Many have done it and able to hold compression and not leak. The problems comes later if the two halves dont align the crank properly. May not able to tell when install but if incorrect the misalign crank can cause problems at some time in the future.
This is what Iím hearing from other forums... that the two cases are machined together so by mismatching them that leaves the possibilities of layshaft, crankshaft and driveshaft not running straight and would be a no bueno... which makes perfect sense to me... others have said theyíve pulled it off.
I had built a mismatched motor on a friendís Cosa I, cause he wanted his electric starter back (old big half, had a damaged rotary from a small screw he lost down the carbís throat) from his first damaged motor. We couldnít find a decent complete set so we decided to risk it. That was 4 years ago. Itís still going strong with almost 36k Kms on itís back so far. We only do the usual maintenance (engine oil, spark plug, etc.). Cases were 328 & 843. Coincidentally they matched perfect on the cylinder surface although I gave them a lapping just to be sure...
I've got mismatched cases on my p200. I've put 4,000 miles on it in the last couple of years. There is a very slight step (.002") between the two cases where the cylinder mounts. I just cut a piece of shimstock and it's been fine.
Most of the bearings are balls, i can't believe a tiny bit out of alignment is critical
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