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I've got a RostockMax v2 with an all metal hotend, and got a deal on a dusty FSR kit.

I don't have a duet board.

Is this a (how shall I say) "sane" thing to try? Am I gonna mess up a working rostock Max and regret trying this? Anyone gone through a similar upgrade?

You'll need a Duet if you want to use the FSRs. If they don't work, you can use a Duet Smart Effector (if you don't want to replace your hot end) or an SE300 hot end assembly (if you do). The SE300 and Duet Smart Effector use the same technique for detecting the nozzle touching the bed.

A Duet makes for an absolutely amazing upgrade.

g.