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MG MGB Technical - Cylinder head hole repair

I have a '74.5 Rubber bumber "B" and I just got the head back from having a machine shop skim it and discovered a slight coolant leak between the head and the head gasket under #4 plug. It didn't look like a crack so I removed the head and found that they had skimmed the head so far as to open a hole in the coolant channel between the 2 coolant holes under the 4th plug. This may sound foolish but I'm not sure why this actually caused coolant to leak out. There was no obvious path to the outside, rather just a hole between these two inlets. Maybe I missed some small contaminant. Hence, coolant was only in contact with the head gasket yet this leakage started almost immediately. Note: I had not yet even added oil or turned over the engine after a complete rebuild and was just adding the coolant. I have looked through the archives but the references are to cracked heads not over-skimmed heads, so I have lots of questions.

1) How do I confirm the head version as Clifton did above (cylinder head casting # 12H2709 I removed from an 18V block)? Is it cast into the head and plainly visible?
2) I measured the head thickness after skimming to be 3.08" with my non-digital calipers. I'll confirm tonight w/ digitals. Is it correct to measure the thickness at the bolt passage holes and what is the minimum allowed thickness?
3) I now have a hole slightly smaller than, and in between, the normal coolant at plug 4. Does anyone have experience fixing something like this or is this a lost cause? I've thought of JB Weld, which might clog the hole and is only good to about 500F. How hot does a head get in this area?
4) Why are the holes so near the plugs, are these particularly hot spots?
5) Slightly unrelated, is it recommended that the head studs be re-newed every time the head is off?

Thanks,
Tom
Tom Gillett

Hi Tom

This can occur on heavily skimmed heads, we have not known one to leak after fitment, we use wellseal around the metal parts of the gasket as an insurance policy. Ensure the stud threads have not bottomed out from too much skimming and not allowing full torque on the head. The Payen/Felpro head gasket with the red sealant beaded along the plug side of the head would be a good idea too.

If you do decide a repair is the only way, you could solder the area and file it down gently.

Unless you are using special posh studs such as ARP I reckon the originals are probably better quality than most aftermarket ones, so use your old ones unless the threaded sections are 'necked'.

Peter
P Burgess

Thanks Peter. I can get another old head but I would like to experiment with this one before abandoning it because it just came from the machinist with new valves and reconditioning. I'd really like to know the temperatures that might be seen in the head near the plugs. Upon inspection I see that my head is a 12H2389 version so this might be the older, thicker head and it should never have been skimmed this much. I guess that much is obvious! I've attached a few pictures, some of poor quality I'm afraid.

Tom Gillett

Hi Tom

I havent a clue as to temperature on a B, we used Devcon for years on special TR4 heads in a similar place and never had enough heat to cause any denaturing of the Devcon. I do prefer solder if you decide to fill. Do remember what I said about sealing using just the gasket with Wellseal, this has always been succesful for us.
How deep are the chambers?

Peter
P Burgess

The temps there will be the same as your water temp. You could drill the hole out so it is round and press or thread a brass plug in with Loctite, then dress the plug off flush with the face. Such plugs are used from the factory to block holes that are left from casting or machining operations. In fact,the competition tuning book has instructions on blocking some extra holes in the head face with aluminum plugs; I prefer brass for such as it stands corrosion better.
There's a good chance that if you had gotten it hot and retorqued the head it would have stopped leaking anyway. Be sure to pay attention to Peter's comment re the stud threads - it is pretty usual to find that the nuts bottom on the thread if the correct (or thicker) heavy washers are not fitted after heavy skimming.

FRM
FR Millmore

This thread was discussed between 20/09/2010 and 22/09/2010

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