Cheers stot
yes i have a grp 4 coupling i will have a go at what you suggested. Bit difficult to explain but my rack has a splined section then a resessed area then another splined section i presumed the ressesed section was for the 8mm bolt to go through the coupling so it could never pop off the rack even if the bolt became loose. mine does though
i may have to weld the holes up and re drill the holes so it works as it should