Jaguar XK mystery cylinder head

My 1953 XK120 is fitted with an XK140 engine block, so obviously, at some point, there was an engine swap. When this replacement took place, I see only two options regarding the cylinder head : whether the XK 140 head was part of the swap as a complete engine, or the original XK120 head was retained if only a short block was exchanged. In both cases, the head would be most likely stamped with an engine number, which would allow me to know which one of the heads was fitted to the car. However, there is no number at all on the front of the head. I thought that maybe and for unknown reasons the number could have been ground, but it would take a great deal of “shaving” to make such a stamped number disappear. Furthermore, upon close examination, there is no way anyone could have done such a good job at grinding the number, as the head is smooth as a baby’s behind. So, my next thought is that maybe a new replacement head was installed, because in the valley of the head, there is no letter A,C or N present, and no stamped letter or numbers at all. The only number present on the head is at the very back, on a slightly raised rectangular little platform apparently cast at the same time the head was made. I can’t provide this number right now as I am away from the car, and I forgot what it was (senior moment?) but I will look it up again. This is not a life and death matter, since everything worked well together and the engine was running strong when bench tested. But, as many enthusiasts, I am always curious (and horrified) to learn how much my car has deviated from pure originality.


I can easily identify your head, even without stamped in ENGINE No.
Photos are best, but if not, can you at least advise:-

What is the number stamped in at the rear of the spark-plug-valley - this is what I call the HEAD SEQUENCE NUMBER, and allows me to approximately date the head.

What are ALL the letters/numbers cast in on the underside of the head - these are all foundry codes that tell me exactly what type of Head it is, and from which of the two foundries that supplied Jaguar actually cast your head.

There are a couple reasons why a head wont have its ENGINE NUMBER stamped in, but your photos/answers to above will reveal most likely explanation.


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Thank you so much for your prompt answer. I will take a picture of the numbers cast on the head very shortly and will post one here. When you mention the underside of the head, do you mean the face in contact with the head gasket, in which case I will not be able to access that since the head is installed on the engine. I may have to take it off at some point since the engine builder forgot to install the ignition wire conduit and I don’t believe one can just casually remove 2 head bolts out of sequence. But in the meantime, an approximate date of manufacture would be great. So as soon as I can, I will take a take a picture, hoping this will be helpful. Thanks again. Bernard


All the relevant cast numbers are visible when head is fitted onto block - they are all in the cast overhanging fields between the machined surfaces also overhanging. If Engine is out of car its easy to see/photograph, but quite a challenge, albeit still possible if installed in car with all manifolds fitted.


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You can see the numbers we are looking for here.

I, too, have a head without a serial number in front of the front spark plug, and I have always assumed it was a dealer ordered replacement from factory stock.

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Having realised as early as June 1978 the numbers stamped into the valley of the head though not the engine number itself and in later years the similar number stamped into the blocks and once again not the actual engine number itself meant something I coined the expression “Serial number” and had always thought due to the way the later ones appeared that it was done at the foundry as they are literally pressed in.
Going though my archives the Jaguar factory actually refer to the numbers stamped into the head either into the centre of the valley or at the extreme rear of valley as
Cylinder head serial number
Cylinder head number
Cylinder head series number
The serial number system started on at least the 2nd Alloy RHD 120 and continued through until late 60’s

When the “Straight Port” head was introduced early 1958 they started a new serial number system at A1
And for D type wide angle heads they started a serial number system at C1 the highest number I have seen is C12 from memory

Over the years I have photographed literally 100’s of these serial number stampings and it has proved invaluable to help determine cars stated to be matching numbers where it can be easily shown that the head was cast after the car was built! It is a lot harder in fact probably impossible to remove these numbers without it being obvious as opposed to the engine number on a block or head where it can be removed almost without being visible and then it gets back to the argument of what fonts the factory used to stamp the engine numbers.

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Hello Roger,
Here are the pictures I took of both the cylinder head and the engine block. I think things may become a little clearer now. Once I discovered the part number C 6733-1 cast on the head, I rushed to my X120 parts manual. There is a C 6733 cylinder head listed for the XK120, but no C 6733-1, which leads me to think that the head on my car could possibly be also from an XK140. I don’t own an XK140 parts manual so I cannot establish that fact for sure. So, thank you for any light you may be able to shed on the matter.

Hello Rob and thank you for the info. I was able to photograph all the relevant numbers as my engine is still out of the car. I still had to lay on my back to take a picture of the underside of the head. These engines are anything but light to move around. I just sent the pictures as an answer to Roger Payne on this topic, so I would love to get your comments on those too. In any case, restoring an XK120 is an adventure and despite half a century spent working on British cars, I still learn something everyday. I enclose a picture of my cylinder head where I believe the engine number should be stamped.

