I have 2 ProSpin aluminum spinners from Mpi, 3" and 3 1/4" respectively. I have experienced bolt failure with both. This usually happens after only 1 to 2 flights, they just break. Same thing happened with the original stock spinner that came with my OS 1.60 FX powered GP Gene Soucy Extra, I suspect it is made by the same people. I don't know if these 4mm bolts are just too weak and need to be replaced with some kind of hardened bolt, but this is driving me nuts. I already lost the original Gene Soucy spinner in flight once and was able to recover the ProSpin replacement that I got for the same plane twice. Now the same thing happened to a brand new 3" spinner I just put on my Seagull Laser 200 (OS 1.20 AX), fortunately this time the plane was still on the ground. I don't think I'm tightening these bolts too much, even though I'm not really sure how tight is too tight. I wonder if anyone out there has had a similar problem.

PC:

While the Pro-Spin and Spin-Right brand spinners are both good liooking and inexpensive, they are also usually badly out of balance.

Have you checked yours for balance? Might make all the difference in the world.

You could also drill the adapter nut out and retap, replacing the 4 mm screw with a 10-32, or maybe an 8-32 socket headed cap screw.

Bill.

... And make sure the screw is cut short enough that it's not touching the crank.