This is a real gotcha. We installed a new prop extension on N79RA, which was not manufactured by Brock. We then installed a Brock spinner on this extension. The center hole in the spinner backplate was a close fit on the center locator on the prop extension. Unlike a Brock prop extension, this one had a rather large radius machined at the flange face, which did not allow the spinner backplate to slide all the way on, see sketch below.
This is a dangerous situation, and difficult to detect because the flow guide prevents a visual inspection. We installed the prop, torqued the bolts to the 18-20 ft/lbs we normally use, and tracked the prop tips accurately. At this point, the prop bolts were tight, the prop ran true, but the prop was not being squeezed against the prop extension. Rather it was pressing only against the spinner back plate, in this case a very stiff 1/8" thick backplate, that was able to support the torque value of the bolts, without going back against the prop extension.
We flew the airplane for several hours, blissfully unaware of the problem. Last week Dick used N79RA to take a business associate to the Santa Monica airport. On his way back he got to within 10 miles of Mojave when a strong vibration became apparent. It steadily worsened to the point that he elected to turn back to the closest airport at Rosamond. The vibration became much worse and then abruptly went away. Dick thought he had thrown a rod, set himself up for an emergency, dead stick landing on the 2300 foot, Rosamond runway. Dick touched down on the numbers, made the turn off and rolled into an empty tie down. Not until he got out did he realize he had lost the prop and spinner.
Looking at the marks on the drive lugs it is plain to see what happened. The 1/8 aluminum back plate gradually gave way, allowing the bolt tension to relax. Without this friction between the crush plate and prop extension flange to drive the prop, it begins to be driven by the drive lugs and prop bolts in shear. This state of affairs can only continue for a very short space of time, before the bolts fatigue and then, of course, the prop/spinner comes off.
This is a very serious situation and anyone who has a Brock Spinner mounted on a prop extension that is made by someone other than Brock (obviously the Brock extension is completely compatible with a Brock spinner) should immediately ground their airplane and check this out. The problem can he cured by decreasing the radius in the prop extension, or increasing the diameter of the hole in the center of the spinner back plate.