Sept 26 – I talked with David last night and he verified that he and Jordan had the stem break. It was obviously too flexy/brittle (before installation it could be bent too easy with moderate finger pressure).
BUT – he contacted Professor Motor and they acknowledged it was a bad batch of guides. David said this was several months ago (early in the year?)
I have been using these guides, on two cars, for a few months and they have not had any problems.
Of course, the Slot.It wood guides are great and usually available at Kings. The most advantages the ProfesorMotor guides have are (a) a stem that can be customized to allow any height (e.g. when you need to add a few shims to get the guide deeper), and (b) a fit that can be customized (e.g. when you have a worn guide-boss or otherwise sloppy guide. Again, my experience has been that they are an alternative to the Slot.It guides JIC someone wants to try something new or cannot get a Slot.It deep-wood guide.