Mk10 header tank questions

I have an ad in the classified looking for an aftermarket 4.2 Mk10 header tank; mine is defunct, as apparently are all the originals. My radiator seems to be ok, and is the style with the tank riding on top, not separate as on later models.
I’ve seen comments about non-original replacements, and have seen photos of such online. Anyone with more specific knowledge of what might be out there? Alas, I’m resigned to fabricating one if needed.

do you have any photos of what you want

There are several different styles of tanks and radiators, with different hosing.

At least 4 variations

I have various original and aftermarket tanks.

After many years of searching, I found the correct original tank for my 420G,
but it has a braze repaired hole, and I have not fitted it yet, as what was on my vehicle since purchase over 20yrs ago is more than satisfactory.

This is a welded SS rectangle shape, I do not know who made these, but I have another that is identical

Since I also have a similar tank to replace, I‘m going to try to shape and solder a replacement from brass. The available ones are rusted thin and through or ~$500 so can’t hurt to have a go (or try), any experiences with making such tanks?

Maybe they can be made to order, too, with a die and press maybe not even that expensive.
Otoh Aluminium tanks from China are cheap and there are a few to choose from, I don’t want one of these though.

Tony, it’s there under the classified section wanted ad, but I’ll post here again. I’m interested in that tank you mention.

As far as manufacturing a replacement, I’m thinking of using aluminum if I have to go that route. Construction parts like filler necks seem more available in aluminum. Here’s what I found from Speedwell motors:

Not every filler neck will accommodate a 7lb cap, which I like to use on older cars. Speedwell also advertises all kinds of other cooling system parts, and there are other suppliers. I can TIG weld, but I’m not that good, so I’m thinking of fabricating the parts and getting a pro to weld it up - shouldn’t be too expensive that way. Again, I’d rather not fabricate if I can find a non-ferrous aftermarket tank that will work. I’d prefer the style that lies on the top of the radiator like my original.

I will post some pics later…I completely forgot about that steel pipe

What a PITA that arrangement is

my late 420G does away with that, so has a different hose arrangement.

Just an ordinary radiator, with an overflow tank

Another area which is problematical in these cars.

I did check, and it is possible to fit a $100 radiator from a Holden Commodore 5.7 V8 VY-VZ with minor mods

here is a pic of my 420G SS tank,

and a spare tank, I do not know its origin,
imo not an original MKX tank as they are somewhat rounded, not square cornered

I have one like you pictured, also a correct original 420G one, my SS one, a spare SS one identical to that pictured, the pictured non-original spare, and another non-original one somewhere

I have some SS pipes that were custom made to replace that thing you have pictured
(I replaced the metal hoses with rubber…420G have a different arrangement)

I would probably sell the spare SS unit or the other non-oem spares.

I do not think I would sell the tank you pictured, as the radiator is very good, and is my best spare, but the top tank doesnt look so well at all…much worse than yours…it is also steel, not brass ?..I will remove and check it, been on my to list for ages.

only now recovering my vigor following health issues

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Ok, thanks. While my top tank looks great from the top, it is badly perforated on the bottom, and is thoroughly corrupt inside when viewed through the hose pipe. I would doubt that your spare original would be worth your time to repair should you need it. I have a good idea how I would make a facsimile of the original, but would be interested in your spare SS pipe unit if you would like to pm me with a photo. As always, thanks for your insights. Tony, it seems that you are the main one in our Jag universe willing to share knowledge about these rare and interesting cars.

I will try and locate it this weekend

There are not many of these cars left, we have a few guys on here, especially @peder who has a few, not sure how many do a lot of work on them personally

I do everything myself, and have had everything apart over the last 20yrs

There is a few things that are unique to these models, and it is difficult to get information

I had a couple of 420G gurus, we had a Jag club member who ran them as wedding cars, a super clever fellow, he taught me a lot,

also a couple of posters on here who have unfortunately been long absent

I try and make available what I have found as it may be useful to someone in the distant future

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Thanks again. I was mentally gearing up for fabrication, but I stopped to think about whether I should convert to the arrangement that you have. While the radiator is a little different, it would not be hard to do. While I like the compactness of my system, the header tank looks smaller than yours. I’m doubtful that it makes any real difference, as the cooling system is large (around 4 gallons), and the car runs cool when I can keep coolant in. Just thinking the problem through before ordering components.

