Strut inserts are determined by the thickness of the thread that goes into the strut top mount. The early models have a smaller diameter hole. You need to make sure that the two match ! Later struts off Mk2 and Mk3 Capri's can be used and will have the larger sized hole in the strut top mount.Hope this helps ?
Had this reply by email from another club. found the answer to problem and have fitted Mk2 strut inserts into Mk 1 and changed top mounts to fit. Great job done at last.