Contraction of pot metal vs brass at low temperatures

[EdgarFB]

Does anyone know what the level of contraction and expansion of pot metal vs brass is?

I have a very stubborn HMV 5A I need to get the back off and I'm considering freezing it as I've had good results with other metal parts in the past.... this will loosen the back IF pot metal contracts more than brass at low temperatures... if its the other way round and brass contracts more than pot metal it will crack the brass so I won't!

[physicist]

This will depend upon the actual composition of both the brass and the pot metal.

The coefficient of linear expansion of various brass alloys ranges between 18.7 and 20.5 μm/m-°C
The most likely alloy used for the pot metal is zamak3 with a coefficient of 27.4 μm/m-°C

I would expect the pot metal to shrink more than the brass on cooling.

[Inigo]

I used to repair this by carefully cutting the front ring with a Dremel, making the cut in the middle between two of the screws. This freed the ring and allowed to disassemble the soundbox without damage. Then, the swelled back must be filed out to is original size, just the outer surface of the ridge which seals the back against the front ring. It's a small quantity only. Then you can assemble back and front, interposing a paper or plastic film strip just under the cutout area, to provide an inner backing, and refill the cut with metal epoxy to weld the ring. The paper or plastic avoids the epoxy to glue where you don't want it to do. After the epoxy is hardened, you can remove the paper.
The circular ridge of the back must be grounded out carefully step by step, to reduce again its expanded circumference, because you want it to recover the original size, fit again in the front ring, and the four screws fit in place without forcing them. You grind a bit all around, assembly and test, grind again, etc, until all fits in place with no tension. It's at this moment when you prepare the backing paper strip, the epoxy, etc, just when you are sure that everything fits in place.
I've repaired successfully two 5a's this way, and they work like a champ.
I'll add some photos later.
I think I talked before about this, and show photos years ago, but they will have been lost...