With a 2.5 and a 4.5 I'm not sure they needed much else in the way of enlargement. Real anorak stuff now - weren't those engines designed by Philip (not sure) Turner? Who also designed the Triumph Speed Twin bike engine? And if you measure the bore/stroke/distance between bores etc you find that the 2.5 is basically four Speed Twin engines stuck together? Anyway, that's what I was told in the sixties by a bloke who knew him (Barry Ryerson, editor of Motorcycle Weekly, founder of the VES and all-round Good Egg).