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The question of the correct length of these arms has come up several times here and in other forums over the last 2 years. The Dozuki documentation states they are 290.80, while the official Repetier firmware has always specified 291.06mm.

I emailed SMC support, and received the following prompt reply (edited for brevity):

Our ball cups arms for the Rostock have always been 291.06

and

I checked the firmware and measured one of our arms. Length on all the ball cup arms for the Rostock Max is 291.06.

I'll wade through the documentation and make sure it gets corrected.

So, they've only been one length for the entire time they've been sold. Hope this helps someone.

Is it advised to stick with that measurement, or is it plausible that (Duet auto) calibration changes that slightly. I think I have pretty nice dimensional accuracy right now, but pretty sure my arm length value is not exactly this 291.06mm. I do however feel that I have a rather big deviation during calibration. (+/- max about 0.1xx mm which is half a layer). I do not realy have first layer problems however, because I have the bedleveling active.