I'm hoping to plumb the expertise on this forum again. A while ago I was adjusting my Seiko 7456-802j when the crown came off in my fingers. At the first opportunity I removed the stem. A close look revealed that the threaded end looked too short as there were only about three threads on it. I ordered a couple of spares and, this evening, tried to screw the crown on one but to no avail. I might have been too gentle but I didn't want to force it in case I caused further damage.

I've had a look inside the crown post to see if there was a broken piece of the stem still in there but it doesn't look like it. I've also had a feel in there with a screwdriver and a rough comparison with a new stem shows that there is enough depth in there to accommodate six or seven threads which sounds like a more sensible amount.

When looking into the post I can't see much, if any thread although it is difficult to tell. Could the thread have become stripped? If so would it simply be case of running a tap down to create more thread or would the removal of further metal make the crown useless?

The crown itself is a screw down crown with a spring loaded post. I think I have tracked down the part number for the crown but a quick search indicates there aren't any spares (not surprising, the watch is over 30 years old). Would there be a generic replacement that would work?

Any advice would be greatly appreciated.