I managed to overtighten and snap the union on my AVL hybrid leaving the bottom part stuck in the cases, though I was able to unscrew it with an improvised stud extractor made from a 5mm Allen key. The thread in the crankcase was 5/16" British Cycle Thread rather than 8mm, but there seem to be different threads for the top half of the union depending on the model. The broken union has some odd 7/16" thread, but the replacement supplied from Hitchcock's has 1/8" BSP (British Standard Pipe Thread, parallel, not tapered) which looks identical to the union in your photo.
Any help?
A.