Very early XK120 engine parts I’ve never seen before

I’ve just stripped an XK 120 engine. Think it came back to the UK from America.
It has some features I’ve never seen!A rather crude top chain adjuster with two peg I like plungers? The crankshaft has massive slug plugs? And the timing gear plates have no provision for top chain dampers? Also the four centre plug recesses are odd! Has anyone seen the like before? I’ve even consulted an ex Jaguar experimental apprentice friend of mine from the Swallow road days and he’s never seen the like???m

Those plug wells are characteristic of early heads. I have two like that. I expect 3.4 engines to have a rather small sludge trap plug but often people have trouble and drill them out. But the size looks absurd.

Hi Art
Yes sludge plug size seems ridiculously big! Don’t know how they’re fixed either! I’m hoping they won’t have weakened the structure of the crank.
Thanks for your comments on the spark plug wells, maybe it’s just my age and I have seen them before and simply forgotten! After all it is over 34 years since I left Jaguar!

Screw-in plugs C.4283 for crankshaft journals were first used on engine W.1747 so it appears that before that they were using Welch plugs. Urs Schmid has a picture of that in his book Vol 1 pg 81. If you are cleaning out that crank I suppose you will have to drill those out and make new ones, unless you can flow some solvent through it.

The C.4126 upper chain damper was first used on engines W.1914 to W.5416, then an improved damper C.6701 after that.

I also have those wide oval plug wells, which are sometimes called siamezed wells.
Jaguar was buying heads from two different foundries at that time, one of which did these oval plug wells and the other did normal separate round wells.
Notice my head has number E5 near the center. If yours has such a number there we would be interested to hear about it, along with the W engine number.

The pin plungers C.2188 were used on engines W.1001 to W.2330 to lock the chain tension adjustment plate.

Have a look very closely at your head in same position as the E5 stamping in the photo provided by Rob.
Its not clear in your photo but it looks like A639 or maybe A689 or very similar.
This is what I call the HSN (Head Sequence Number) indicating the sequence order that bare Cylinder Head Castings entered the Jaguar Head-Shop and were machined up, and assembled into completed Heads, starting presumably at A1 to A999 then B1 to B999 and so on, so if yours is indeed A639 that indicates only the 639th head made. Research to date says that these A1 etc numbers were allocated to all of the 4 and 6 cylinder experimental over-head camshaft aluminium casting heads, but not the earlier Cast Iron heads as used on Mark IV and Mark V saloons. So again if yours is A639 then that doesn’t mean actually the 639th XK120 head made, as a lot of the earlier numbers will be 4 cylinder heads, other experimental heads and indeed ‘spoils’ that would have been scrapped/unused.

Does your cylinder head have any ENGINE NUMBER stamped on the vertical surface at the front of the spark-plug valley? This will of course definitely identifies head, and thus the XK120 car originally fitted to, bearing in mind the earliest XK120 Heads did not have any Engine Number stamped in place - that was a later addition.
But if yours does not have the Engine Number stamped on Head, can you advise the ENGINE NUMBER as stamped on the Block - on the flat section where your oil-filter is mounted.

Regardless, your Head does appear to be one of the very first types/variants of XK120 head, Jaguar Part Number C2242 of which only 1345 were made/and fitted/ended up on an XK120 (possibly a few more made for spares or scrapped/spoilt), before the slightly revised second variant Jaguar Part Number C2242/1
The actual castings that were machined up to make a C2242 Head were all (mostly) produced by the foundry William Mills (Birmingham), and all featured this ‘siamesed spark plug’ arrangement. Later on, a second foundry - West Yorkshire Foundries Ltd - also supplied head castings to Jaguar, about the time that larger volumes of heads were needed for the new model Mark VII saloon, albeit both WM and WYF castings were supplied in parallel and allocated to both XK120 and Mark VII engines, and both went through the Head machine shop and allocated their HSN in order, regardless of the WM or WYF casting mix.

So there is a whole host of interesting features with your Engine/Head - but would be nice to confirm the HSN and indeed the stamped in Engine Number (on head and or block)

If the Head is off the block, can you provide good photos of the underside surface, as the cast and stamped numbers tell us a lot more about the head and its age.


great pic of the valley of the head showing the number A689
there is a secondary number stamped above this S3 although 3 stamped on its side
Would it be possible to get a real good close up photo of the A689 and the S3 and it is not unusual to find this second number stamped on these early heads but not the real early heads.
This crank should have a 6 bolt flywheel?

