Cylinder Head Identification

A later head has been fitted to my 1950 XK120 some time in the past. I’m interested to pin down exactly which head I have.
Underneath on the exhaust side are the part number C12500, plus two other numbers: C697 and B680. On the opposite side is the mark for West Yorkshire Foundry.
I once found a reference to C12500 as being for XK150 or Mk VIII. Is this in fact true, and if so, do I have an A or a B Type head? B-Type would make a little more power, I understand?

C12500 is a B 150 head. for a 3.4 and was painted duck egg green, and had a cable operated tach. up to May 1959

Good to know, Morris - thanks! I wonder if anyone can decipher the other numbers… Maybe a date code in there??

Chris, as noted already your pictured C12500 is the earliest version B-type head introduced with the standard 3.4 litre XK150, Mark VIII saloon and 3.4 litre (Mark 1) saloon in 1957, and only used a couple years before being superseded by the second version B-type head C14956.
Can you advise, and provide a photo of the top surface of the head, noting both stamped in ENGINE NUMBER (as on the vertical face at rear of timing chain housing, and also the HEAD SEQUENCE NUMBER as stamped at the vert rear of the spark plug valley. These numbers will help give you an approximate date, rather than being 1957>>1959.


Thanks, Roger. I will make a note of the engine sequence number next time I go to look at the progress of the engine rebuild. I do know that the head has been stamped at some time with my car’s original engine number W2604-7. I don’t know if this head might have been a factory service replacement, or was fitted by a previous owner at some point in the past.

Just an opinion, and you can ignore it if you don’t like it, but the vertical grinder marks above and below the number suggest to me that it once had another number which has been ground off.

Service replacement heads would have had no number at all, like this C.6733 head:
The horizontal grinder marks were done by the foundry to remove flashing, but here the ground area is not large enough for there ever to have been a number there.

We have also previously discussed the situation where in some jurisdictions, if a car is registered by engine number, and the head is changed, the licensing authorities required that the new head be stamped with the old number.

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I had considered this, and you may very well be right, Rob. Thanks for the photo - I have never seen a service replacement head. #660405 has always lived in the UK or Jersey (briefly) and I’m not aware of any such rules over here. It received a new body F4576 c1952 (original F2500) and I’m told this body number was never assigned to a car. Several of the 1953 pics I have which were given to me by the original owner’s son, show the foot well vent doors and side exit exhaust. The wing doors had not appeared when this car was built in Dec 1950, of course. He told me that his father hit a cow on a rally in 1951, and I have the bill for extensive repairs to body & paint, dated July 1951, but I can only assume that he wasn’t happy with the results, and had a new body fitted. Bill Lyons was a family friend, so I was told, which may have helped there.


OK Chris,

As you are aware your Engine Number stamping W2604-7 is not original on this head, and has been added at some stage. The font used for the number-stamps is quite different from the factory number stamps, albeit a better effort than most. And I agree with Rob, that the grinding marks suggest removing another number, before stamping in the W2604-7. AS before, the C12500 casting, tells us original stamped in number would have been V1001 >(XK150), N6001-N9999, NA1001> (Mark VIII) or KE1001>KE9999, KF1001> (3.4 litre Mark 1). That exact original number would help date head, as would the Head Sequence Number stamped at rear of spark-plug valley.

Regardless this C12500 Head is 100% OK and interchangeable for your XK120, subject of course to head being in good/serviceable condition, and as you are aware gives you some additional HP over your original XK120 head. The stamped in W2604-7 is a reasonable effort that only a few people would pick up.


Thanks, Roger. There is no way now of knowing what the original engine number stamping on the head was, but I will get the Head Sequence Number this week, hopefully. I’m rather glad in a way that I have a later and more powerful head on the engine. If I’m correct, the inlet valves are 1.750", exhaust are 1.625", and the cams are 0.375" lift. With the 8:1 pistons, I’m not sure if this would result in a theoretical 190hp or 210hp (depends what you read) but in view of the fact that I’m running a single pipe exhaust system, I guess it’s bound to be a bit less. In any case, it’s going to be better than the original 150hp with A head and 7:1.
My main aim with this car is to keep it visually as close to original appearance as I can, though as mentioned on another thread, I have the plug leads routed over the cam cover as in later XK120s, and the coil in the later position on a bracket above the front carb. I swapped the cam covers for a “studless” pair with another XK120 owner whose car is later than mine.
I plan to keep it for a LONG time, so I’m not really concerned with matching numbers. These changes made over nearly 70 years are just part of the car’s story, to my way of thinking!

