XK120 - Fitting the most powerfull cylinder head on a budget

Hello all :slight_smile:
I have discovered this fantastic forum and I have already learned a lot from it. I hope I can do a humble contribution with my messages.

I am the proud owner of an XK120 FHC from 1952 (679882), since this summer.
Even if the car is matching numbers (engine is W5572-8), my current cylinder head is earlier than my car (W3423-7). (This head actually belonging to 679050, information found on from XKdata.com). Unfortunately this head is so old that I cannot put straight 3/8 cam lift on it.

I bought the famous book from Des Hammill that I have just started reading.
My understanding is that a XJ6 3.4 S3 would be the most powerful cylinder head to use.

My questions to you:

  • Does the XJ6 3.4 S3 also has the big valves (like the XJ6 4.2 S3, but apparently this one wouldn’t work as the cylinder spacing is different)?

  • Is the inlet manifold also straight port even if it has only 2 carbs originally? Would it then be the best manifold to fit with my twin HS8 carbs? (I have already bought these carbs)

  • What would be the loss of performance compared to a Triple set of HS8 with a triple carb manifold? (Ideally I don’t want to fit a triple carb set in my XK120 bonnet as I think it’s too crowed and it is needed to cut inside the wing, but maybe I am wrong).

  • I hope that the modifications needed on the XJ6 S3 head are feasible without too much trouble. My understanding is: Closing the 2 cooling holes - Adapting the old cam covers to keep the old look - Adapting the mechanical drive tachometer to keep my old tachometer - Adapting the cooling system/cooling hoses to keep my original thermostat/thermostat hose/radiator/temperature probe.

Many thanks in advance for your wise answers.

cannot see any reason why you can’t drop some 3/8 lift cams ex MKVIII XK140 XK150 straight in it.
just done this in a virtually identical head also gave it light port

Thank you for your answer but I have read multiple times in some XK120 books and also on this forum that the cylinder heads prior W4483 were not ready for the 3/8 lift cams. Jaguar has done some machining on the heads built afterwards to make this possible (valves guide and tappet guides modified).

can you direct me to any of these posts or comments?

surely you don’t believe what you see on the web!

I have checked it again and this is in the book “Jaguar XK120 in details” by Anders Ditlev Clausager

Would you have any inputs on questions on the XJ6 3.4 S3 head ?

what pages is it save me reading though thanks
the head you note XJ6 3.4 S3 head is a very poorly rough cast head could have holes for the hot air pumped into cylinders different water holes back of head. will need later inlet manifold early won’t fit

If I remember correctly, Nick Hudson referenced a Service Bulletin about this. Apparently the very early heads with 5/16 lift cams had tappet guides without the chamfering on the inside edge. On these heads, the tappet guide edges need to be machined (chamfered) before a 3/8 inch lift cam is used.

If it were my car, and I wanted to increase power on a budget, I would send the early XK120 head to a machine shop to replace the valve seats to take the bigger valves used on the C-Type head and machine the tappet guides to take 3/8 inch lift cams.

Des Hammill’s “Power Tuning” book is not particularly positive about the results of increasing the carb size up to 2 inches.

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Correct…it is in the Tech Service Bulletin #105…prior to W4483 but it is more complicated than just that number…as the mods are in production sequential in prior to W4012, as to just how the early 5/16 lift cyl heads must be modified as per thr TSB…the later heads do not. The early head will be fine…if you do the mod work…There are MANY Service Bulletins that APPLY and must be noted, followed in any cyl head rebuild…later parts that supercede…and must be used with other later parts in the valve train, pistons, …even in the depth of studs. Do NOT rely on the Factory Service Manual which does NOT include the later Service Bulletin information. This is one reason a qualified, experienced Jaguar shop should be selected for work…one would hope they know all the relevant TSBs…but I would have them in hand…and confirm. Nick

further…I would not vary much from the basic “allowed per TSB” engine/cyl head mods as for example in SB 95,97 -rods, 105, .117, 129 valve springs, …as the rest of the system is designed and in fact well designed as is. ie water pump and cooling, damper, . Later mods require other mods as well to make it all work. There are TSBs re so many engine parts…valve springs, valve size and valve stem length , guides. A TSB set is mandatory reading. Don’t go rogue beyond what is within tested norms. Nick

Because I have been down this path, I would BEG you to not molest that car! Make it mechanically reliable, then DRIVE it. Enjoy it for what it is. No amount of cylinder head and cam swapping is going to amount to a huge treasure trove of undiscovered horsepower. To do that, you will need to significantly increase the operating RPM. To do that, at great expense, you will need to replace virtually every component in the entire engine. And then you will find that so many other things will not be up to par with your gee-wiz engine. Like tires, brakes, road springs, dampers and on and on.

