What was found
(CAN) AIRCRAFT IS AN AMATEUR BUILT, ENGINE AND SUBJECT OIL FILTER ADAPTER ARE USED ON MANY CERTIFIED ACFT. DURING ANNUAL INSPECTION SUBSTANTIAL OIL LEAK WAS NOTED ON BACK OF ENGINE. TROUBLESHOOTING SOURCED OIL LEAK TO CRACK ON OIL FILTER ADAPTER EAR. CRACK COULD NOT BE SEEN WITHOUT SOME NDT PROCESS. NEW PART INSTALLED, OIL LEAK WAS FIXED.