Hi Roger. I went to see the progress on the engine today (crank back in block, valves & springs in head, spark plug threads helicoiled, pistons & rods assembled and weights matched, etc - all looking good!). The number at the back of the valley of the head is CJ428. I don’t know if that sheds any light…?

Factory replacement heads were not normally polished at the front, and would not be stamped.

The pictured CJ428 is indeed the Head Sequence Number I was asking for, and is in effect a serial number of the Heads that progressed through the Jaguar Head machine/assembly shop starting at A1 to Z999, then AA1 onwards regardless of whether being an A-type, C-type or in this case a B-type head. You will note that the CJ428 is stamped onto a purpose provided rectangular platform, this is a characteristic of the B-type head casting, so that in itself tells you it is not an original XK120 head, but such detail is beyond most to notice/determine. Unfortunately I cant improve my exact dating any more than saying this B-type head dated from 1958, but realistically that’s enough. Your engine number stamping has of course been stamped in, and in the scheme of things has been fairly well done, albeit the font of the number stamps are not all that accurate, but again not too bad, so it doesn’t stand out as being wrong to anyone other than a few of us who have studied such detail. The grinding marks are another indicator, but again detail not usually seen.

I would be happy with still using this head ‘as-is’, assuming it is in good serviceable condition - it provides you with about 20 bhp gross additional power, with zero downsides apart from not being ‘authentic’ for your XK120. In the mean time, if it matters to you, keep an eye out for an original A-type head, that you can recondition and one day substitute, at your leisure.

Hi Roger,

thanks for this info - nice to be able to date the head to 1958. It’s obvious that someone has at some point been at pains to try and disguise the head to simulate the original, but in fact that worries me not at all. I’m happy to have a more efficient head with an extra 20hp! I noticed that the W on the engine number stamp is a bit different to the correct Jaguar font as well. The head is currently undergoing a rebuild, as is the whole engine. The spark plug threads have been helicoiled, as a couple of them were getting really sloppy, and new valves and springs have now gone in (one spring was discovered to be broken). The cams are fine.


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HI Im new to this forum my name is Clayton and I’m form Malta. I just removed a 1952 cylinder head xk120 and the casting number is D731 which I never met. Has someone seen this casting number? This head is studless and there are no other casting numbers on the head.


Where on the head is this number D731 located? Front, rear, middle, underneath?
Can you post a picture of it?

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The significant head number is at the front of the valley between the cams on the vertical area, it should be a few letters and numbers dash 7 or 8 or 9 indicating the compression ratio (originally).

Hello. Wanny say hello in this Forum. May there can be some help to identify this engine and head.

The Head shows V-1576-8. If I´m right this is 3,4l head from a XK150 between1957-1961 ? Any ideas the “8” in the end stands for? Is there any official information which can used for homologation?
It also shows GC 479 on the end in the plug valley near the bulkhead. This stands for?

The block displacement shows 4.2 litre and the number 7I67531-S next to the gearbox flange.
It also shows “F245” on the block. I have no idea but for sure 4.2 litre engines were built only from 1964. How to identify?

Thank you for your help!

Welcome to the forum!

If the cylinder head were really V1576-8 it would be from a 1957 XK150 3.4L. However, the stamping on the cylinder head in your photo has a few problems. Firstly, the font appears to be too small to be from the factory. Secondly, the factory didn’t put a hyphen between the V and the first digit. Thirdly, the factory did not use an inverted “A” in place of the “V”.

The 8 at the end of the number indicates that compression ratio was 8:1 of the engine to which the head was originally fitted. NOTE that the compression ratio on these engines is determined by the size of the dome on the pistons, so the heads are all the same independent of the compression ratio.

This is a serial number that was believed to be used by the factory to track heads during manufacture, and not documented in the official build records.

The 7L67531 block serial number indicates it was from an XJ6.


I wasn’t sure, but that was my first thought: that number doesn’t look like any number or font used on any Jag Head that I’ve seen.

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