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

The aiplane will act as a shock mount, wings included, when balancing the prop, are you balancing the hub also? very essential to do that.

I usually break engines in on the airplane. Get it adjusted enough so that it's reliable and then go fly. It's not like a glow engine and will tolerate being out of tune quite a bit before actually quitting on you. There is no mention of what brand of prop you are using. Some prop designs have not been updated to accomidate multi bolt hubs. Vibration harmonics is a complicated topic and the combination of prop, mount and RPM may be putting it into a destructive harmonic. The other thing is the prop bolts on a wood prop should be tightened to about 45-50 in lbs and checked frequently. Over tightening them will cause them to fail. Some manufacturers also supply grade 12 bolts and in some cases they can be too brittle. I always swap out bolts to stainless grade 8.

Send it back to where you bought it. Have them test run it & let them figure out what is wrong. A twin should be fairly smooth. Mabye the pistons are not coming up to TDC at the same time. Who knows???

More...could the timing be to far advanced causing pre-ignition placing shock to prop bolts???

Curious what prop brand was used but my guess is the prop bolts were just not tight enough. I seriously doubt this is an engine defect issue though tuning could certainly contribute to the problem, especially if the bolts were loose.

This is my first twin engine. The DLE instructions are very limited. There was no indication of which spark plug wire went to which spark plug. Does it matter?
i agree with the loose bolt theory. The way one of the bolts sheared off indicated it had loosened and come at least 1/8" out.

Are you sure they are 5mm screws and not 10-32? Did you see if the prop hub and been compressed or not?

Does not matter which spark plug wire to which spark plug. They both fire at the same time.
BCCHI

Very interesting, haven't heard of this on the DLE's yet, I just tightened the hub screws a little at a time in a cross pattern obviously, not even lock tite, and she has been solid for over 3 gallons.

Awesome engine btw! Here it is on my big stik.

https://www.youtube.com/watch?v=tK6tp1X7WhE