Mk IV compression

Hey All,
I’m an etype/XJ guy and rather infrequent visitor here as i am getting distracted by 2002 BMWs.

Anyway, here in Malaysia a friend is helping another friend with a badly running Jaguar MkIV, 1947. Compression test shows zero on 2 cylinders, so off with the head to find a burnt gasket (good) but then a weird ‘channel’ between 2 cylinders. Can anyone explain what that U shaped channel is?

I would think this is burning the result of continuing to run with the leaking gasket.


That is THE WORST burned path I’ve ever seen between two cylinders. I cannot even conceive how it got this large!

Engine will need to come out to be repaired properly.

The channel appears more like a machined groove rather than a burned path for whatever reasons.
Was it to reduce a 6 to a 4 cylinder for some regulations or tax ? Did the head gasket or the head show any kind of burn through ? The top of the piston appears too clean with little use. Interesting forensic evidence that require more engine inspection including estimated mileage. Definitely rebuild and restore.

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I wonder if the head had been burnt and then machined to take a filler that has since fallen off?


That may explain the very regular and consistent shape of that U: I cannot see any kind of actual use would have burned such a perfect U.

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So if the gasket burned through here and exhaust gases are being blown back and forth between these two cylinders, it would gradually blow away block metal until you get this channel shape. How about the head, does it also have a channel?
I think a good welder could weld that up and then you would rebore and surface it.

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Thanks for the inputs, the head seems undamaged.

As said, on removal of the head, the original gasket burn looked like a normal failure i have seen before on MGs and Triumphs…

Dunno how to quote here…you said it looked like a machined place for a filler…

I also wondered that, but theres no mechanical damage from a ‘D’ shaped lump of steel flying around…

Fair point! Seeking to explain what looks like a very regular passage is the problem. Perhaps someone thought to fill it with something other than metal?? Chemical metal or some other amalgam might have burnt out.


I am trying to find out some history of the car/engine…was it a quick fix for resale or has it spend 30 years in a barn…

I suspect that’s right: in a pinch, someone good with TIG could repair it in situ, then hand-file the block surface flat.

I am inclined to believe this is the original failure presentation, that this is the type of wear you would expect from a long period of rapid hot gas flow alternating in both directions. Take your air hose and blow straight down on sand or loose ground and you will get the same round pattern in the displaced material. In my Fluid Mechanics course it was called the Venturi effect.
Initial cause is more speculation, but a defect in the gasket would be a candidate.


I am sure it’s possible, but should probably be left for the engine shop to do and very likely reline all bores. Both engine shops in Southern Finland that deal with cast iron blocks told me they will not touch a block someone has tried to weld. I am still waiting to get my MKIV block back with the new cast iron liners fitted.


Yes, I should have stated that better, a professionally trained welder, not a beginner.

I had exactly the same problem on my MKIV 3 1/2 litre engine, same cylinders too. My problem appeared to be caused by a cracked head which had been repaired with braze which (of course) didn’t last long!

I had the block repaired (welded) and they re-sleeved the two cylinders. The weld apparently was a lot harder than the block because when they tried to bore the block out for the liner it kept breaking the tips on the cutter. four in total as I recall. The machinist was not a happy bunny. That was 37 years ago and the car still isn’t finished. The engine gets started and run for half an hour or so on sort of regular basis and runs just fine.