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MG Midget and Sprite Technical - Oh Calamity!

The head gasket has gone yet again. It always breaks between 3 and 4. The head has been skimmed twice, and the latest gasket only lasted a month. Clearly there is a fault in the block deck. That was skimmed when I rebuilt the engine many moons ago. I think the time has come to rebuild the spare engine. It is actually the original so it would be nice to get the car to matching numbers. At the same time I will try to find a better solution for the rear main bearing oil leak. Meanwhile another gasket will go on tomorrow, giving me no more than a month for the spare engine. Wish me luck.
Les Rose

It will be Sod’s law that you will start the build the replacement engine and the next gasket will last forever . That’s classic cars for you
Mike Fairclough

I feel for you - and Sod's Law will always apply.

I find HGs and HGFs so annoying, such ancient technology, so much work for what often is just a bit of paper.
Nigel Atkins

Curious to know where it was skimmed anywhere local?
James Paul

Which gasket have you been using Les? A set of ARP studs and a BK450 from Minispares usually does the job. Run it up without any coolant for two or three minutes then re=torque when cold.
David Smith

The regular place is between 2 and 3. Repeatedly between 3 and 4 suggests perhaps a local heat issue, either in the head or block. Are all water ways completely clear?

As well as David's stud suggestions, which will elliminate this anyway, have you checked your stud threads/nuts for binding? You might have been getting false torque readings.

anamnesis

Payen got took over a while back, not sure if you'll find BK450 stock, not with Mini Spares at least, though others may(?) still have stock.
Nigel Atkins

IIRC Les' engine is a 998 so BK450 isn't the gasket to use as that's for big bore engines.
David Billington

Just realised. This being the engine with the fast idle speed 'mystery'. Could this hg failure be linked to that? When hot, hgf worse, idle speed increase.

anamnesis

Just catching up with this - thanks for suggestions.

The block was skimmed by https://www.saunders-motorworks.co.uk/.

The head was skimmed by https://www.lamb-engineering.com/.

The last 2 gaskets that failed were so-called competition copper ones from Minispares. I have a composition one on now, from Sussex Classic Car.

Head nuts spin on easily, and I usually use assembly lube. In my mis-spent youth I didn't have a torque wrench so just bunged the nuts on (without lube) and guessed the torque. Never had a failure on any of 4 small bore engines.

I am now wondering about hot spots. I cleaned the cooling system recently and no gunge came out, but I should do it again. It appears that Prestone is the best cleaner, but it can't be bought anywhere unless in massive quantities.
Les Rose

I've used old battery acid to clean the water passages when the engine was apart and that worked very well with the passages looking pristine after. I used an old thermostat housing, heater tap, and water pump to seal those apertures and plenty of Vaseline on the head and block surface to prevent local rusting and all thoroughly rinsed afterwards.
David Billington

I can't speak for the particular gaskets you used, but there were std bore MGB copper gaskets with the same problem. On the other hand the 1950cc bore gaskets I've used in my own competition B engine for 20+ years have not had that problem (dangerous thing to say, invites trouble!)

It comes down to the quality of the filler material. Originally asbestos, great material for the job but killed people. When that went out, some of the substitutes weren't up to the job and collapsed when they got too hot. Others didnt. Which makes such gaskets a lottery.

So it could be the gasket. But might not be. It will be interedting to see how the composite one fares. Also to hear others' experiences with these particular copper gaskets.
Paul Walbran

Always a Payen BK450 for me, 1275 with 60 over. I used to use those copper ones and had repeated 2/3 problems.

I've just done the head gasket on mine (because I wanted to change the rockers) and there was just the tiny hint of some possible leakage between two and three. I think that head gasket had done about 10k miles but I do drive my car hard and take the revs beyond 6.

