The RA strut is about 25mm longer than the TA22 strut , so when you fit the TA22 strut cartridge into the RA strut there is a gap of about 25mm between the nut and cartridge ( big nut with a hole to fit over the cartridge shaft and screw into the top of the strut to hold the cartridge in , so a spacer is needed between this nut and the cartridge .