I forgot we weren’t talking about a Disco drop arm. Your new one should still have a section where it looks like the splines are missing, and there should be a corresponding area on the output shaft. Have you definitely matched these up ?
So I pulled the drop arm off and checked the splines on the shaft and arm . All looked good . Then I measured the diameter of the new arm at the top and bottom with a vernier caliper and compared it to the old one . Measurements were identical . The only difference in the splines was the old ones were flat at top and the new ones were sharp . I'd be inclined to think several decades of use could have caused that . Then I have the old arm's splines a good clean and tried to refit it . It would only go on the same distance as the new one before getting tight ! So just decided to put on the new one and tighten it up as much as possible using a breaker bar with a length of pipe over it . Tightening it up as much as I can I have about 3mm of splines visible at the top and two full threads showing at the bottom . Looked at several videos of drop arms being replaced and none of them ended up as far as the one that had been replaced . I think this is best I can do , subject to a recheck after giving it a test drive .
