AFTER PERFORMING ROUTINE ENGINE OIL AND FILTER CHANGE ON A CESSNA-172; MECHANIC IS INFORMED THE ENG SUMP SCREEN COVER HAD SEPARATED DURING FLT TRAINING; CAUSING OIL PRESSURE LOSS AND PRECAUTIONARY LANDING.

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DURING ROUTINE OIL CHANGE; I ALLOWED AN ASSISTANT TO REMOVE THE ENG COWLING AND DRAIN THE ENG OIL WHILE I PERFORMED OTHER MAINT FUNCTIONS. THE ASSISTANT ALSO REMOVED THE SAFETY WIRE AND LOOSENED BOTH THE OIL SUMP SCREEN COVER AND OIL FILTER. AT THIS POINT I TOOK OVER THE OIL CHANGE; CHANGING THE FILTER AND REPLENISHING THE ENG OIL. THE OWNER OF THE ACFT INFORMED ME AT THIS POINT THAT HE WOULD LIKELY NEED THE ACFT ASAP TO FLY TO A BUSINESS MEETING. AT THIS POINT; I PULLED OUT THE ACFT AND PERFORMED A RUN-UP AND LEAK CHK WHICH WAS GOOD. I DID NOT NOTICE THE OIL SUMP DRAIN COVER SAFETY WIRE WAS REMOVED AND MADE THE ASSUMPTION THE SCREEN WAS CHKED; REINSTALLED AND SECURED BY MY ASSISTANT. I THEN INSTALLED THE COWLING AND RETURNED THE ACFT TO SVC. THE OWNER DECIDED TO NOT USE THE ACFT AND IT WAS USED LATER IN THE DAY FOR FLT TRAINING. DURING THIS TRAINING FLT; A LOSS OF OIL PRESSURE WAS NOTED BY THE INSTRUCTOR AT WHICH TIME A PRECAUTIONARY LNDG WAS MADE IN ZZZ1. UPON LNDG; A MASSIVE OIL LEAK WAS NOTED. LCL MECHS REMOVED THE COWLING AND FOUND THE SUMP DRAIN COVER MISSING. LACK OF COM BTWN MYSELF AND MY ASSISTANT PLUS THE TIME PRESSURE TO GET THE ACFT RETURNED TO SVC FOR THE OWNER WERE CONTRIBUTING FACTORS IN THIS EVENT. I HAVE DISCUSSED THIS MATTER AT LENGTH WITH THE ASSISTANT AND WE HAVE AGREED TO IMPROVE OUR COM AND DISCUSSED THE IMPORTANCE OF PASSING ALONG A COMPLETE JOB STATUS UPDATE WHEN TURNING WORK OVER AND THE DANGERS OF DISASSEMBLING ACFT COMPONENTS WITHOUT REASSEMBLY OR SOME TYPE OF DOCUMENTATION OF PARTIAL WORK ACCOMPLISHED. VALUABLE LESSONS WERE LEARNED FROM THIS EVENT BY BOTH MY ASSISTANT AND MYSELF THAT WE WILL CARY WITH US IN THE FUTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE CESSNA-172 LYCOMING ENGINE OIL SUMP DRAIN COVER HAS A 1 INCH NUT THAT SECURES THE COVER; WHICH HE FAILED TO SAFETY WIRE. BECAUSE OF THE LOSS OF OIL PRESSURE; HE RECOMMENDED THE ENGINE BE DISASSEMBLED BEFORE FURTHER FLIGHT. THE ENGINE CRANK JOURNAL WAS FOUND SCORED; SO A COMPLETE ENGINE OVERHAUL WAS INITIATED.