A discussion about the broken valve collets. And shimming in general. I had 5 broken collets, 4 of which were intakes. These all failed within the ducati valve train operational specs but all were looser than assembly specs. The closer clearance range on the broken ones was 0.005 - 0.008 inch (0.125-0.20 mm). The valve stems neck down slightly in the section where the shims seat but as they were measured it was clear that the valve stems were essentially identical. Then the stem holes in all the closer shim were measured. These are also being held very precisely. But they do have clearance. For the collets to break in the middle like they do, the middle has to be able to be distorted relative to the ends which would have to be constrained so that force can be applied to the middle. Two ways this can possibly happen is the ends of the collets are slightly bigger than the middle or the clearance between the closing shim and the valve stem allows the closing shim to cock slightly on the stem and if the collet is oriented such that the middle of the collet is in line (90 degrees to the centerline of the rocker face) with the repetitive forces of the valve bouncing off the seat relative to whatever clearance is there. Recognize also that the opener clearance is based on the change in valve stem length. And that the closer clearance gets bigger as the stem lengthens.
The valves that had broken collets were reassembled with unbroken (but run in on the other valves) collets so the change in closer clearance contribution from the broken collets could assessed. This was found to be between 0.001-.0015 inch (.025-.038 mm). I'll do this again when the new collets show up. This will allow understanding of the contribution from running in new collets. When I reassemble, I intend to orient the collets such that the gaps are in the same plane as the rocker face. This may or may not help as the possibility exists that the whole bit rotates in operation. But at least they have to find their way there first.
Valve shimming. I remember being part of a discussion on here about valve shimming and how to deal with ones outside of the operational specs. Showed a total lack of understanding of the valvetrain dynamics. The bottom value of the assembly specs (1103) on these are 0.004 (0.1 mm) inch for intake openers and 0.005 inch (0.125 mm) exhaust openers and 0.002 inch (0.05 mm) for all the closers. If you want longterm valvetrain reliability these should be adhered too. I intend to take the closer springs off the heads so I can actually assess the needed closer clearance. The closer clearance is primarily driven by the clearance needed to ensure that there is not any operation drag caused by interference of the opener and closing cam lobe profiles. If the machining allows me to run the closers at 0.001 inch I intend to do this as I have with my other bikes (have to look at some thermal characteristics first). Loose closers allow the valve to bounce off the seat as it closes. Bigger gap bigger bounce and more bounces. Ducati is using coil springs with the coils stacked on one another instead of hairpins on the closers because the drag between the coils faces provide a level of damping but the banging about still happens.