I have one of these early cranks from a double digit RHD XK120 and it also has the large core plugs. There is a number in fact 2 cast into the webs one I believe is the part number visible in this pic and it would be great to get photos of these additionally there is a stamped number into one of the main bearing side pieces (last one before scroll) which is also just visible in the photo below it would be great to get a good pic of this number as well. This number relates to the engine number or the block number and appears basically to be a job number.

The 6 bolt Flywheel on the engine if it is there will also have the early part number C2211 stamped into it and once again a stamped number that relates to the assembling of the whole motor these numbers continued through until at least the early 1960’s

And I would love to get a photo of the actual engine number
thanks terry

Hi Terry
I’ll look it over again tomorrow ( got a hangover to repair today!)Its interesting to know the large core plugs are standard.Can you tell me if they’re Welsh washers or threaded plugs? I can’t see how to remove them without drilling into them. But if they are threaded maybe I could tap them out with a centre punch?
It has been reground to 20/30” and the faces are perfect.

Roger photos of cylinder block

Photos of cylinder head hope all this is of interest?
Best Regards
Nigel Boycott

nigel over the hangover!
thanks greatly for the pics and great to see the one showing date of manufacture ie may 1949
The most interesting one will be the one on the back of the block close to the top of block on the oil filter/carb side of engine. So to the right of and above the core plug in the back of the block it will also be A### and will guess at something in the 400’s ie A400?
Did it come with flywheel? and does it still have the 6 bolt crank?
What is the story on the engine and what are you using it for?
thanks terry

Very difficult to see the number in the back of the block but looks like: S 386?

Sorry! 286! It’s my age! The crank is a six hole mounting on flywheel flange. I got the engine from a friend of a friend to build for his mate! They are rebuilding a complete car but not sure if this is the original engine.

Is there any significance in the engine number (W1383-8) being stamped in a different font on the head and the block?

knowing these early engines so well the number should be A286 and I think I can see the A there ever so slightly stamped between the S you note and the 2 and even closer view and photo of this area would be wonderful
thanks terry

this motor is so early there was the chance the head may bot have even been stamped guessing the 2 numbers applied at different times.
The next exercise is to see what car the motor is from

This has the hallmarks of a VERY early engine, possibly worth a fair amount of money, to the proper person!

W1383-8 indicates that it is the 383rd XK120 engine built. Someone with access to build records could figure out what car it came from. Probably an early steel body car, not an alloy.
The -8 means 8:1 compression ratio pistons were installed.
A689 means it was the 689th XK head to pass through the hands of the guy that was stamping heads.
The different fonts for head W and block W means there were two sets of number/letter stamps probably at different points on the engine assembly line.
Without much else to go on, my guess would be that the block received its W number when it was assembled and passed inspection, and the head received its W number when it passed its inspection and was mated to the block.
I don’t know that anyone has begun to make a study of the S 286 number at the back of the block. My guess is it is the 286th block to pass through the hands of the guy that was stamping blocks.
Either that or it means put 2 pounds 8 shillings and 6 pence on Swansea against Manchester United.

Hi Mike,
The reason the font is different maybe that the head is stamped with a smaller font due to the available space? I haven’t studied them both too closely. The other reason could well be simply that two blokes at the Jag had a different set of stamps!

