on a 1966 harley davidson 4 speed transmision,what happens if the wrong main schaft is installedfo a diff,year

Answer 1

In 1965, (the last year of the Panhead), Harley came out with an electric start. This necessitated that the primary cover be cast aluminum, instead of the old stamped steel, or "Tin" primary, for support.
With the electric start, there was also the need to have a longer transmission shaft, with the end supported by a bearing in the inner primary case.
You can run the longer shaft without the primary, but it needs a support bearing to eliminate the longer shaft "whipping".
Here's an example of what I'm talking about:

http://www.jpcycles.com/productgroup.aspx?GID=54A08443-D5B8-48B2-9E1D-B0C180F0F3D3&search=support+bearing&store=Harley&page=1

Answer 2

It will probably cause it to lock up.