Having said that, I know you are still not convinced. Here are some things that may simplify your efforts.

I would encourage you to keep a type B head. Your existing manifold will fit it, as will your tachometer drive. If you are dead set on the straight port head, forget about the S3 XJ6 head, there are easier candidates. S3 heads have an extra water jacket at the rear and are longer than the XK block, so those extra coolant passages would need to be plugged off. The extra length may also foul your firewall. Find an early 4.2L head from a 420. Lacking the extra water jackets, they match the 3.4L block length.


Wiset words, yet!

A stock 3.4, in that oxen cart of a chassis, will provide PUHlenty of thrills!

That is one gorgeous car Jean-Nicolas! I have a ‘54 OTS but love the FHC shape.

The XK120 was the fastest production car in the world, so even in unmodified original trim (in good working order), they’re good for a spirited drive, noting the design is 70 years old.

A chat I had recently with a highly respected Jaguar restorer here in Australia, who also has long term experience building and racing Jaguars for his personal enjoyment, resonates with me. He is happy to accept money from customers for mods like power upgrades and disc brake conversions but basically he said Jaguar engineering is excellent, so a well maintained Jaguar is an extremely solid, strong and reliable machine. They were designed with sport in mind so will take a bit of hard work. Unless I was chasing lap times or grid positions, I would not modify a well-sorted reliable factory-spec car.

That said, good luck – Jaguars and motorsport in general can be addictive so completely understand your desire re the head :grin:

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If you study this excellent book closely it advises many things, but among them is that;

  • installing 2" carbs instead of 1-3/4" will only give more power at high RPM
    it will not help with mid-range torque, and make idle less stable

(triples worsen this effect)

  • the street driveability of the car will not be improved without other engine and related mods…and the above still applies

in my opinion, unless you bought it especially to race or drive at high RPM, it is better not to thrash a 70yr old long stroke engine much above 4500RPM

the other users on this forum are much more knowledegable than me, my ownership is of heavier Saloons, I would listen to them closely

VERY well-said! …and 20

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I only very rarely took Tweety past 4500: in any gear other than 1st, doing so would exceed some speed limit.

Being familiar with the XK’s internals, excellent engineering notwithstanding, I could never not think about those looooong rods, thrashing around in there.

Thank you all for your answers. I really do appreciate our exchanges.
Nevertheless there are multiple points I would like to raise:

  • To Mike S : Many thanks for the effort in your comment. Why would you advise to use a 4.2L head where, I think, the cylinders spacing is different and then the combustion chamber wouldn’t be aligned with my 3.4 block? That would cause some issues right?
    Otherwise, thanks for the B head mention. I think it would be the easiest, I fully agree. Would all B head be a direct fit in ?
    But to have maximum power, maybe I should go for a Straight Port B head like the XK150 S or E-Type 3.8… ? (I would then fit 3 HS8 and cut slightly my inner wing) Would there be some issues with the rev counter with this type of SP head? (I think I read somewhere that the late XK150 had a electronic rev counter and that the E-type is taking the rev counter on the Inlet camshaft electronically…)
    Also, are you saying that ANY head would NOT be a huge boost between 1000RPM and 4500RPM ? (which is the rev range I only intend to use)… I had the impression that the B head was giving more torque anyway…(maybe even better with the SP head ?)

  • To dmckay5: In general I do agree, but when I see that the XK150 3.4 S has much more power with the same block, I just want to match the same power or to have something not too far… If Jaguar has done this optimisation, I can copy it :slight_smile:

  • To awg and Wiggles: I agree and already today I am not going past 4500RPM. I can “feel” it’s a safe maximum for this old engine.

