with no part of either key 56 a and 56 b exposed to the crushing chamber 36 capturing the keys 56 a and 56 b entirely within the head nut assembly 50 prevents the keys 56 a and 56 b from being subject to impingement of the crushing material this prevents the keys 56 a and 56 b from being knocked out of bores 68 a 68 b 78 a and 78 b which would allow the head nut 52 to rotate independently from the burning ring 54 causing the problems described above additionally the use of the dowels 56 a and 56 b to key the head nut 52 to the burning ring 54 prevents the need to weld to the head nut 52 welding to the head nut 52 can distort the head nut 52 requiring the head nut 52 to be cut off when it is to be removed possibly damaging the threaded shoulder 44 of the bearing sleeve 32 in the process damage to the head nut 52 can thereby result in a great expense and associated downtime while the head nut 52 bearing sleeve 32 and possibly the shaft 18 are repaired the bottom face 76 of the burning ring 54 is generally contiguous with an upper surface 80 of the mantle 34 mantle upper surface 80 is typically wider than.