That just made me think of something. If the bottom of the countersunk
hole doesn't match the profile of the fastener where the head meets the
shank then there's is your problem. The fastener has a bit of an
interference fit at the transition point, and right away the bearing
surface
under the fastener head isn't fully making contact because of it.
However,
the aluminum "flows" from the stress and lets the bearing surfaces bear
down
on each other.
SNIP

Another very good point. We actually countersink the aluminum housing on
the other side of teh plate to avoid this very problem. When we drill teh
flat head off teh screw, the "shank" can be taken out with bare fingers...
Most of the time so I'm 100% sure it is the flat head screw's angle on the
flat head coutnersink within the plate.
--


Regards,
Joe Agro, Jr.
http://www.autodrill.com
http://www.multi-spindle-heads.com

V8013

My eBay: http://tinyurl.com/4hpnc