H8 carb accessories

Hi All
I am still mulling over the upspec to H8s as I do not have any carbs with the car - only the original C Type Head and block.
Moving to the inlet manifold - I understand the manifold is the same casting/ part Number for the 2" H8 at C7462 as for the 13/4". XK140 Explored says the H8 manifold becomes C8479 when bored out but no new number is showing.

The manifold inlet holes must be 2" I presume but are the internal voids bigger too. I have been told that this dia at the internal web should be 1".
Is this a machinable modification to a standard manifold? Anyone gone into the details of the manifold spec for H8s?

Love to hear from you



Hi Goeff pics are of genuine H8 manifold. 2inch carb side 1 1/2 inch head side JOHN

Hello John

Many thanks for these photos - great quality. Your manifold looks in great condition too.

I can just make out the end of the C8479 number, amongst others, stamped in the left side proving it is a genuine part.

Lord knows if I will ever find one!

Do yo have a view on whether the internal cavities are different to the 1 3/4" manifold C7462?

Good to chat


Hi Goeff My manifold does have the part no stamped on it.As this is the only one i have i can not compare it to a C7462 ,JOHN

Hi John

Thanks again. If the manifold is still off your car perhaps you could peer into the centre internal area where there is a casting bulkhead. The hole in the bulkhead should be about 1" dia.

Hi Goeff that hole is the same size JOHN

The internal hole in my original H8 manifold is 1 1/8 inch. You will also need a number of other changes or modifications to meet the full H8 specification, but they are not all needed to make the car run with H8s.

Hello Bob - Thanks for this confirmation I had felt that the hole would be at least an inch. Yes there are a number of other mods to do. This process will not be quick I am sure.

Hi John - Thanks but do you mean same size as standard inlet manifold?

I measured the ā€˜balanceā€™ hole between the two carb chambers of a standard XK140 manifold. It is only 5/8 inch in diameter.

Hey Mike
Thanks for this it confirms what I had recently learnt. There is a difference between the two manifolds inner voids.
I will now have to embark on a search for an original item rather than set about getting a standard one machined.
This is the great thing about the forum. I learn much more than the parts catalogue reveals.
Once again many thanks.

Geoff, I think a standard XK140 manifold can be bored fairly easily with the right tools. The 5/8 inch ā€˜balanceā€™ hole is directly below and between the two central (round) ports on the reverse side of the manifold. Thereā€™s plenty of material to allow the hole to be enlarged to 1 inch or greater. Likewise, thereā€™s plenty of excess material to bore the carb holes on the front of the manifold to 2 inches in diameter.

Some years ago, I thought about converting my XK120 to H8 carbs. In several years of searching, I only found one original H8 manifold for sale. It was priced at nearly $2,000.

Hi Mike
That is interesting. I guess I am not over confident that anyone would part with a genuine H8 inlet manifold.
Maybe the only option will be to modify the standard manifold.
They can be obtained here in the uk for between Ā£300 to Ā£500. Your item price would allow some room for the machining costs.
I would prefer the genuine part of course but time pressure would start to bear ultimately.

I will put out a request search for one and see if anything comes up.

Thanks for your time on this topic.
Speak again no doubt

Perhaps I am mistaken, but my understanding was that the original inlet manifolds were machined to the H8 specification. That is why they display the same part number. In addition, all these alterations are completely invisible. You are probably better off trying to find the unobtainable coil mount which would be mounted front and center above the carbs.


Please appreciate that when I co-authored XK140 EXPLORED back in 2009/2010 the book was intended as a comprehensive originality reference, and not a workshop/engineering manual detailing design and manufacturing as many people now seem to expect. Accordingly when I did all the detail on Plate 5-a1 I deliberately chose and took two only photos intended to show the identification details of original XK140 Inlet Manifolds - so when Viart turned my photographs into line drawings, that allowed me to fully mark them up and detail the key originality identification features as per books intent, and not go into the engineering detail of an H6 manifold versus an H8 manifold as you are now seekingā€¦ Actually I have fully resercah 100% of what is involved in turning a factory equipped XK140 with H6 carburetters, into what the factory did to produce a factory equipped XK140 with special option H8 carburetters, and there is a lot lot more to this than just the Inlet Manifold. One day I may publish it allā€¦
Re the Inlet Manifold only - the first thing you must appreciate is that Jaguar purchased in Inlet Manifold raw castings from two different English foundries, that the XK140 Inlet Manifold was shared with the Mark VIIM, being a significant efficiency and cooling capability improvement over the Inlet Manifold as shared by the last of the XK120 and Mark VII - period test results indicate a net 10 bhp improvement and a significant part of XK140s improved cooling system (in conjunction with other upgrades). Both these foundries would have been contracted to provide castings to suit a Jaguar Engineering Drawing for Part No C.7462, which would have comprehensively detailed required final dimensions of the castingā€¦ and typically this would include a requirement to identify the finished casting with the actual intended final Part Number C.7462, noting just the bare raw casting is not yet an actual C.7462 Inlet Manifold until it was machined up/and partially assembled to full C.7462 specifications by Jaguars own internal machine shop.
In XK140 EXPLORED I advise that the cast in C.7462 part number can be found in three different locations. What I dont say is that there were at least five different castings produced over the 1954 to 1957 period by the two different foundries - WILLIAM MILLS and WEST YORKSHIRE FOUNDRY, with those produced by William Mills all exhibiting also a cast in WM6034 on their castings - being an internal William Mills Part/Project No of no relevance to Jaguar, but a very obvious identifier for us, with West Yorkshire Foundry castings have a very subtle, often difficult to see/read WYF logo adjacent to the C.7462 part number underneath the thermostat housing.
Of major relevance to your question is the detail I have shown in Plate 5-a1, but although mentioned in a note at lower-left of page, I did not elaborate. But there are three combinations of the perimeter shape of the carburetter mounting pad

  1. All sides straight, as illustrated in the lower photo/now drawing
  2. Two sides and bottom concave and the top straight - as illustrated in the middle photo/now drawing.
  3. Top and bottom straight and the two sides concave - not illustrated.

