I drove a truck with a SM465 a friend had recently bought,and he told me the "clutch never felt right" in this truck,and sometimes the pedal got "stuck" on the floor a few times and he had to pick up on it with his toes to get it to come back up (mechanical linkage)--he thought maybe the throwout bearing was not sliding on the collar on the input shaft properly...
I though the cab mounts were rotted enough so the linkage was binding on it myself...but you could feel a "rough spot" pushing the pedal down sometimes..
When he went to investigate,he noticed the bolts holding the transmission to the bell housing were quite loose, on the top two especially--tightened them up and it never had the pedal get stuck down again...but the disc in it,despite being not too old,acted like it was glazed almost right away--you could rev it up and dump the clutch ,and the tires would not break loose,the clutch didn't really "slip" ,it would stall the engine doing a stall test,(put it in 4th and rev it up and dump the clutch)..but it didn't engage quickly or as firm as it should have it seemed..it took it like 4-5 seconds to stall it..
One day going to work he gassed it to pass someone,heard a clinking crunchy type noise,then the engine sped up suddenly..an autopsy showed the disc had the hub tear out on the metal part of it between the lining and where all the anti-chatter springs go- (much as you described!)..
He ended up buying a complete clutch kit and had the flywheel turned,put it all back together and the truck was much better...he showed me the disc he removed,when I went to pick it up he said WAIT!--look which way the splined hub is pointing--its exactly how it was when I removed it...
I took note,then he shows me the instructions the clutch came with,it had a picture of the cross section of the clutch setup,and plainly noted "flywheel side" on the disc...sure enough,whoever had replaced the disc just months before,had installed it facing the wrong way...
The lining on the clutch looked like a rock hard shiny glaze,and some of it was fluffing off around the rivets...
I was surprised it even worked that way,most vehicles, if the disc isn't in facing the right way,it either makes you aware of it the first time you try driving it,or it wont even bolt together...
I find buying aftermarket clutches can be a crap shoot,even some of the "better" brands have had their share of defects ...
I put two 12" 3 fingered clutches in my 72 K5 and finally ended up with a used 11" Centerforce clutch I got cheap at a swap meet..the 12" ones were crap Borg-Warner rebuilts,I would never go with a 3 finger style pressure plate again--the diaphram type are much less pedal effort,but grab as good as anything else does..and way less failure prone too..
