Quote Originally Posted by Jonathan View Post
Four M6 bolts would probably handle several hundered times that before shearing! As mark said, you don't need the bolt holes to be deep - bear in mind you'll struggle to tap M6 more than about 25mm deep anyway. The strength of the joint depends on the friction between the surfaces, and thus the clamping force and surface finish. Increasing the length of the bolts beyond 20mm of thread wont make much difference to the force you can apply.

If you don't mind me asking, what is the part for? It can help a lot to know since we might be over examining this, or worse vice versa.
Jonathan, I wasn't worried about shearing the bolts, I simply thought that 80mm long bolts will offer more support for the horizontal plate.

As for the use of the part, it will be part of a coffee grinder with these used to crush the beans:

http://www.portafilter.net/uploaded_..._LL-752918.jpg

This means there will be mostly clockwise turning forces acting on the mount and the carriage, although I don't know what the torque is yet.

Regards,
dsc.