This is an all Meccano ball race, suitable for fitting inside ashtray tyres. The outer race is a pair of suitable curved 5.5" strips, overlapped so there is a 2 hole gap between the ends of the inner strip. They are bolted to the outer curved strip through holes 2 and 10 of the inner strip - this makes the diameter of the whole unit correct. The two races need to be spaced 0.75" apart, which is done using the plastic 5 hole spacer, as can be seen in the photo. The gap between the inner ends of the inner 5.5" strip is filled with a curved fishplate, to prevent the balls from falling out. The inner race is a pair of bushwheels or wheel discs spaced using washers.