One other question while I am at it. I was poking around in the left front engine compartment and found the screen washer bracket and original wiring for the plastic bottle (found one in my spares that works!). There is a Jaguar burp tank(same as e-types) that you said is non-original blocking the screen washer. Is this unit the correct one for my car?

I looked at a lot of cars on saloon data, and this is the only one that seems to show a similar bottle.

I think it’s the correct one.

not sure Ron…I will refer to my Parts Manual later

I think conversion to the later system makes sense

will have a look at the pipes and hoses, for the reason being;

you would not believe it, seems like the radiator and tank I bought same appearance as yours are not original, but have been replaced sometime in the last 20-30yrs or so lol.

I bought it loose of a guy that told me it was out of a 3.8MKX that he wrecked for a D-type replica (saw it)

The radiator looks perfect, and the reserve tank has a perfect base, with a part number written in chalk!..unfortunately, it has at least one pinhole in the upper surface

therefore I can hopefully work out a way to pipe that up as a future spare for mine

dont know whether I can repair the top tank or not, it would not be ideal.

I would prefer to use the very nice looking rad core, and plumb it up to my 420G style header tank, when needed… think my radiator may be the original to my car

I went ahead and picked up that overflow tank on Ebay. I think it’s original and for $25, I’ll take the chance. If you do run across something in the parts book, let me know please. It is not in my 4.2 parts book under DD491 nor did I find anything under that # online.

I’ll have to spend some time thinking about the radiator and header tank arrangement.

I dont think that plastic item is a Zenith (MKX/420G) item

Not shown in any of my parts books

…and it doesnt make sense …for more than one reason

our systems are pressurised from 7-14PSI…at the header cap

refill is via the header tank, in all of the bewildering number of different arrangements of MKX-420G

My parts book shows the overflow going nowhere, and after the pressure cap

I cannot tell whether the plastic item is built to take a pressure cap ?

finally, almost all suppliers know fk all about these cars, and will blithely tell you something is right for our cars when it is not !

A major brake manafacturer advertised for years on Ebay the front brake discs from an early MKX as fitting a later one…they do not

I even sent them measurements, took them years to stop, probably had nothing to do with me, but returns!..I think you should have the 1/2" discs, not 3/8" (hat height is very different)

dont worry, we will get the header tank situation sorted out

In most cars, the burp tank is not pressurized. It’s simply a recovery tank for the overflow, which as you point out, goes onto the ground on most Jags. The radiator top tank, or header tank in the case of the mk10, has to have the right double gasket cap to allow burped fluid to be sucked back in from the recovery tank. It also has to have a tight coolant system, and the leaky header tank is why my current overflow tank doesn’t work. The only car I know of that has a pressurized recovery tank is the VW vanagon with the unusually cooled waterbox engine, but there may be others. My mk2 radiator stays full to the brim with it’s aftermarket unpressurized recovery tank.

If there wasn’t one car in the archives with a similar tank to the one I bought on ebay, I’d not have risked my $25, but you are likely right. I’m thinking about the options for a header tank. There is plenty of time, as I still have to pull the rear suspension and send all the calipers in for rebuilding. I don’t have room in the shop for the loose rear end until I finish the 3.8 e-type engine rebuild - blah blah, etc, etc. So, I’m just keeping my eye out for stuff as it comes up and planning. I get more shop time as the rains start each fall.

Hi, the cooling system is one thing I havent had a huge amount to do with, and atm, my 420G has a S2 XJ engine in it, so plumbing is not standard

Having said that, I cannot see how the overflow tank is not pressurised,
as it is connected directly to the radiator body with a 1-1/2" hose, and the radiator has no other cap, or opening, wouldnt that mean it must be at the same pressure as the rest of the cooling system ?