Hello Terry and thank you for your participation into this discussion. I just posted a whole bunch of pictures of my cylinder head, in an attempt to establish if it originated in an XK120, XK140 or something else. Like you, the thought that it might be a factory replacement head crossed my mind. In any case, you are welcome to gather these pictures for your personal collection if you desire to do so, Best regards, Bernard


The numbers certainly reveal all.

But you have nailed it yourself = the C6733-1 part number cast on head tells you this is a normal ‘A’-type head, that had a minor casting modification made from the earlier C6733 heads that did not affect interchangeability/functionality – thus simply a -1 suffix and not a totally new part number. (The modification was to do with the amount of metal provided for drilling and tapping the Inlet Manifold Studs to avoid rare instances of the studs breaking through to the water-jacket)

Given there were casting lead times, stock warehousing, Jaguar machine shop times, and allocation to either an XK140 (standard, not C-type) engine build or a contemporary Mark VII saloon engine build, or a contemporary 2.4 litre (Mark 1) saloon engine build, it is not possible to give an exact build date of this head – but a good guide is the first use of the C6733-1 head casting ended up being built into XK140 Engine Number G6912, and from other data sources, we can estimate G6912 (or a later XK140 engine number) was first fitted to an XK140 with a build date of ‘about’ FEBRUARY 1956, albeit noting these C6733-1 castings were still being used up until the last XK140s built in January 1957.

I think that’s all that matters to you – its an later variant A-type head that suits either a standard XK140 or a Mark VIIM built between Feb 1956 to Jan 1957, and realistically will also totally suit your 1953 XK120, albeit would have originally been supplied with a C6733 head and not this C6733-1 head.

If you want more the B235 number cast underneath the C6733-1 number tells me that this particular head was cast/supplied by West Yorkshire Foundry, who was indeed the more dominant of the two foundries supplying Jaguar in 1956, the other being West Midlands foundry, noting both were supplying Jaguar during XK120 period as well.

The E509 cast number – if I have read this correctly, is the CASTING SEQUENCE NUMBER (as I call them) as allocated individually one by one incrementally by the West Yorkshire Foundry, and the closest I can advise from my records, is that a C6733-1 head with CSNumber E111 has Engine Number N4064-8 stamped on it – a Mark VII saloon engine number, so that only tells me closer to Feb 1956 than to January 1957.

The BL190 number stamped into the rear of the top of the head was applied by Jaguar when they were machining up/assembling the head casting into a completed cylinder head, noting this number is independent of what type of head casting, and is just based on sequential allocation to all heads going through the Jaguar factory cylinder head shop, indeed the latest C6733 head casting I record is BF280, interestingly, a supposedly factory new replacement head, retrospectively allocated and stamped with an XK120 engine number, but again suggesting more early/middish 1956, rather than Feb 56 or approaching Jan 57.

Now in amongst these head photos you include a couple of Block photos – C8810 over FF – this being a block casting for a later XK140, from Engine Number G1908 onwards, albeit nothing I can tell whether actually a C8810 or a C8810-1 part number block as the difference is in the machining, not in the casting, but regardless not inconsistent with the G7214-8 Engine Number stamping, that is for a standard XK140 built approximately/around MARCH 1956.

So all of this tells me the HEAD is totally consistent with being fitted to a early/mid 1956 standard XK140, and actual detail numbers aside, totally suitable/interchangeable technically with the head that would have been originally fitted to your 1953 XK120, albeit the G7214 block has the improved hydraulic timing-chain-tensioner as fitted to XK140 from 1955 on, being a much improved design than the C.4820 block that had a spring-blade timing-chain tensioner as fitted to your 1953 XK120 and indeed the earliest 1954 build XK140s. (and Mark VII saloon equivalents)

Your decision, what this all matters to you.


I think Roger meant to say William Mills foundry, although it was located in the west midlands.
We’ve discussed head markings several times in the past.


Of course you are right to correct me Rob - its most definitely WILLIAM MILLS foundry.


C6733-1 William Mills head as detailed above with the part number cast into the head
Has anyone any thoughts on the same part number on a WM head except only the C is cast the complete rest of the number is stamped doesn’t even look like there where any numbers cast onto it ie old part number later ground or chiselled off like some heads. What is interesting is the is a small cast A in the valley.

See photo - is this what you mean Terry?

The pictured head is a brand new head, never used, that was a spare part from Brysons.
It has been fully checked ready to be fitted, but wasnt - change of plans, so now surplus.
And its available and for sale if anyone is interested.


same thing but yours clearly shows removal of cast numbers by grinding. And yes A in same spot.
photos below