G’day Nigel.
Thanks for all the additional photos - a rare chance to get such detailed pics of a very early XK head, block and other parts.
If you have a copy of Urs Schmid’s XK120 book Volume 1, page 68 to 103, he has some 46 pages of excellent quality photos and text on all aspects of the XK120 engine, but takes great pains to fully show all the details of the very earliest engines such as your W1383-8.
But there is a lot more detail to understand, for those of us obsessed with such matters, thus the questions and photo requests.
Basically - and I will talk HEADS first, the very first variant of the XK120 Head was Jaguar Part Number C2242.
There were only 1345 of these C2242 heads made, plus I dare say extra spares and spoils/scrapped heads, but 1345 that actually got allocated and fitted to an XK120, with records saying numbered W1001 to W2483 (with some overlap in W numbers with the second-variant head C2242/1)
These C2242 heads were initially assembled/built-up from aluminium castings as supplied to Jaguar by initially just the WILLIAM MILLS (Birmingham) foundry, but later augmented by supplies of castings from WEST YORKSHIRE FOUNDRY. The head castings from WM are quickly identified by having spark plus No2 and No.3, and also No.4 and No.5 within the same cast recess oval - simplistically referred to as ‘siamesed’. No.1 and No.6 have their own individual recess. On the underside as in your photo the WM castings also have XK774 and C2242 lettering cast. (XK774 is an internal engineering development/project code for the new XK engine and C2242 is of course the allocated Jaguar Part Number for the final machined/assembled head). So your head is unambiguously one of these earliest WM head-castings - so Variant 1 head!
Next - the number A689 as stamped into the top of the head, was allocated and stamped in place by Jaguar at some point during their ‘Head’ subassembly process. Head castings were received in from the WM and later also the WYM foundries, stored briefly, then drawn from store into the Head-Shop. AS head-castings were introduced to the Head Shop, they were stamped with this A689 style number, in numerical order as introduced, thus A689 means yours was the 689th head casting to be introduced (I call this the HEAD SEQUENCE NUMBER - HSN), bearing in mind they could be WM castings or WYF castings in random batches, one-off or batches of ‘experimental’ heads or indeed one or batches of heads for 4-cylinder XJ or XK engines. So the base C.2242 identified casting, was machined up, and assembled, and during process received the HSN stamping, plus other stage stampings including Quality Control inspectors stamps, thus if anything was found defective, or done incorrectly the head at any stage could be reworked or indeed scrapped.

Completed C2242 Part Number heads, that were suitable for Engine Assembly were then allocated their ENGINE NUMBER, initially all heads were the same C2242 and all were destined for XK120 engine allocation, with Engine Numbers starting from W1001 onwards. Now the head at this stage was independent of the block-build and the desired complete engine build, but when the engine number was stamped in it had been allocated to a specific Engine Number build of 7:1cr pistons or 8:1cr pistons, thus W1xxx-7 or W1xxx-8 (for instance was stamped)
Now the very earliest C2242 Heads did not get any Engine No stamping at all, thus the questions/photos of your Engine Number. Yours clearly does have its W1383-8 Engine Number stamped in, which was expected for the 689th head machined/assembled, but sound photographic proof reinforces the research. I have not yet established exactly when Engine Number stampings were introduced, and indeed reliable evidence is so rare/difficult I doubt an exact demarcation will ever now be established 68 years later, unless some obscure internal factory engineering/process document is uncovered.

There is similar logic/process with the BLOCK, albeit reliable proof/data is even less available than for the more readily accessible/visible head.

And indeed for all other engine components.
The CRANKSHAFT as you picture is also the first variant - part number C2239 as was used up until Engine Number W1746 (then superseded by revised crankshaft C2239/1 - same forging, revised machining)
You asked about the ‘massive slug plugs’. page 81 of Urs Schmid’s book provides an excellent photo, and notes “oil sludge holes in the crank webs sealed-off by shrunk-in steel discs”. So that tells you, you will need to carefully cut/drill/pry them out if you want to, albeit I think I would be talking to a competent machine shop about how to replace them with new ‘steel-discs’ before doing so. The six-threaded hole flywheel mounting is a characteristic of a C2239 crankshaft, increasing to 10 threaded holes for C2239/1.

And so on, with many other parts evolving/changing early on - as above, refer to Urs Schmid’s pages 68 to 103 for pictures and details. CAMSHAFTS are another major change, as are cam-bearing housings,

Now this discussion can go off in several different directions, depending on agenda/interest of various people - the various Water-Pump changes is a popular topic.

But, I for one, would love to see photos/details of your INLET MANIFOLD and twin SU H6 carburetters - two more of my detailed research projects.


Hi Roger
Thanks for your very informative reply. I will certainly send you some photos later this morning ( it’s 6.30am here now) when I get to my workshop. The manifold and carbs are very dirty at present so please excuse this!
In the fullness of time, I’ll send you some “ after re work” pics as well
Best Regards
Nigel Boycott