My overflow tank has the 38mm pipe, a 3/8" recovery pipe comes in from the inlet manifold and a 3/8" outlet that must pass the recovery tank pressure seal, and is dispersed on the road, via a hole in the inner guard.

I have a vague recollection of someone talking about recovery bottles under the guard

In the link you sent me, I could not see the plastic bottle your referred to ?

$25 wasted is nothing if you own one of these cars

As a catch tank it should have one hose going in through the bottom and one out the top. This thing does look like an expansion tank, but I don’t understand how. And the catch tank has no need for any marks, full/ low.
The XJ S3 had a catch tank ahead of the left front wheel.
I wonder if a tank sealer would work with smaller leaks? Tape over the hole, fill in the sealer and make sure the tubes inside aren’t clogged?
I will go ahead with trying to fabricate one. I‘ll put up pictures in about a year, ha ha.

The recovery tank in question is hard to see on the link I previously sent, located on the left front inner fender well. It’s the only photo that seems to show the one I bought on ebay, and I looked at hundreds.

I think the semantics of the cooling system are confusing, and Jaguar has some extra hoses, the purpose of some of them obscure to me. Let’s call the pressurized tank attached to the radiator by the large hose a header tank. It’s functionally the same as a large top radiator tank on a conventional old car, just remote in location. That describes the original configuration of the mk10 and 420G. The mk10/420G radiator by itself has no header. When your header tank is overfilled, it burps onto the pavement if there is nothing to catch it. With a recovery tank, that tank catches the burped fluid - then the header tank will suck the fluid back into the system when it cools down, provided there are no leaks and a proper double gasket cap is in place. With a stock single gasket cap, there is no provision for reversal of coolant flow, so it won’t work with a recovery tank. So, there is no need for the recovery tank to be pressurized. See the photos of my mk2 (conventional radiator with intrinsic header tank) with a junkyard recovery bottle and double gasket cap - note that the cool radiator is completely full. The recovery tank really only needs a hose coming out the bottom.


I can’t see how your setup, according to the photos, can work. But I admit this is an area I have never had much to do with.

You have the ‘sprung’ cap in your radiator so when the water in the radiator wants to get out it compresses the spring and out it goes into your plastic tank. But when it wants to get that water back, it can’t as it has no way of compressing the spring.

I seem to remember that on Austin cars of the 1960s the cap on the radiator had no spring, just a rubber seal above the expansion pipe, and the sprung cap was on the plastic tank, which was bolted to the inner wing or the bulkhead (firewall) somewhere - I can’t remember whether this was mounted higher than the radiator, which could be important. This allowed free flow of the coolant into and back from the plastic tank, which had the sprung cap as a safety feature in case the pressure got too high. You topped up the plastic tank, not the radiator, and to the correct level, not to the top, which may be significant as it would pressurise the air in the plastic tank as it heated up, then back to normal is it cooled down again.

But it was all a very long time ago & I’ve slept since then so I might be remembering it wrong.

Surely if you’ve got a sprung cap at the top of your radiator, there’s no way back for the coolant?

I think it must be possible to do what Ron is thinking,
as that is the way many vehicles work, with a non pressurised plastic recovery tank, and a pressurised rad cap so the coolant is expelled and drawn back as the coolant expands and contracts

When they overheat, they spew coolant out the plastic lid, so the only benefit I can see is a visual check on the coolant level via the markings on the plastic bottle

I noticed I have a 7psi cap…thought the 420G has a 14lb cap specced, so I will check that, mine does not overheat, buts runs a bit hotter than I would prefer

Ok ok I get it now.
At least on the E types they had different pressure caps, depending on AC but I think they eventually all got the stronger caps. Higher should always be better, no?

The plastic recovery tank on the XJ S3 is much lower than the expansion tank with the sprung cap which should be at the highest point. The coolant gets sucked back through the sprung cap somehow, and the tube for this is between the sprung part and the top seal of the cap. Maybe there’s a valve supposed in there; I lost my original cap!

The S1 XJ has a steel expansion tank with radiator-and water rail bleed entering at the bottom and a pressure cap on the top. The cap is the highest point in the system and it vents to the ground. The tank is likely half full as in the E types and under pressure.