What was found
MAINTENANCE LOOKED AT ACFT BEFORE FLIGHT. COWLING WAS REMOVED AND EXHAUST SOOT WAS SEEN AROUND LOWER SIDE OF NR 4 CYLINDER. FURTHER INVESTIGATION REVEALED A CRACK AROUND THE EXHAUST RISER THAT WAS ABOUT 270 DEGREES AROUND THE PIPE. HEAT EXCHANGER PORTION OF THE EXHAUST HAD BEEN REPLACED DURING THE PREVIOUS 100-HR INSP ACCOMPLISHED 30 HRS PREVIOUSLY. THERE WAS NO PROBLEM IN ALIGNMENT DURING RE-ASSY OF EXHAUST SYS. SUBMITTER STATED THIS IS THE FIRST CRACKED RISER WITH THIS STC'D SYSTEM. OTHER AREAS THAT CRACKED WERE RE-DESIGNED BY MFG AND HAVE NOT HAD ANY OTHER PROBLEMS. (X)