I am installing a rag joint to a coaster brake rear wheel. As soon as I get the nine bolts somewhat tight and spin the wheel I hear a pop. The first time it happened I brought it to a bike shop and they said they replaced a spoke and trued the rim. The second time around the same thing happened. What is going on? Do I need heavier spokes or am I doing something wrong? The first time I installed the sprocket I used both spacers, one on the inside and one on the outside. The second time I only used one on the inside. Any suggestions?