Overall, I do not intend to make it a race car (I have a Ferrari Testarossa and an Aston Martin DBS V12 to enjoy “speed”).
It’s just that, with the same block, I think it’s such a pity to be that far away from a XK150 3.4 S power (250 SAE gross)…

I think it’s reasonable to say that my stock XK120 has something like 130hp DIN, and I am expecting 170hp DIN with a 3.4/3.8 SP head + 3 HS8. Would that be a direct fit and a correct power expectation?

PS: I am adding some pictures to thank you all :grin:

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beautiful car

sounds like you can afford the petrol money to drive with foot flat to the floor for the while

if that is an original matching numbers car, perhaps you could consider obtaining a used MKV11 or later engine, having that built for max power, and substituting it, whilst preserving complete originality, as the old motor will be on hand to bolt back in

I have several used MKVII motors, and one was offered to me recently for $350

Thank you Tony. Yes, I am a very lucky and young 30 years old owner :slight_smile:

I have thought about changing the engine, but I don’t want to go this route as it’s working properly and I don’t want to make so much work. (I am doing everything myself).

That’s why I just would like to exchange the head with the engine block in place… I wonder how hard it will be to lift up the current one…

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You have a few areas still not fully understood.

  1. An XK150S does have a ‘straight-port-head’, and yes if fitted to a 3.4 litre block, the head, and the associated different inlet manifold and having triple HD8 (2inch) carburetters does give additional power over the same block fitted with a B-type head and associated inlet manifold and twin HD6 (1-3/4inch) carburetters.

  2. A straight port head has combustion chambers that have same spacing as on A-type and B-type head, so fits properly aligned with a 3.4 or 3.8 litre block.

When the 4.2 block was introduced, the cylinders were respaced, so the block now didn’t align properly with the unchanged straight-port head. Jaguar didn’t believe the slight misalignment mattered so saw no need to revise the spacings of the combustion chambers. Indeed, later on they believed the slight misalignment created some additional swirl aspect, and was indeed a slight improvement.

The biggest issue is any A-type, C-type or B-type head will happily interchange on any 3.4 litre or 3.8 litre block and retains choice of interchangeable inlet manifolds for twin H6 or HD6 carburetters.

If you fit a straight-port head it will fit on the block OK, but you need a compatible straight-port head Inlet Manifold, and nearly all Straight-Port Head Inlet Manifolds provide for triple 2inch carburetters, which simply do not easily fit, if at all in the engine compartment of an XK120/140 without surgery to the inner body work, and or compromising other underbonnet fitment.

One solution is to source an inlet manifold off a 420 (Saloon 1966-68) which fits the 420s straight-port-head, but provides for only two two-inch carburetters, and can be made to fit with a lot less problems.

The 420 Inlet Manifold also has a thermostat housing that can be connected satisfactorily into an XK120/140 radiator.

The XJ6 (series 1 ) inlet manifold also fits straight-port head and suits twin 2-inch carburetters, but has a totally different thermostat housing arrangement, that does not allow for a satisfactory connection to radiator.

And that the MAJOR other issue you need to consider.

Anything you do to an XK120 to increase horsepower, further compromises the XK120s already marginal cooling system.

Part of the improvements made to an XK140 – a better cooling system that wasn’t marginal, and happy to accept the upgraded power of a C-type head option.

XK150 further upgraded cooling system.

The twin HD8 carburetters with 420 Inlet Manifold and straight-port-head, is a good upgrade for a Mark 2, which already has 1960s cooling system capacity, so all works good.

But for an XK120, I would upgrade to XK120SE spec and no more – little benefit, for a lot of aggravation, and simplistically no real point.


Roger Payne





Thank you Roger for this very well detailed answer!
About cooling, I have already changed the fan with a 11 plastic blades and I have added an electrical on demand fan. So I think it’s a start… But I agree that a faster water pump or bigger radiator would be nicer.

Then, my solution is to source a 3.4L/3.8L SP head (or a 4.2 SP head, as you said it would be ok even considering the misalignment but I would have also the 2 extra water holes issues and the tachometer issue? ) + 420 manifold! I will then fit my twin HS8 carbs on it. (I agree that the triple carb setup looks super crowded)

Would anyone have this for sale?