I just purchased my second gas engine. A DLE 40 twin. I wanted to break it in on a test stand so I bought a PSP test stand and put the motor on that. After getting it started it ran for about a minute and the prop flew off and all four bolts on the prop hub snapped. So I bought a second prop, 4 more bolts and the guys at the hobby shop said not to over tighten the bolts. On the second run after about a minute the prop split in half and flew off. When I found everything in the yard it looked like one of the bolts had worked itself loose. I have a couple of questions:
1. the test stand has a shock mounting pad on it, but it seems like the engine vibrates. Could that be a problem?
2. Do the bolts on the prop need to be really tight?
3. Should I just break it in on the plane?

any other suggestions would be greatly appreciated
thanks
peter