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MG MGB Technical - Cracked Head??

Number 3 plug is sooty black. Misfires at lower rpm's.

Has had current valve job. Replaced all ignition components
trying to cure problem. When I drained my Antifreeze I drained
slightly over a gallon. How much should have been there? Head casting
number is cam1106. I may have jumped the gun, I already pulled the
head. Compression was good on all cylinders. But I did not perform a leakdown test.
MBF Frazier

MBF,

According to the Haynes manual, the cooling system capacity is 11.4, 13.8, 12, or 14.4 U.S. pints, depending upon year.

Since the head is off, take it to a machine shop and have it pressure tested and magnafluxed.

Charley
C R Huff

Charley,
Got the head with me today, machine shop said they could pressure check
it for $45. That seemed reasonable.
MBF

Good Luck. Hope you find it is well.

Charley
C R Huff

MBF, what was the result of the shop test? I started a thread with similar issue. I haven't narrowed it down yet.
kids1

Heard back today from the machine shop. They could find no leaks.

He did say that a hairline crack may only leak after the engine is warm, so it still may be cracked.

What to do next? I only drained about 10 pints of coolant so it is going somewhere.

kids1, what all have you tried so far for testing?
MBF

Did they magnaflux it? That should pick up any cracks.

The Wiz

No, they said a pressure check would find a smaller crack than magnaflux.
MBF

Magnaflux will not find cracks in a Head. Try Dye Penetrant
Geoff F.
Geoff Farthing

Before taking it for pressure testing I did a dye test. Could not find anything. I think the leak may be inside the head. But then the pressure test should have found that. I am puzzled.
MBF

I know when I was getting the same symptoms as you a magnaflux picked up a very slim crack through the #3 exhaust valve seat. There was nothing visible on the outside of a head but after bead blasting it is possible to see the crack with a magnifying glass.

The Wiz

The wiz,
So you had the same problem. Sooty #3? I feel certain even though they
didnt find anything it is cracked.

I found another head. Took it to see what they find with it. Did a new head take care of your problem?
MBF

I threw in a complete new engine from an early car, I wanted the higher compression ratio.

However, I have just put a big valve head on the block that had the cracked head and dropped it in my GT, it should be running again this evening. I'm not expecting any problems though, the issue was obviously the crack.


The Wiz

You say your symptoms were identical though when you found the crack?
MBF

Yes, plus it was down on power and sometimes it was just reluctant to start.
The Wiz

Wiz,
You get your GT fired up yesterday?
MBF

Nah, just got to connect the cables and plumbing though, it will run tonight.
The Wiz

I am at a loss. The machine shop did not find a crack in my suspected head. But the one I took as a replacement, was cracked. Should I put the original back on. May have been a head gasket. The gasket did show where water had been under it....and above it. (block side and head side looked burned around outer edge of head gasket)
MBF

All you have to lose by trying the original again is time and a head gasket, I would just make sure it is flat ans throw it back on and see what happens.

The Wiz

Looks like that is what I will be doing. I ashamingly also will admit I did not use a payen gasket last time but a composite generic gasket.
MBF

MBF,

From what I understand, cracks in MGB heads are pretty common, so I'm not surprised that the second one you took to the machine shop showed up cracked.

However, if your original one did not test as cracked, I would be inclined to put it into shape, and put it back on with a quality gasket.

Before you put it back on, I would at least check it with a straight edge and feeler gauge to see that it is not warped, and have it cut if it is.

Also, I never quite understood why the black sooty plug you mentioned is an indication of a cracked head. To me it sounds more like over fueling or oil fouling than a cracked head.

If worst comes to worst, I am the proud owner of a new-in-the box pre-smog head that I would sell if you need it. However, proud is a euphemism for expensive. Since I own three MGBs myself I am not that anxious to sell it, so I would want similar $ to what you would pay for a readily available new head for a modern car.

Charley
C R Huff

I fired the GT up last night, it's running beautifully, the new head solved all the engine's ills.

The Wiz

Charley,
Fouling because of the head was the only possibility I could come up with when this was posted. I had eliminated fuel and ignitions problems. A valve job had recently been done so that was just a next step to check. (learning as I go, I should have done a leak down test first)

Since the head has checked out good they did surface it and it is ready to go back on.

2 more possibilities have been thrown at me now though. While the engine is tore down this far I was told I should go ahead and check the rings on that cylinder. (however the car does not smoke or burn oil)


The other possibility is a bad lobe on the camshaft. (the camshaft has less than 5k on it though so I dont think that is it)


I guess a little more exploratory surgery is in order before reassembly.
Thanks for all the support. Any and all suggestions are appreciated.

MBF

MBF,

When the new cam was installed, were new cam followers installed, and were the lube and break in procedures followed?

I have been told that cams can have a rather short life span if this is not all done correctly. My 68 GT came with a new cam (~2000 miles) and it was already scored. The rebuilder had not installed new followers.

Charley
C R Huff

Charley,
Yes, new cam, lifters, and cam bearings. I dont really think the problem is in the cam. I was just passing on what others had suggested. They were saying a flaw in manufacturning. Either opening the exhaust too soon or holding it open too long. If it still does this when put back together i guess that would be the next check with a degree wheel.


kids1,
If I ever figure this out I will put a follow up post on here. If you find the fix before me, please let me know what you find.


Thanks all!
MBF

MBF Fraze: My 79 MGB had a cracked head right between 2 & 3 cylinders. It cracked through to the valve guide. MGB cracked heads are sadly quite common partly due to the fact the exhaust valves are siamesed between 2 & 3 thus creating a hot spot. In the end, I replaced the head with an alloy head at great cost, but it is about as bullet-proof as you can make it.

Let us know how this adventure plays out.

cheers

Gary :>{D
79 MGB
gnhansen

Head cracks on an anally prepped and flowed CI head ala Burgess are a heartbreak. I spent 300 to have a guy heat the head in the kiln to 1100 * then weld the crack. This head BVFR from Peter 5 yeears ago is now back on the car with a P 285. SWweeet sweet. An alum head as sold by On The Road Again, or Pierce , Morgan Hill and Gilroy ( resp) california will really be a step up if flowed and cvut to about 10:1 ACR. A distant second: I have recently hear Alumiseal or HD Bars leak can do the trick for a while or a long time. Luck.. Vem
vem myers

This thread was discussed between 25/08/2008 and 06/09/2008

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