Citroen XM Forum

You will find the head gasket is responsible for metering coolant flow, this port may be open on one block and restricted on another but i would guess the head gasket hole is the same.
In short, it doesn't make any difference and the head gasket will create the desired restriction.

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Thanks Dean for confirming that. That's a relief! I was thinking those pin style holes were critically important. Almost like creating a jet of coolant. Maybe they are, but as you say the headgasket doesn't restrict them and neither the head. Curious as to why those three were designed to have such small holes, where as all the other coolant passages are much larger.

Presumably to create an adequately high coolant flow at the other head/block ports, and to avoid an "easy path"/short circuit. Have seen this before on non-PSA engines.

Make sure they are part of the casting though. The reason is even flow, if all the holes were the same size all the way through the engine it would change temps very quickly when the stat opens but also parts of the block would have high coolant flow and other parts would have very low flow or even stagnation, by creating this obstruction you increase pressure upstream of the gasket (downstream of the water pump) and this ensures even flow through the whole block.

^^^^^what Chris said

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Just give all the coolant passages a scrape out with a small flatbladed screwdriver then they will perform as intended.

Dean wrote:Make sure they are part of the casting though. The reason is even flow, if all the holes were the same size all the way through the engine it would change temps very quickly when the stat opens but also parts of the block would have high coolant flow and other parts would have very low flow or even stagnation, by creating this obstruction you increase pressure upstream of the gasket (downstream of the water pump) and this ensures even flow through the whole block.

^^^^^what Chris said
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They are definitely part of the casting and not corrosion built up. They are meant to have very small holes compared to the other coolant holes. There's only three of them, but as you and Chris say Dean, they must be about controlling coolant flow then.
The headgasket really does reduce the flow on all the other passageways, I had a good look....apart from these three, where the block does it (before th HG) rather than the headgasket.

I suppose it's worked okay so far..... so hopefully it will continue too.

Dieselman wrote:Just give all the coolant passages a scrape out with a small flatbladed screwdriver then they will perform as intended.

Excellent. I have been doing that.

That's it then, the coolant metering is in the block, not the head gasket. Hows it coming along anyway?

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Just don't do what I did back in my teens, when I decoked a friend's 6cyl Cresta engine... Used a 1/4" round file to clean out the waterways in the head, and didn't notice that a small piece of the file had broken off, and disappeared into the head waterjacket. As the head was put back on the block, the 1" of file end must have dropped out, unnoticed. Next day, when engine was started, it punched a hole in a piston. Live and learn...

Dean wrote:That's it then, the coolant metering is in the block, not the head gasket. Hows it coming along anyway?

D

Yes. I would upload a pic, but the forum or my iPad has decided it doesn't want too!
It's doing just that really coming along. Had a slow week last week and didn't get much done due to others things cropping up.
So the head is off. As well as most things that need to be.the gasket had been changed to an unknown MLS type one. Trouble is it's managed to take chunks of head with it when I removed it. Anyway, that's not a problem as I'm not using it.
I have been cleaning up all the different gasket facings. Lots of grey sealant has been used. Worked at preventing leaks, but blimey it's everywhere. The thermostat was sealed in, then the o ring on top then more sealant for good measure. Great!
Unless of course this is normal.... doubt it somehow.

Strange anomaly. The coolant screw at the back of the block is ...well ....plugged. By that I mean I have opened it so many times trying to drain coolant via it and not a drop, being quite confused by this, I stuck a screw driver up it and it met solid metal. It appears there is no drain hole. It's not brown rust, it's grey like the block. Almost like it never got drilled out when it was made. Very odd!

I do have more concerns. I know, I know! !... number 4 piston face, I.e the outside one on cambelt side, is well errrr... pitted and quite sooty. I will try and upload a pic. All the others are great as you would expect. This one.. not so much. All the pistons move laterally in the bores by the same amount, but by a teeny tiny amount. I would expect this on cold pistons. I think. My reasoning is they expand when hot. Anyway it's concerned me! Piston rings?.... hmmm maybe. But it's a 137k engine. It will have some wear. Bores are all wonderfully smooth.

But apart from that and the fact this poor engines coolant has been clearly neglected before I owned it, all is ready to reassemble I think, barring those issues making it not a good idea.....

Doubtful whether gaskets are ever used to seriously regulate gas or fluid flow: the wear on their exposed edges has got to be suspect.