Naf,

God Bless Ya!  I'm glad that ended well!

I had this happen to me while taxiing to the runway a couple years ago.
The Matco supplied disc  was apparently not annealed after tooling
(according to Mech Eng. Types observing the pieces).  Matco was gracious
enough to send me a pair of new ones for free.  More importantly, the
bolt attach points are now a huge part of a thorough preflight on my
discs.  New discs have over 150 hours (bunches of landings) and remain
crack free.

I don't know for a fact if it's an annealing issue or not but prudence
dictates that Matco drivers at least preflight the attach points
carefully.

Larry