I have PM'd Stuart a more detailed description but the article Len is referring to is MPH 561 October 1995 'A Thousand And One Uses'. Basically I sealed the Dynamo drive and everything else in the chaincase so that it was completely oiltight and airtight. The timed breather was working perfectly and the engine had recently been rebuilt by a reputable expert. The result was that the engine pressurised the chaincase to such an extent that it forced oil through the chaincase and then through the gearbox mainshaft bearings into the gearbox and raised the gearbox oil level until it spilled out of the filler cap. Subsequently it turned out that the engine builder had dispensed with with the drive side main bearing reversed oil scroll and substituted it with a plain spacer. He had then fitted an outer drive side main bearing of a sealed type after removing the inner seal. It transpired that in practice this 'half sealed' bearing acted as a flap valve opening to allow air/oil to enter under pressure on the down stroke of the pistons but neatly closing on the up stroke and so trapping the pressure. With every down stroke of the pistons the pressure in the chaincase increased exponentially with the results as above. Only a rebuild and reversion to a standard drive side main bearing set up solved the problem.
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