4. Proper socket sizes for bolts of M39 and larger (Reference) For the proper sizes of sockets or spanners (= width across flats) for tightening hexagon bolts of M39 and larger, see Table 5. Table 5 Unit: mm 21 Thread diameter Proper sizes of sockets or spanners M42 65 M45 70 M48 75 M52 80 M56 85 M60 90 M64 95