If you don't already own this lens, you can readily purchase a
new Helios 85mm 1.5 on Ebay for $300 that already comes in the M42 mount.
If you already own this lens in the Nikon mount, it probably would be easier (and cheaper) just to sell it and then purchase the same lens in M42 mount.
But if you're like me and you like a challenge and enjoy Frankensteining a lens, go for it - give it a try. It's not like it's an expensive lens. While you might destroy the lens in the process (I typically destroy at least one lens each month with my mad experiments), you will likely learn much about experimental lens adapting that will likely serve you well on future adapting challenges. Besides, if you end up destroying and parting out the lens you will likely find good uses for the parts in the future (provided you keep on adapting lenses). A large parts inventory from parted out lenses is very valuable to those who like to tinker with lenses.
In the last few weeks I've adapted 3 old Leica projector lenses to Pentax K mount, Nikon F mount and Fuji X mount. These projector lenses are a Walter Mandler design. The projector lenses cost me about $75 USD each. They have the exact same optics as a Leica R lens that costs around $400. However, each of these lenses required a couple of parts from my "lens parts bin" to complete the conversion. You just never know when old lens parts will come in handy.
Dave
Last edited by Fenwoodian; 12-01-2019 at 11:22 PM.