Open & shut case
“Question for you,” said ‘Cuppa Tea’ Steve at Kempton Park. “Do crankcase halves have to be a pair or can you use mismatched ones?”
Ideally they should be a pair – but I’ve got away with it before. When you think of how crankcases are machined, there are jobs that can be done with the cases assembled and jobs that can’t. Taking a pair of pre-unit Triumph cases, for example, the cam shaft bushings were likely line-reamed – a pilot fitted into one side guiding the reamer concentrically through the other – and the cylinder to crankcase joint face will have been milled with the cases joined. But the main bearing bores and the crankcase joint face are machined from inside separated cases, so realistically any two should be machined accurately enough to fit together. You can check this using a mandrel (see ‘How To, above’) but to match two Triumph cases you may only need to fit and line-ream new cam bushes and maybe true-up the crankcase mouth with an oilstone, so: “Yes, Steve, you can”. “Good!” he said, “’Cos a mate of mine has a 250 Superdream and the bottom crankcase is broken.”
“Oh, sorry, in that case, no Steve, you can’t!” The Honda’s plain main bearing shells are housed in the two horizontally split halves and these will certainly have been machined as a pair. But so many Superdreams were sold that it should be easy to get another engine or pair of cases. It’ll mean changing the engine number on the log book, but that’s OK – it doesn’t cause the same kind of registration problems that changing a frame creates.