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MG MGB Technical - blown head gasket it was

Thanks to all who helped me in trying to figure out the readings I was getting on my 72 gt. tried them all and the final one with the compressed air is what answered my question. When I did the test on #2 with all the plugs removed I could hear air, but not from the intake as I had also removed the intake manifold. It was coming out of the #3 spark plug hole. I knew then it had to be the head gasket. I also made fitting out of a spark plug extension (the ones they use when spark plugs keep fouling or getting wet. I threaded a 1/4" nipple into it then to make sure I welded it up. Went a step further and put a T with pressure gauge and a ball valve so I could control air at fitting. I removed the head and sure enough the head gasket was blown between #2 and #3. Now for more advice from you all. Head, valves and cylinder walls look great. Do you recommend that I have the head re-done (new valves, seats, valve guides and re-surfacing of head? Also what head gasket is the best. I had a copper head gasket from Victoria brit. Any other work that I should do while the head is off? I am ready to get this car on the road again. Thanks again to you all. What wealth of info you gave me. I hope this will help someone out there in mg land.
RP Padilla

Check the valves with kerosene in the combustion chambers to check for seal. if any leaks, lap them by hand. Check the head with straight edge for warps. Replace or install valve stem oil seals on inlets only.
Art Pearse

The Payen gasket is recommended, follow the instructions tothe letter and use a torque wrench as per the manual when pulling the head down. The best thing I ever did here was borrow a surface block and go over the block withfine emery paper. /this showed up "pouting" around the studs where I had blithely heaved on them before, thinking I couldnt hurt a lump of iron like that. I was wrong PS glad to be able to help, i did not suggest a between cylinders fail as your odd readings were at either end of the block, but my car went between 3 and 4. (in the 1970s !)
Stan Best

Hello. Not arguing with your discovery of the head gasket between 2 and 3, but if #2 was zero, why was #3 145PSI and why is #4 50PSI as listed in your original thread? It sounds to me that you have more than one problem. Sorry to be all doom and gloom.

As to what to do next, removing the head and not, at the very least, grinding the valves in is not a good thing to do. Once you have gone to the trouble of removing the head do all you can to eliminate potential future problems.

Tony Oliver


You say "oil seals on inlets only", that would be my first thought . After problems with 2 cracked heads I finally got one that has been rebuilt and it's got seals on the exhaust as well. Should I remove them? I'm suspicious this is just a way of disguising worn guides. What's the general opinion on having them on the exhausts?

R.A Davis

Agree with Tony that this is not a head gasket problem otherwise number 2 and three would have read the same low pressure probably 7o psi. So you have anoyher problem to find before you reassemble and it's not so easy to diagnose with the head off. Check all the valves by supporting the head level on the bench and then pouring kerosene into the combustion chambers and see how much leakage you have into the ports. If leakage is minimal I'm afraid that you have a piston and rings problem but if there is significant leakage you can try regrinding the valves and refitting the head. Whilst you have the head off I'd be inclined to have hardened seats fitted to the exhausts to stop recession and fit seals only to the inlet valves. In addition I'd be inclined to have the head refaced just to be sure then refit with a Payen gasket and torque down as recommended and then keep your fingers crossed.
Iain MacKintosh

I agree with Tony and Iain that the readings you originally posted are not consistent with a simple head gasket failure. But, it may be some form of odd ball head gasket failure and it will be difficult to determine until the engine is reassembled. (I keep wondering about piston rings being part of the problem, or even a bad compression gauge giving erratic readings.)

As to the head, the majority of the ones I have tested or had tested over the last 10 years have been cracked. I would not recommend reinstalling a head without crack testing. There is a tech article on my website,, that covers the dye-penetrant form of crack inspection which is suitable for hobbyist use. Better still would be to have the head inspected by a machine shop.

As to how much work should be done to the cylinder head, that has been a matter of discussion, both in the automotive and aircraft fields, for more than the 40 years I have been associated with them. One group feels that rebuilding the cylinder head, as a stand alone item, is acceptable. The second group notes that engines wear as a system and, when only the cylinder head is rebuilt, you are bringing back the upper end into factory original state, then mating it to the older, worn cylinder bores, piston rings, and connecting rod bearings, often putting more stress on them than they had to cope with before the cylinder head rebuild.

The decision is yours. Were it my car, I would follow the advice of those suggesting a light lapping of the valves to their seats, and new oil seals on the intake valves, then reinstall the cylinder head.

The reason the intake valves get the oil control rings, or umbrella seals, is that there is negative pressure (vacuum) on those valves as the valve is open and the piston is lowering to draw the fuel/air into the cylinder bore. With the exhaust valve, there is positive pressure (over pressure) present because the burned exhaust gasses are being forced out of the cylinder due to the rise of the piston. Hence, in the first case, there is a potential to draw oil into the cylinder while in the second case there is none.

Les Bengtson

So sorry Tony and anyone else that might of been misled, In re reading my original thread I see I goofed up on the firing order. It should have read #1=150,#4=145,#3=50,and #2=0.
RP Padilla

Sorry again for another goof, the readings on original thread were right. After looking up firing order the readings were #1=150 #3=145 #4=50 #2=0. #1 as to book is the one closest to the radiator is this correct? Also you suggested using a PAYEN head gasket. I looked it up to order one from MOSS and they show one for 1975-80-fits all years. My car is a 1972. Would it fit or should I talk to MOSS about it? What about the manifold gasket any one better than the other? I am going to take head to a machine shop and have it completely checked over. Again thanks and sorry for my goof on readings and firing order mix up.
RP Padilla

Now I Know where the goof was on the start of this thread it said that I put air in #2 as I did and said the air was coming out of #3. Wrong wrong it was coming out of #4.I will post some pics as soon as I can of head gasket and tops of pistons. Thanks again for bearing with me.
RP Padilla

No reason to take compression tests in firing order, take them 1, 2, 3, 4 like everybody else does and save the confusion. throttle blocked open.

If air is coming out 2 cylinders away, it is running through one of the manifolds to get there, so pointing at valve problems as the original readings showed. This would normally be the intake manifold with throttles closed, or with SU carbs and throttles open even. Exhaust will do the same to a much lesser extent, unless mousie has blocked the exhaust system.

Rings rarely if ever give zero comp readings; holes in pistons or open valves do.

I have seen head gaskets where the gasket is blown leaving a loose piece that acts as a check valve, giving more compression in one of a pair of adjacent cylinders, but it is rare.

The major wear point in any IC engine is valve seats. With older seat/valve materials, a well maintained engine can easily go through 2 or 3 head rebuilds before attention to the bottom end is required. Driving without oil/filter changes and good air filters might invalidate this statement.

FR Millmore

The Payen head gasket, indeed, fits all B model years. The intake/exhaust manifold gaskets are all pretty much the same. I run a stock one, on my supercharged engine, and it has held up very well for 8 years now. RAY
rjm RAY

This thread was discussed between 10/03/2011 and 11/03/2011

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