H12 EXHAUST MANIFOLD

[Michael Green]

When I purchased my 1928 heavy twelve four of the fixing lugs on the exhaust manifold had been welded on where they had previously broken off, two lugs subsequently cracked through.
I purchased a second hand manifold which had not been repaired and checked both the block and manifold faces for flatness. When fitting this replacement manifold using brass nuts and a box spanner with a tommy bar only about 6" long one of the lugs cracked through at the side of the stud hole. Since then two more have cracked when the vehicle was in use. I understand this is a common problem with this manifold.
As far as I can see the only cause of these fractures must be the casting being subject to a bending moment at this weak position. The bending moment being created by the copper gasket not being the same thickness throughout its length, i.e. the thickness is greater where the copper returns around the exhaust hole.
Can anybody advise if in 1928 when the car was produced the gasket was cut from a flat asbestos sheet which would remain flat when compressed ?
Or any other suggestions.