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MG Midget and Sprite Technical - another head gasket failure thread,

As Chris H, has replaced his HG and is still having problems over in thread: "Compression - should I worry?", I thought it would be best/fairer to him to post here in a seperate thread.

So Paul New Zealand,
To my comment that my spare engine hg was blowing between 2-3, you asked me "what makes you think it was blowing through?"

Actually I said earlier there were visible signs on the block. In fact I remembered that wrong, and it is more on the head. See picture below.

The block is +30 overbore. The distance between 2-3 is about 5mm. The distance between 1-2, 3-4 is fractionally over 6mm. I'm wondering if this is the cause/contributing factor, on my engine of hg blowby.

The hg was definitely shot, though not completely destroyed between 2-3. There were thin carbon deposits on the surface of both the head and block between 2-3. Can't show you the hg as I dumped it.

A straight edge doesn't show any recess of the head or block, but I guess there could be, if the edge I have isn't accurate enough to reveal it.

And that's also why I am interested in this, copper wire round the fire ring solution, as an alternative to having the head/block skimmed.



Lawrence Slater

Looking at the fire rings you'll notice there's less of an impression on the lower half than the top, suggesting the torque was less over the rocker pillars, and the clamping force was insufficient to maintain a seal. If the rocker assembley had been removed in situ - perhaps to replace a worn shaft, that could account for this failure.
F Pollock

I asked because I have often heard people mistake the black colouration on the block as signs of leakage. I've seen it on lots of engines which haven't been leaking there so I don't think there is any correlation.

Lawrence - have you tried the BK450 gasket? Apart from saving the hour of scraping other types usually need to remove them off the head and block, they seem to work very well. A lot of race cars use them here, no problems.

One thing about compression leakage between adjacent cylinder: remove the plug from one of the suspect cylinders and you clearly hear the compression of the other farting through the gasket if it's leaking.

You can hear no 2 or 3 exhaust valves leaking this way too thanks to the siamese port. To differentiate between valves and gasket, the gasket will be heard on either plug removed (not both at once or there's no compression), whereas the valve will be heard only on one or the other.

Paul Walbran

Lawrence
The headgasket that Chris shows in the other thread has them little (approx 35mm long) metal strengtheners between the cylinders
WELL in my opinion they are trouble with a capital T
When the head is tensioned down it pulls down tight on these little plates which in theory is ok I guess but it leaves a weak spot at the end of the plate and guess where they blow, right at the end of the plate every time
I've had a cow of a run with them on MGB's and now use multi layer shim gaskets with great results---
Cheers Willy
William Revit

Paul,

That is really cool advice on the head gasket vs valve leakage

Thank you...im writting that one down

Prop

Prop

Hi Fergus,
Yup it does look that way I agree. I definitely didn't take off or loosen the rocker posts once the head had been tightened though. I learned that lesson years back.
The engine had done a lot of miles though, easily at least 90thou, before I swapped it for my spare. So it's possible that the head became less clamped on one side I suppose, since I didn't re-tighten the studs at all after the initial re-torque.

Paul,
That's the Black Payen hg isn't it? I haven't tried that before, but I think that's the one everybody says is good, so when I rebuild the engine that's the one I'm going to use too. It would have been a Payen of some type when I built it back in the '80s, as that was all I ever used, but I don't know what number.

I only pulled the engine out because it had done a lot of miles, and was starting to lose power, and I have a spare engine. Apart from the signs of the hg blowing, the rest of the engine is amazingly free of wear. The bores hardly show any lip at the top. I'm pretty sure all that was really wrong was the hg. Still had good oil pressure and didn't smoke - usually.

I say usually, because this engine is one I have asked about here before with regards to a weird oil problem. When cold, in the winter, it would suck oil up the crankcase breather, straight into the carbs. As soon as it did this, if I stopped the engine and waited a minute, the oil drained back, and when I started again it would be ok. I never found out why. Other than that it had no problems.

Anyway, I'll have to have this head converted to unleaded though. The last time the engine was in the car was over 10 years ago and you could still get lrp fuel then. It's a good head, Austin 1300GT so worth doing.

William,
The hg I took off had the plates you talk about. You can see the marks on the head. But given that given the mileage I got out of it before any trouble was prett high, I'm guessing that wasn't the cause of mine blowing. Maybe it was just high mileage. But as it was 2-3 and not elsewhere, I think there must be something else going on too. Maybe I should have re-torqued it once in a whilem, as has been suggested on other threads.

I think I'll try the copper wire trick though.

Lawrence Slater

If its a 1300GT head then check it carefully for cracking in the valve seats, putting hardened seats in will sort out any minor cracks, but big valve heads can be prone to major cracks...

Get the head and block skimmed lightly to straighten it up and if I were you I wouldn't use copper wire just the Payen black gasket.
James B

hmm a thought about Payen gaskets

As I was considering pulling Lara's engine this winter I asked in LeaceyMG's for the black one and a full (conversion) set

They only have the silver ones

I shall be trying Unipart for mine now



or simply leaving the bugger alone and standing for the oil flooding out for another year



not a happy option, I want to see if Peter May's rear seal is part of the problem
Bill 1

This thread was discussed between 02/11/2011 and 04/11/2011

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