Oh and yes I run ARP studs.
Jeremy Cogman

1275 30thou over. I used a 10 quid head gasket from Sussex on my Sprite. I used one on a std 1275 midget too. No problems with either.
anamnesis

Same as Anam. I use the standard gasket. engine is plus 30 running 10.75cr. I do have 11 studs but they are not ARP. Never had a problem to date
Bob Beaumont

Don't over torque the head studs. It crushes the fire ring too much, spreading the load over the whole gasket face and reducing the contact pressure. The idea is to focus that pressure on the fire ring so it is firmly clamped to create the seal at the fire ring rather than the whole gasket face. Unless you use a solid copper one, that is!
GuyW

I measured the new composite gasket and it is thicker than the copper one. It didn't compress down to the same thickness as the copper as the valve clearances were wider afterwards. I retorqued the nuts cold after about 30 miles. They are standard studs with flanged nuts.

As you say, wait and see. I'm just getting quotes for machining the spare block. Goodness, costs have gone up a lot since last time. £420 + VAT and parts seems excessive for fitting cam bearings.
Les Rose

Les,

IIRC the early 948 cams ran in the cast iron block, only later ones had replaceable bearings. Are they quoting to bore the block to take replaceable bearings?
David Billington

Yes Dave, they are quoting to line bore the block. It's normal practice to fit shell bearings if a hotter cam is used. I had it done on the existing engine and it was about half the cost of cylinder reboring to 998cc. This quote is about 30% more than the cylinders. But I have other quotes that are more reasonable.
Les Rose

More calamity. The composite gasket lasted two weeks. Something is badly wrong with the block. I am now committed to rebuilding the spare engine.
Les Rose

2 suggestions but you may have already covered them. First with repeated skimming you risk running out of threads when you torque up - had this happen once and on the last 1/2 turn the nut was essentially cutting its own thread and giving a false torque reading. Secondly did you chamfer the stud holes in the block and have you checked the threads haven't pulled up?
f pollock

The head had 25cc chambers when I bought it, and it has been skimmed twice. It was warped 0.004" so that is what was taken off. I don't think that's enough to bottom out the threads.

Yes I did countersink the stud holes. These are good points but I don't think they apply here, as the this string of failures is recent, after years of good service. Something has changed.
Les Rose

I have the head off now. The composition gasket stuck to the block so I have had to clean off the debris. The studs have lots of thread left, about 3/16". Checking with a straight edge and bright light I can't see anything wrong with the block in the area between 3 and 4, where the gasket always fails.

This time there was some water lost, about half a litre. But I can't see that the gasket failed near to a water passage. I wonder about 2 options:

1. Grease the gasket to stop it from sticking.
2. Use Steel Seal to reinforce it.

Thoughts?
Les Rose

Burning through is usually due to insufficient clamping allowing the flame to erode the fire ring or if the fire ring overhangs the bore/chamber. I'd suggest you need to remove the head studs to check the threads haven't pulled up. The BK450 and similar composite gaskets are supposed to be sticky - I add a light smear of Wellseal too. You could get your torque wrench checked although you really only need about 45 ft/lbs to seal a head gasket. Can't comment on greasing a gasket or using Steel Seal.
f pollock

That should read block studs - mea culpa
f pollock

I had the studs out today and checked the threads. Made sure all the holes were chamfered. I could not see anything odd about the block surface anywhere.

In my misspent youth I didn't have a torque wrench and just guessed when fitting heads. Didn't have a gasket failure for 40 years. This is a recent problem.
Les Rose

Another long shot here Les, . . . have you checked your valve lift? I wonder, if your cam lobes were worn on the #3 and #4 exhaust valves, could that cause excessive cylinder pressure to build up? Induction still drawing in a charge but exhaust not fully clearing ?
Sort of thing Willy might know about?
GuyW

Sludge in the block accumulates around cylinders 3 and 4. Are you sure the block is clear of sludge here? Try removing the block drain tap or plug and see what is inside. If it appears it hasn't been drilled out it will in fact be sludge build up.
Rob
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This thread was discussed between 10/09/2022 and 28/09/2022

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