There are several people here that have run them with no problems. There are some people here that have had motors crater, and they happened to also have a UD pulley. I have talked to several engine builders and every one of them has said they would not recommend them because there is no dampener. In fact, one of the shops had actually done some harmonic testing on a crankshaft that had a UD pulley on it, and they found that between 2200rpms and about 3500rpms there was a very noticeable harmonic vibration on the #2 and #3 rod bearings. So, my choice is to either use stock or a Fischer pulley. But that’s just me.