[ Ref. 110411 MMDigest, "Seek Unusual Metric Size Tap & Die"
> Does anyone know of a source for an M4.5 x 0.90 tap and die?

Might the artifact in question be of British (or British Colonial or
Empire) origin?  If so, it's remotely possible that the screw thread
is 'No. 1 BA' (British Association).  BA threads (officially obsolete
since 1966 but still widely used by U.K. model makers) are based on
metric rather than Imperial dimensions.

No. 1 BA has a pitch of 0.9 mm (approximately 28.2 threads per inch),
'effective diameter' is 4.76 mm, 'major diameter' is 5.3 mm, and
'minor diameter' is 4.22 mm.  It's clearly nowhere near an exact match
to 4.5 mm, but if you're measuring a very worn or damaged bolt that
could (maybe) account for the discrepancy in diameter.

BA taps and dies (and nuts and bolts) are easy to source on the Web.

Barrie Land, UK

 [ British Association Standard Thread dimensions are given at
 [   http://www.britishfasteners.com/threads/ 
 [   http://www.gewinde-normen.de/en/ba-thread.html 
 [   http://www.conexmetals.com/thread/ba-thread.htm 
 [ -- Editor (Robbie)