Now for C.7462 castings that were initially intended to suit, and always primarily suit being machined up into an H6 C.7462 Inlet Manifold then all inlet manifolds from both foundries and of all three carburetter mounting pads remained suitable when the 1-3/4" hole was bored out, leaving enough meat to the adjacent mounting pad edge, regardless of whether straight or concave - to ensure a sound/robust surface for complete sealing of an 1-3/4" hole gasket, and insulator black and second gasket. On the very very few rare occasions when Jaguar wanted to supply special option H8 carburetters, the same C.7462 Inlet Manifold casting was used, but this time the raw casting was machined up by Jaguar to become a C.8479 Part Number - and the relative difference of the C.7462 and C.8479 Part Numbers tells you that the C.8479 came significantly later. I speculate that when the C.8479 was first designed and prototype manufacture, they realised that all the earlier C.7462 castings with Concave sides, became marginal and unacceptable to have a 2" hole bored in, with minimal sealing edge for the 2" hole gaskets and insulator blocks, thus presumably a revision to the casting drawing, stipulating that all four sides should now be straight, and no longer concaveā€¦
So of the 20 + original factory C.8479 Inlet Manifolds I have seen first hand, or viewed photos off, ALL have had platforms with four straight sides. The only ones I have seen with two or three concave sides, on closer examination, have all been home/aftermarket modifications of factory H6 inlet manifolds. So thatā€™s the first MAJOR thing you need to be aware off - if you want to modify a factory H6 Inlet Manifold into a reproduction H8 inlet manifold, make sure you start with one that has all four sides of the mounting platform straight. Both WM and WYF made such castingsā€¦
See below photoā€¦

The top manifold is a William Mills C7462 casting, machined up by Jaguar to C7462 Part Number H6 specifications.
The middle manifold is a West Yorkshire Foundry C7462 casting, originally machined up by Jaguar to C7462 Part Number specs, that about ten years ago I modified in Australia, to bring it up to an exact replica of C8479 Part Number specs, and this was done by close examination and exact copying of a friends factory original WYF H8 Inlet Manifoldā€¦
The bottom photo is of a William Mills C7462 casting, that was originally machined up by Jaguar to C8479 H8 specifications, as supplied in a new XK140MC fitted with special option H8 carburetters delivered new to Australia. I obtained this factory H8 inlet manifold a matter of weeks only, after completing the full exact reproduction middle pic manifoldā€¦ Note this factory H8 inlet manifold DOES NOT have C8479 stamped on it anywhere, indeed every factory original H8 inlet manifold that I have closely examined, similarly has no C8479 markings, albeit I cannot categorically exclude the possibility that some factory H8 Inlet Manifolds may indeed be so stamped C8479, albeit in other casting reidentifications, usual Jaguar practice was to grind off/overstamp the cast in casting number, in this case the C7462. But who knows for sureā€¦
Now whatā€™s involved in turning a C7462 machined up H6 inlet manifold into a replica of a factory C8479 H8 Inlet Manifold is four major stepsā€¦ the first one is to find/select a suitable four-straight-side C7462 machined up H6 inlet manifold in sound/non-corroded conditionā€¦ Then there are THREE distinct modification steps/processes that all three must be done properly if you have any desire than your final H8 installation will work/perform as Jaguars Engineers intended in periodā€¦
These days, the number of XK140s now fitted with both period/aftermarket or second-hand H8 carburetters or more commonly now reproduction H8 carburetters (as made from at least three different suppliers) dominate, but being mostly show cars, are upgraded to varying degrees of completeness, accuracy and successā€¦ as is always the case with reproduction partsā€¦


There was a discussion a couple of weeks ago about converting to H8s, and Roger P explained very comprehensively how it was not a good idea unless you were only interested in racing, not street driving.


Out of curiosity, do any XK140 owners with original H8 setups find there are drivability issues and if so, are there steps you have taken to successfully address those problems? When Joe Curto rebuilt my H8s (which have still never run outside my garage), he suggested leaner needles might be an option.

Hi BOB All i have done with mine was fit new kits.Since then only the odd ballancing and mixture tweek ,which goes with any SUs.Float level is critical.Theres only been highway cruising and around town and they have operated with no vices at all hot or cold.Always a conversation point when the bonnets up Havnt been game to check fuel consumption but i know its up thereI have the origonal gauze AC filters which i felt are inadequat for a dusty environment ,so replaced them with the sports filters which i feel also look the part JOHN

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Bob, hereā€™s the story of 660702, an XK120 that received all the latest speed modifications in 1956. This included H8 carbs, special intake manifold, C-Type head, changes to axel ratios etc. The owners of the car kept meticulous records on all the modifications and maintenance issues. There donā€™t seem to be any regrets about the H8 carbs.


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Hello Roger

Straight away may I say a big thank you for your intervention into my little discussion topic. Your comprehensive and learned summary of the manifolds gives me the detail I was looking for. The book you have edited with Bernard Viart is an impressive publication and acquiring it in 2011, some 20 years into 140 ownership, I soon realised it would be the light I needed to shine on my full restoration process.

I am still reading your text to fully absorb the comments and will post any response I have in the near future.

Once again a big thank you