?

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Cineman Oliver Stapleton
London, UK   GBR
1952 Jaguar XK120 "The Jaaahg"
I bought an XK120 fhc a while ago, and have now discovered the engine is a 4.2L block from a 1981 XJ6. There is nothing wrong with the engine but my question is this - for re-sale purposes how important is it that with a non numbers-matching engine is at least from the series i.e. a 3.4L or 3.8L engine from an XK120 to 150? The car is fully restored and in immaculate condition, but this later block strikes me as an "unworthy" addition to an otherwise great car. Thoughts?

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erlewis Avatar
erlewis Eric Lewis
Madison, USA   USA
I learned there is a difference between XJ6 engines and E-Type engines in the following way, there may well be other differences.

In my 4.2 E-type engine head bolts pass through the water jacket and thread into the block in the same area where the crank bearings are located.

Head bolts in XJ-6 engines thread into the top of the block where the head and block are joined by the head gasket.

Obviously the block is much more structurally stiff around the crank shaft than it is at the top of the block so this may be a requirement for the higher compression pistons that may be fitted on an E-Type engine.

None of this may apply to your older sports car model 3.8 or 3.4 but, in general, I would say that a 4.2 is more advanced (better) than a 3.8 or 3.4. I suppose a collector would object and you would lose points, I expect, in a show competetion for the newer motor.

I installed a 5-speed transmission and electronic ignition in my '69 E-type, preferring drivability over showability.

formercat1 Avatar
formercat1 Mack Besser
Maple Park, IL, USA   USA
Of course on resale, any non-matching # driveline is going to be a detriment to value.
However, the 4.2 XJ motor is probably the best non-matching motor you could have in a 120.
If you still have the original Moss box, non-synchro first, this 4.2 motor will pull from a stop in 2nd gear.
If is so smooth at low speeds, one rarely needs first gear.
We have had this combination arrive at our shop in the past & once sorted, makes a fine driving automobile.
The negative is only on resale value for the #'s matching fanatics.
As previous gentleman said, 5 speeds are good. However, wouldn't that go against the above named individuals?
BTW...we have just completed a '63 E-type w/a GT40 5.0 V-8 bolt on electronic fuel injection & a Tremec 5 speed, Corvette brakes & dual brake boosters.
Whatta ride!!! OOOPS... the # matching gods are angered! Big Deal. Mack

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Cineman Oliver Stapleton
London, UK   GBR
1952 Jaguar XK120 "The Jaaahg"
Thanks guys: more or less what I have come to think. I looked into changing the engine for an "original" and the cost seemed ridiculous as well as the thought I would be loosing torque.. I had the engine (which has triple webbers) tuned on a rolling road recently and it has made a big difference to the performance as well as much smoother starting so thanks to Dave Baskerville of North Devon (UK) for that.
Only remaining niggle is some "bump steer" which seems to have originated when the rack-and-pinion conversion was done. However Guy Broad - from whom I bought the car - has agreed to look at it (for free) so there's no arguing with that!
Oliver S.

rlowe reg lowe
norwich, norfolk, UK   GBR
1973 Jaguar SS-100 "The Beast"
I think that to change the 4.2 for a 3.4 or 3.8 will be a backward step in terms of drivability and after all that is why we have theses silly old cars. The 4.2 is no more powerful than the 3.8 but has better torque figures which makes for less stressful driving.
Another point to think about if you did change to the correct capacity engine, as shown on the Heritage Certificate, is that it will still not be matching numbers which IMHO is the curse of the current classic car scene.
Finally, if Guy Broad built and sold the car, this is all the kudos you need. Drive and enjoy.

abohica Peter Cook
Toronto, ON, Canada   CAN
I have an XK120, from numbers it is a 52. It is I would say un restorable condition, everything flat is gone, but many good parts.
Bonnet and boot lid are very good and doors are easily fixable.
The engine is 696538 or 896538. Is it possible to find the car it came from to see if someone needs it for their vehicle?
The chassis number is679750.
I bought this many years ago for spares and never looked at the car.

Coeshow Avatar
Coeshow Mike C
El Cajon, CA, USA   USA
Xk engine

Edited 2 time(s). Last edit at 2018-07-03 03:07 PM by Coeshow.

jdrubin David Rubin
Rochester, NY, USA   USA
One more vote to keep the 4.2 engine. The 3.8 engines are sleeved and frequently crack between the cylinders. To avoid this, they moved the cylinders in the 4.2 block so they are no longer centered on the combustion chamber in the head. This causes additional turbulence, resulting in better torque at low speeds. However, it costs power at the top end. For any reasonable use, the 4.2 block is a better way to go. However, it may require better cooling than the original radiator in the XK120.

Coeshow Avatar
Coeshow Mike C
El Cajon, CA, USA   USA
In response to jdrubin, the 4.2’s are also sleeved from the factory. The reason Jaguar altered the bore spacing was to improve cooling of the cylinder bores when increasing the bore size of the 3.8 from 87 to 92mm for the 4.2. From a performance standpoint, there’s likely no benefit or added ‘“swirl” from this change that would improve low RPM torque. If anything, the misalignment hurts performance at all RPM’s. Cylinders 3 and 4 suffer the greatest amount of misalignment. Besides, many 3.8’s were bored and sleeved to 92mm long before 4.2’s became available as original equipment. Cheers!

Coeshow Avatar
Coeshow Mike C
El Cajon, CA, USA   USA
I believe the difference in the head stud length was introduced at a certain time, not between models. All early 4.2’s had studs that screwed into the top of the block and were @ 10 inches long. The newer 4.2’s have the longer studs that locate deeper in the block and are @ 15 inches long. Jaguar didn’t make blocks specific to the etypes alone.

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