2 people on briskoda had the single collet ones and both engines have just blown to pieces caused by the valves when reved to 9k
cant remeber the exact detail but one is being rebuilt again by jabba......
maybe worth a look on briskoda for the exact prob but deffo them valves where the
problem
from what i have heard, mix of single groove valves and std springs is one failure.. and on one of the failed engines, springs showed signs of coil bind/contact
the valves were not the
problem... a poor mix of parts and/or assembly most likely were
another failure i have heard was a mix of 3 groove valves and supertech Ti retainer/spring kit.. (same setup I have used since 2005/6) - a collet failure reported.. popped off. weird failure.
too many variables to blame any one thing
One comment, is the suggestion to use Schrick is flawed, as they are a two piece valve still.. Supertech intake stainless steel valves are one piece, and only the inconel ones are 2-piece (because you cannot induction harden the collet groove)