[xk] Block cleaning

I cleaned my block a couple of years ago. It was packed with all sorts of
build up stuff from years of deposits. With much preparation work, I poured
in muriatic acid. Same stuff used in swimming pools. It is very dangerous
stuff to work with and requires extreme care. It will disolve aluminum so
the head was removed as well as everything else on the block. It was leveled
on the engine stand, filled to the brim with acid, and let sit overnight.
The next day I turned it over, dumped out the “broth”, and revealed an
interior so clean it almost sparkled. Tons of crud as washed out with a high
pressure hose. A sandbox size pile below the stand. A second new radiator
finally fixed the overheating problem, but, the block cleaning sure helped
too. I can give you details of how I plugged the holes if your interested.
This technique was recommended by a professional Jaguar mechanic.
Aloha, Rob

I don’t plan to take the engine apart at this time but the acid bath sounds
like what might done to my spare engine. The original engine runs well and
I’m going to try the “circulation treatment” as a stop gap measure. I just
needs a good cleaning because it went into dry storage, empty, in 1973 and
prior to that had no over heating problems. Had the radiator boiled out and
the shop that did it said it really hadn’t been necessary
Thanks for the tip


A very interesting narrative. I would not have thought of using muriatic
acid on metal as it’s almost exclusively used on masonry, here in New

Happily. my 20K mile XK120 engine is still in the car, doesn’t overheat,
except in midsummer traffic jams (which are infrequent on Martha’s
Vineyard, Mass), you just park it and walk.

I’m sure your Island is much larger and warmer than mine. A very good
tip if I ever have a problem. Our water is very soft, so I doubt I’ll
need to turn the car upside down in my lifetime

Regards, John M.

I have a 3.8l ,1965 block, full of dirt, and rust.
I used chloridric acid to clean, and used a pressure jet. Cleaning is not finished.
My problem is that i wonder where the water goes out toward the water pump (only the one introduced in right side of the car, near carbs) . Where are the holes for the exit water?
Impossible to find them.
Can someone help me please?

Pascal, the XK block features two separate water jackets. There is no communication between them within the block casting itself. The one on the exhaust side of the engine is shallow and contributes little to cooling, acting as a conduit to the water pump through the timing cover (red arrow). The communication between the two water jackets is through the head.

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Is the oil pump body cast iron?

Yes, though the cover is cast aluminum.

Interesting: that must be a 120 thang, that I long forgot about.

Thanks very mutch for the reply.
I understand water goes out there yes, but the question is before, « the two water jacket… » where is the main exit for the water coming from the carbs side ? There is only exit from the exhaust side comin to the big hole with arrow !
Toward the top to the exhaust side? Where are the holes ? impossible to find and clean :frowning::frowning:


The hole with the arrow is where the water flows into the engine from the water pump. It then flows around the cylinders in the block and up into the head through holes in the block deck. It then flows out through the head into the intake manifold and back through the thermostat into the radiator, or if the system is still cold diverts directly back into the water pump.

Just an edit, in my early engine the water can flow from the exhaust side of the block to the intake side of the block between the cylinders.


The water pump supplies coolant exclusively to the exhaust side of the head…there is NO block circulation. I discovered this several years ago while cleaning my block. Even the well-known Jag expert I called was not aware of this.

That’s my understanding as well. The communication between the two water jackets in the block is through the head. It’s as it should be. The head absorbs a great deal of heat so preferentially receives the cooled water from the radiator, which then flows into the larger water jacket in the block then circulated back to the radiator.

If I understand what you are saying the water enters the waterpump, then exits into the exhaust side of the block, up into the head, across the head, into the block and back to the radiator. But, is not the exit to the radiator through the water rail which is attached to the head? How does the water get into and out of the carb side of the block? A nice little diagram with red and blue coolant paths is needed!

I’d appreciate a schematic too. As I understand it, the coolant flows from the exhaust side into the head then down into the big water jacket that surrounds the cylinders, then flows through the head again Into the intake manifold and heater before being returned to the radiator. Circulation of coolant is maximized at the top of the engine. Maybe why that larger water jacket in the block aeventually gets crudded up.


I don’t have my block here at the moment so I am going on memory. I will check when I get the parts to the machine shop. If what you say is true then the coolant entering the block on the intake side must flow down through some of the passages in the head and back up into the head through other ones.


The water pump circulates the coolant from the bottom of the radiator into the exhaust side of the block, where most of it is channeled upwards into the exhaust port cooling passages, then across the head to the intake side, out through the coolant passages in the intake manifold, then through the thermostat housing and back into the top of the radiator. There is very little coolant circulation in the cylinder block, nor is it required. The vast majority of the cooling is for the benefit of the exhaust ports in the cylinder head. The intake ports are cooled by the induction of the sub-freezing fuel-air vapor charge.

The general low or no velocity of the coolant in the lower regions of the block are why the debris settles there. The fluid velocity is too low to transport it. Block cooling is mostly by thermosiphon and splash cooling by the massive amounts of oil spraying off of the spinning crankshaft.


Greetings All.

I’ve used muriatic acid on furnace water coils to clean out chemical deposits.

One thing, to the guy that did the block…what percentage muriatic acid did you use?

But Mike surely thermosiphon is not possible due to the water high up allways being hotter than the low down block water. You might recall that I am using two electric pumps on my engine , one to circulate
the block water, also fitted is a dual water gauge with a head and block senser bulb.
Have not fired up the engine yet but getting close. I suspect that the head / block temp will show quite
a difference, with the block lower than the head, whereas a block temp higher than the head is required for thermosiphon.The result as I see it is that only half the water quantity is doing work
As installed the water from both pumps returns to the rad via the head, but I am considering having
a take off on the inletside of the block returning direct to a second connection on the rad thereby mixing the cooler water from the block with the hot head water., hopefully reducing the temp of water
overall, moreso I feel than pumping all the water through the head. My preference is to keep the XK engine at around 70 degrees, I start to feel uncomfortable when the temp gets into the 80 / 85 range.

If your cooling system is in good nick, 85-90C isn’t an issue, and the engine is more efficient.

Until boiling occurs, nothing bad’s going to happen.

The oil temperature is normally much hotter than the coolant temperature. Since so much of the lower block and cylinder walls see almost constant oil spray, I’d say there’s plenty of temperature differential to establish circulation down there. The critical cooling in an IC engine is always going to be the exhaust ports.