If money isn’t an issue, keep the OE engine…well, OE, and find/build a warmed-up 150-spec 3.4.
Win-win!
I have thought about changing the engine, but I don’t want to go this route as it would be much easier to just swap with a nice cylinder head.
Sounds like you are determined to give the old girl (a) head!
One of those RestoGusto ones may be your best bet.
Its difficult for me to concede that the standard XK120 isn’t fast enough. I bought my FHC because I think it is the most beautiful of the 9 XK variants, though of course there are 8 other opinions among the friends here. 
But if you really want more power and just want to change the head, your best route would be a C-type head used mostly on XK140 or a B-type head used on many thousands of 3.4 saloons. Then you could use the same tachometer drive, same intake manifold and carbs, so there would be no difficulty with changing water hose arrangements.
But a word of caution about twin exhausts; I tried it and found they are incredibly loud in the FHC, like riding inside a tympani, and the factory stopped offering them on the FHC after a few months.
BTW your car is too low in the front; the body line should be parallel to the road. You can crank up the torsion bars, but be sure to jack up the car first to take the weight off the front wheels.
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Rob are you positive the front is too low, to me it looks as though the rear is riding high?
The back of a lot of XK120’s ride way to high as a the modern aftermarket springs just don’t seem to be right classic example is this restored car.
Hello Jean-Nicolas
The XK 120 engine is amply sufficient for serious road use.
A five speed gearbox, plus a 123 electronic distributor, will make it an excellent touring car. And you are not detracting much from originality.
The five speed gearbox is a must for serious driving.
The 123 distributor offers several ignition curves which are much more sophisticated than the original one, and will increase the engine responsiveness. I even have not tried all of them.
If as I understand you are in France, go and see Jean-Sebastien Vienne in Nanterre. He has been my engine shop for many years and knows everything about these engines. What he will suggest is just fit the larger valves as per the C type head, nothing else (if you are not into racing).
Best regards
Francis Thibaud
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I suggested the straight port 420 head because you had already mentioned the S3 XJ6 head, which is also straight port. The 420 head requires fewer modifications to swap.
Irrespective of displacement, all XK engine cylinder heads have the same center-to-center combustion chamber spacing. That spacing matches the center - to-center dimension for 3.4 and 3.8 blocks. When the 4.2 was created the 3.4/3.8 bore spacing provided insufficient space between the cylinder bores for the desired bore diameter, so the center-to-center spacing was increased. Jaguar judged the slight mismatch inconsequential, so the heads never were revised to match.
Here’s a 3.4L with a 420 head, an S2 E-type manifold, manual choke H8’s, S1 E-type sump, and magneto ignition.
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Thanks for sharing your nice engine pictures !
- Would this S2 E-Type manifold make the connection relatively easy with my XK120 cooling system?
- Did you have to modify the 420 rev counter system?
It certainly has been a loooong time since I’ve driven a 120, so color my statement as such: it certainly seemed to be MORE than adequate a fun ride, with a good 3.4 in it.
I do understand others’ desires for more ooomph, so I understand the OP’s desire.
That said, we’re it me, I’d want far better brakes, than more horsepressure.
Could be both. I re-arched my springs and then adjusted my torsion bars with the rear wheels off and 7-1/8" blocks under the chassis at the leaf spring front attachment points.
Happy memories there Paul, is that how you got the Wig in your name !!
Peter B.
Here`s my centimes worh Jean-Nicolas.
When series 1 XJ6s were imported to France I think the 2.8 outnumbered
the 4.2. The early 2.8 and 4.2 were fitted with HD8 carbs and a soleniod
operated starting carb, if you find one of these way to go. The later 2.8 / 4.2 were fitted with HS8 and the ,to be avioded, AED.
The 2.8 inlet manifold , a type that are American friends are not so familiar
with, is fitted with a different thermostat housing that makes coupling to the
rad easier (photo) but the water bypass connection needs to be deleted,
not a problem in my opinion. The problem of the extra two holes at the rear of the head is solved by tapping the holes 1/4 BSP and screwing in 1/4
BSP pipe plugs, used this method since the mid 70s…
Photos show a series 3 head with the tacho take off placed in position
the series 3 does not have the tapped holes, not a prob, you just drill and tap, allthough some of the late series3 heads need some extra macnining.
The series 1 head , however, is allready tapped to accept the tacho addapter. I mention the 2.8 as you are more likely to find this model in France rather than the 420.
I am in 49.
Peter B
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Depends on your definition of easy. I used to get light machine work and welding done for free. I discarded the E-type thermostat housing and bolted the E-type coolant hose connection directly to the manifold outlet. I took an unusable, very heavily corroded XK-120 thermostat housing, cut the crumbling flange (and bypass port) off of it, fabricated a new flange from aluminum plate. When I welded the new flange to the T-stat housing, I rotated the hose inlet so that it pointed towards an intersecting axis of the E-type hose outlet. The result is a simple 90 degree coolant hose. Both the E-type and 120 upper hoses are the same diameter.
I use an electronic tach compatible with the magneto.
what Roger and others have said: …follow the Tech Service Bulletin info for performance: …I have found noticeable seat of the pants improvement feel…with…modern fuels, a distributor re set to modern fuel and spark plugs, a clean-smooth flow dual exhaust with virtually no muffler but a “resonator”… (if you are ok with that sound)… This with a C head will provide plenty of power…then tune performance with the final rear end drive ratio of your choice, and or the close ratio ear box or a modern 5 speed…I like top end low revs…but you have a wide range of ratios available. Plenty of old original road test specs in the collector magazines…you can do in the 80mph s in third…120mph or more top speed if you dare. You will not do 0-60 in sub 4 seconds…use your Ferrari.
Nick
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I have a 2.8 head and was going to mention that…the other one in the back of my mind is a 2.4 head (for some reason ?)
I think the gent has mentioned he has obtained HS8, the dreaded AED can be overcome, either with an available AED choke kit, or remove AED entirely, and install full manual choke
I have done this and like it, not liking the ASC.
In your excellent triple carb conversion post, you detailed your reasons and work though, and I greatly admire and respect your knowledge
Much appreciated Tony .
You probably know this.
The 2.4 head if natural colour(silver)
(MK1) is essentially a XK120/early MK7 head, if green it’s a B type.
The 2.8 head suits 4.2 also of course.
Your HS8 conversion is for a MK10 as I recall, now in the car?
Peter B.
The OP has a perplexing amount of info available for him to mull over
My triple HS8 (onto a S2 XJ engine), but in a 420G is built but not fitted…
priority to do some repair to A-pillar area corrosion to keep the car always registered and on-the-road. I suspect this will take all the available time I have, as new metal will have to be fabricated (by me), and it will be difficult. Salty Ocean air does terrible things
I did like your tri-carb linkages…is that fitted yet?
very important linkages be taken into account with any swap, they have to be spot on
Be careful with 2.8 or 4.2 XJ heads. Not sure of exact demarcation, about 1970, but you must get earlier head and not later as later heads have revised water passages at rear of head, and casting bulges further back accordingly, thus will not fit onn/ match a 3.4, 3.8 or early 4.2 block. it’s a huge problem, and can’t be 100% fixed functionally, despite a few trying/knowing better.
And go back to my earlier posting about Inlet Manifold to use. The 420 Manifold works well in all respects. XJ manifolds do not with complex, unsatisfactory cooling system connection.
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Early 2.4 heads are simply age for age, same A Type head.
3.4 (Mark 1) introduced new B type head, which 2.4 also got later on. The last 240 and 340 got Straight Port heads.
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