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MG MGB Technical - Specification question - head gasket replacement

Hi -

Does anyone know of a milling specification for the cylinder head mating surface on an MGB 18GB engine block? I am replacing the head gasket, and there is some slight warping of the engine block, but I don't know if it's enough to merit milling the block.

(I have already milled the cylinder head itself.)

Thanks in advance!

Joe G
JG Gage

Hi Joe

It is good engineering workshop practice to mill the head and the block when practicable. We flycut ours with a single tip tool which gives us a grippy surface. How much warp and where is the warp?

Peter
P Burgess

Hi Peter -

That's helpful. The head should now be very flat, as I've had it milled. The block cleaned up very nicely, but I appear to have somewhere between .0025 and .003 inches (0.0635 to 0.0762 millimeters) of gap between my straight edge and the top of the block, mostly around the left side (facing from the back of the car to the front) of cylinder #3 (3rd from front).

REALLY hoping to avoid milling the block, as it's not in the budget and I'm just about to move, but I guess I'll have to do what I have to do.

Thanks for your advice - very helpful.

Joe
JG Gage

Joe,
Who milled your head? I've just pulled the motor and trans from my '72 and will doubtless need to have the block decked and hot-tanked and head milled.
Rgds
Andy
East Brunswick, NJ
Andy Taylor

Hi Joe

I think I would try a Payen gasket ( the black one) with something like wellseal on the metal parts of the gasket as an insurance policy. We torque to 54/55# cold and do not retorque.

Best of luck.

Peter
P Burgess

Peter, thanks very much for the info. I have a Payen gasket already, but did not know about Wellseal. I will try to find some.

Andy, I used G.N. Auto in Doylestown, PA (just across the river from me) to mill the head. Greg is the owners name, and he has lots of MG experience, though mostly T series.

Cheers, Joe
JG Gage

Peter,
Thanks for the information.
Rgds
Andy
Andy Taylor

Hi Peter - I've got one more follow-up for you if I can... I be very interested to know why you feel that it's better not to re-torque than to do so.

Thanks again!

Joe Gage
JG Gage

Hi Joe

Some years ago I was worried about customers who came for head fits(engine not psycholgical) and didnt know how to/couldnt retorque heads. I contacted Payen and was informed the gaskets were single torque, problem solved. We have had only one failure in head gasket after single torquing in 23 years and that was on a Race Engine that may have got a tad hot. We torque to 54/55# on a calibrated torque wrench.

If you do decide to retorque, make sure you slacken off a nut then torque it before doing any others.

Peter
P Burgess

Joe-
To give you a specific answer, when using a resin-impregnated cylinder head gasket, the surface finish for both the mating surface of the cast iron cylinder head and that of the deck of the cast iron engine block should be 80 to 100 RA microinches. When using a steel-reinforced cylinder head gasket that combines either a fiber composite or expanded graphite layers, the surface finish should be 60 to 100 RA microinches. If a rubber-faced multilayered steel cylinder head gasket is used, then the surface finish should be 30 RA microinches maximum, but there is no minimum.
Stephen Strange

And just to be morer specific, when the mating surfaces of an aluminum alloy cylinder head, intake manifold, or exhaust manifold are resurfaced for use with a resin gasket, the surface finish should be 50 to 60 RA microinches. When using an annealed copper cylinder head gasket the surface finish should be 60 RA microinches for a cast iron cylinder head and 40-50 RA microinches for an aluminum alloy cylinder head. In all cases, the surface finish should be fairly uniform across the entire face of the cylinder head and deck of the engine block, not varying more than 20% from one area to another. In addition, there should be no more than +/- .001” (.0254mm) of out-of-flat across for 3” (76.2mm) in any direction. Pay particular attention to the areas between the cylinders on the engine block, between the combustion chambers on the cylinder head, and where the cylinder head gasket seats around the cylinders on both of these surfaces, as these are the most highly stressed sealing areas. Any surface flaws that are found should be eliminated by resurfacing.
Stephen Strange

Joe Gage,
Sorry Joe, thank YOU for the info.
Cheers
Andy Taylor

This thread was discussed between 30/06/2010 and 16/07/2010

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