I have a giottos carbon fibre mtl 8360b tripod and mh 1302-652 ball head and mh 5001 three way head both of which use the same mh 642 quick release plate. These have served me very well on a d90 and should be able to hold 6kg. However I recently upgraded to a d7100 and I am finding that no matter how tight I tighten the plate onto the camera it is susceptible to slipping when in portrait mode - it doesn't really matter what lens I use. Interestingly I bought the chseap Meike grip, which adds to the weight, but which has rubber on the bottom and this seems to grip the plate better which in turn prevents slipping. Any ideas on how I can prevent the plate slippage?