T5 4TH GEARS

HELP!!!
CAN ANYBODY SHED ANY LIGHT ON THE FOLLOWING PROBLEM??

HAVING FITTED A COSA 21 COG CLUTCH TO MY 172 KITTED T5,I HAVE GONE THROUGH 3 INPUT SHAFTS AND 3 4TH GEARS!!!
THE ENGINE WAS TAKEN TO A PIAGGO MAIN DEALER-WALTON ROAD GARAGE IN MOLESEY-SURREY.I PAID FOR A COMPLETE ENGINE REBUILD(SEALS,BEARINGS ETC) AT THE COST OF £350.
I HAD THE ENGINE BACK FOR APPROX 100 MILES AND THEN THE 4TH GEAR BLEW.THIS HAS HAPPENED TWICE NOW.ON THE 2ND TIME BACK IN THE GARAGE THEY FOUND THAT THERE WAS A HAIRLINE CRACK IN ONE ONE THE CASES-THEY HAD THE CASE WELDED AND MACHINED WITH A BRASS BUSH.
THEN 60 MILES LATER THE SAME THING HAPPENED AGAIN!!!
I HAVE SPOKEN TO A FEW PLACES AND THEY CANT UNDERSTAND WHY.
I HAVE SPOKEN TO THE GARAGE TODAY AND THEY HAVE THE LONDON TECHINCAL ENGINEER(MITCH) TO PERSONALLY OVERSEE THE REBUILD OF THE ENGINE.HOPEFULLY I WILL HAVE THE BIKE BACK FOR THE WEEKEND.

HAS ANYBODY HAD ANY PROBLEMS LIKE THE ABOVE???

ANY IDEAS HOWEVER MAD OR SILLY WOULD BE GREATLY APPRECIATED AS I AM AT MY WITS END AS NOW IN MY MIND THE BIKE IS NOT RELIABLE(AFTER ALL IT’S A VESPA)

HELP!!! !!![?[] [?[] [?[] [?[]