Tiny hole in coolant expansion tank - repairable?

After replacing all the coolant hoses, removing the expansion tank so I could give it a fresh coat of paint as it was looking a bit rough, I find there is a tiny hole in the engine facing side of it, which leaks very slowly. I’m guessing it is original to the car and has rusted from the inside out ( I probably sanded the bit of rust that was keeping it intact !)
My question is : is it worth repairing? How would people repair it? Or should I just buy a new one (£150). I’d rather repair it for now until I can be sure that this car is all fixable.
I’ve seen various types of tape that seem pretty durable, waterproof, temp resistant etc.
Many thanks for any thoughts

I might try using a small stainless steel self-tapping screw. Screw it in, back it out a bit and apply some thread sealer or silicone to exposed threads and under screw head. Screw it back in.
The system is pressurized but generally less than 15 lbs. So whatever fix you come up with, account for the pressure factor

You could braze it up … and the next one too … etc.

I brazed up 3-4 pin holes, put it back in, brazed up another 2-3 pin holes and 1 small crack/split, put ir back in again, … that was probably 10-12 years ago, no leaks since then. I got lucky.

Usually, when pin holes rust through, there are many more not far behind getting ready to show up too.

I’ll be removing mine in a few weeks to give it a good look over and test. I’m expecting results to be ‘past time to replace it’, then order a new replacement for it.

Over the decades I’ve had great luck patching things up with JB Weld.

In this case, since a temporary repair is what you’re after at the moment, I’d conjure up a small patch to cover the hole…a bit of thin sheet metal, some plastic…whatever…and glue it over the hole with the JB Weld, generously slathered. Let it dry overnight.

If things work out well with the car plan on replacing the tank. I can all but promise you that it’ll spring other leaks.


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original expansion tanks for MKX/420G are NLA and rare as hens teeth

It took me 10 years to find an acceptable one, and it has been brazed

I am thinking that coating these tanks internally and externally with POR 15 might be worthwhile, as 100% they rust from the inside ?

It may not be so much of an issue with XJS as you can evidently buy new

It seems the steel is sacrificial in the electrolytic coolant process with alloy

Someone made up a batch of SS tanks, but they are not the correct shape (and not sacrificial which can be bad, as some other metal now is)

For the earlier vehicles, the tanks are different, and in my opinion, you would have no other option but to fabricate one

Do like the do in water hearters - put in a sacrificial anode rod.

I’ve also thought of thoroughly ‘rinsing’ the inside of the tank with a rust converter, followed by a coating of fiberglass resin covering all of the inside of the tank, but not sure if that would actually cover all of the inside, or if it is even a good solution?

A thn lyer around the hole. Ae, mottre are on the way.

Decades ago, faced a similar issue with an old Ford engine sump. About an inch of sandy oil covered the bottom. when i cleaned it, I saw a lot of tiny holes. Oh me, model b tsumps are and were rare O decided to braze. I ended up with the entire floor covered in brass. It looked rather good. Ugh, i did not finnish tht engine, veered course and went to a flat head V8.

Braze or solder oughta work fr the expansion taank.


I considered doing that to the side. I also considered cutting the steel side out to the radiused edges, then braze or solder a brass sheet new side (with an “X” stiffener on the inside) to the radiused edges. I would probably use solder in that case.

Easy answer, no. I had one that had a pinhole. If I tapped on the outside with a rubber mallet, rust would break free inside. I replaced it, then cut open the old one. The whole bottom of the tank was rusted, and I could dislodge sheets of rust. If there’s one pinhole, there are more just waiting to break thru.

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I know it may look unprofessional but square inch of black gorilla tape will resolve the issue. Confirmed lifespan of this fix = 2+ years (easy to re-apply, just saying…)

Duct tape is not for everything … most things, okay, sure, but …

For basically everything else …


I’ve got some T-Rex waterproof rubber tape that is meant to be good up to 200 F, so that might work ok.
I was thinking tho of just getting a replacement, as I’m sure it’s really rusty inside. I’ve seen aluminium tanks for very little money

It doesn’t have a connector for the top hose but I could put one in.
Is there something special about the original tank, or would another work just as well, as long as the hose connections are in similar positions. The original has a very long neck up to the pressure cap. Is there a reason for this ? Space is a bit tight also

Neil. Sizing is based on enabling fluid expansion for a 12 cylinder engine. A smaller “expansion” tank may not provide adequate expansion space before venting over into the atmospheric tank behind the wheel arch.

JimD . . .
Try this little thought experiment: start with a cold engine, cold coolant, and an expansion tank filled with coolant. Start the engine; as it generates heat, the coolant warms and expands. Any expansion of the coolant will cause excess coolant to vent into the atmospheric tank, regardless of the size of the expansion tank. When the engine is shut off the coolant gradually reduces in volume (can I say “de-expands”?) as it cools and causes excess coolant to be drawn back into the expansion tank from the atmospheric tank, provided your cooling system is properly sealed, thus keeping the expansion tank full. Thus it is the atmospheric tank that should be sized to accommodate the expected coolant expansion.

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I agree 100 percent Ron. Pre-supposing the hose to the atmos tank isn’t clogged or kinked, and the expansion tank cap has a good seal so that it maintains vacuum. And that is for when pressure exceeds the cap rating, no?

Just saying to consider trying to use a replacement tank of same (or close) volume

The expansion/header tank I was looking at held 2.5 litres. I don’t know what the volume of the original tank for 1983 car is, but I don’t think it’s huge. I just wondered if a tank similar to above would technically work ok (assuming it fits in engine bay) , assuming it’s got the pressure cap with relief hose to ART, and the other hose connections in a suitable position. Also being aluminium less of a rust issue. Why does the original have such a long neck ?

The whole idea of the header tank is to help get rid of any air bubbles in the cooling system. The long neck (IMHO) is an efficient collection point so that as the coolant expands with heat, the air is pushed out into the expansion tank, and as the engine cools, water is drawn back in.

Outside Box thought- Anybody try a Ford FE Exp. Tank?? (IE. 352-390, to about 1966) Brass, surly big enough, Cold War era thick.- inlet was built to mount to block- so heavy flange to work with- bolt “to block” gaskets cost maybe a buck, Outlet was hose to rad. Takes a standard Rad Cap. Are available new- think a T Bird supplier, otherwise about a million produced…(believe that # includes a generous safety factor…)

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That is exactly the header tank that I used on my JeeType, and it’s worked perfectly.

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Does anyone know if these different versions of the header tank are the same design and size (particularly the holes that mount to the car). I know the later models have a hole for a coolant sensor but I could plug that.
Expansion Tank XJS from (v) 105048 to (v) 115815
Expansion Tank 5.3L XJS from (v) 115816 to (v) 147268
Expansion Tank XJS VIN range: From 147268 to 226645