Early- and cracked heads

Paul from Australia would like to know the difference between early Model 18 and Model 19 heads. Further, how to repair a cracked head?

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I am wondering if anybody can throw light on the difference between Model 18 and 19 heads for early (open pushrod) Nortons. It appears to me that the width of the mating surface between head and cylinder got wider on the later Model 19 but not on the Model 18.

On the left, a 1928 Model 18 barrel, and on right un-numbered Model 19 barrel. Both have a narrow mating surface. I suspect that the Model 19 barrel is early

On the left an undated head, with wide mating surface and wide fins around exhaust port. This head has "19" cast into the nearside vertical fin. I presume this to be a late Model 19 head. On the right an undated head, with narrow mating surface. I presume this to be a Model 18 head.

This (presumably) Model 19 head has a crack that makes itself known at about 60 mph.

Further, I am wondering if any of the readers may know the best solution to a split cylinder head. Unfortunately I have found that the head on my 1927 Model 19 (above) is split through from the nearside plug hole right down to the barrel mating surface. When you get up to about 60 mph it starts to fire out the side of the head. I would like to try to repair it. We have a national treasure here who's 84 years old and still does cast iron welding. He started cast iron welding at about 14 years old in the English industrial midlands, and still does a bit when he's up to it. He can have a go at it, but it means grinding out the crack and re-filling. Is this the best way? Can it succeed? Alternatively I could try to get it brazed, which may be easier, but can this succeed? I would appreciate any advice from somebody with experience of these things.