Rob , Was your elmar 50 mm or more ? I love Hektor is very cheap but 90 mm. What happens with a 90 mm Hektor on Zenit ? Does it magnify too much and work as macro or worse ?
They have different lens to film lengths.
The Zenit, being a single lens reflex, needs to leave room for the mirror to move between the lens and the film.
The M39 Leica thread mount (LTM) lenses are designed to come much closer to the film plane since they don't have to clear a mirror.
The M39 lesnes for the Zenit are few (and fairly rare - all Soviet I think) and were designed specifically for the Zenit: Most M39 lenses are for rangefinders and on the Zenit will only allow focussing very close.