A BEECH 1900D IN CRUISE AT 15000 FT DIVERTED DUE TO #2 ENG UPPER FORWARD COWLING LOOSE AND RAISED; CAUSED BY IMPROPER INSTALLATION AFTER MAINT.

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OUR COMMUTER FLT ON JUL/XY/98 STARTED OUT WITH A MECHANICAL DELAY INVOLVING AN OIL LEAK ON THE R ENG. THE FIX INVOLVED TAKING THE AIRPLANE ACROSS THE FIELD TO THE MAINT RAMP WHERE THE MECH BEGAN HIS REPAIRS BY REMOVING THE MAJORITY OF THE R ENG'S COWLINGS. THE LEAK WAS FIXED AND ALL OF THE COWLINGS; ZEUS FASTENERS; CAM LOCKS; AND SAFETY WIRES WERE REATTACHED; AS I; THE CAPT OF THE FLT; OBSERVED. I FOLLOWED UP WITH A PREFLT; AND THEN TAXIED BACK TO THE TERMINAL RAMP; WHERE MY COPLT ALSO PERFORMED A SUCCESSFUL PREFLT. THE SCHEDULED FLT BOARDED APPROX 5 HRS LATE WITH 3 PAX. TKOF AND CLB WERE BOTH UNEVENTFUL; WITH ALL ENG INSTS INDICATING NORMAL. WE LEVELED OFF AT 15000 FT MSL AND SET CRUISE PWR. APPROX 20 MINS AND 50 MI INTO THE FLT; I; THE PF; AND THE COPLT NOTICED A SLIGHT BUMP IN WHAT WAS OTHERWISE SMOOTH AIR. AS THE COPLT LOOKED OUT HIS R WINDOW; HE NOTICED THAT THE TOP FORWARD; INBOARD ATTACHING POINTS (CAM LOCKS) HAD COME LOOSE; CAUSING 2 SAFETY WIRES TO BREAK. THIS RESULTED IN THE UPPER COWLING SEPARATING FROM THE LOWER COWLING APPROX 4 INCHES. THERE IS NO EMER OR ABNORMAL PROC FOR THIS CONDITION; SO I IMMEDIATELY SLOWED DOWN TO 150 KIAS; JUST ABOVE VG IN ORDER TO MINIMIZE PRESSURE ON THE COWLING. WE RETURNED TO ZZZ; REQUESTING ONLY PRIORITY HANDLING -- NO EMER. UPON LNDG; WE SHUT DOWN THE ENGS BEFORE ATTEMPTING TO TAXI TO THE RAMP. BY THIS TIME THE COWLING HAD SETTLED BACK DOWN ONTO THE MAIN STRUCTURE; AND HAD BEEN HELD ON BY 2 OTHER CAM LOCKS AND SAFETY WIRES. HOWEVER; THE LEADING EDGE WAS UNCOMFORTABLY CLOSE TO THE SPINNER. WE RESTARTED ONLY THE L ENG AND TAXIED TO THE RAMP. THE ORIGINAL MECH MET US AT THE GATE; INSPECTED THE ENG; AND REATTACHED THE COWLING.