base of the barell where four mounting bolts pass through, if barrel not pulled flat down on cases, vibration can crack around mounting points, base of barell actually broke away on one side of the rm465 through mounting bolt, hairline crack around other, i like you was thinking blown crank seals at first and prepared for leakdown test, same symptoms as what your saying, very hard to see so clean with degreaser and get bright torch and put your best magnifying glasses on, look very closely around mounting holes, baseline probably not best wording, sorry freaky