How to modify a Helios 81M lens for a Nikon D5200?

I've just purchased a Helios 81M (2.0f, 53mm) prime lens to use with my Nikon D5200. While it has an F mount and is reported to work with the Nikon D80 without modification, it fits the D5200 mount but can't turn at all to lock into place. Does anyone have any suggestions of adapters I could use or how to modify the lens to fit?

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It should fit, make sure you're mounting it correctly. If it doesn't, get your money back.

Of course, you lose AF and all metering modes, but you knew that, anyway