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Would 3x1mm magnets be suitable for space marine arms/ backpacks?

I use 2mm x 1mm, neodymium magnets, but I think 3mm would fit.

I personally found that 3mm x 1mm worked better than 2mm x 1mm for attaching arms/backpack to torso.

2mm x 1mm was fine for attaching weapons/hands to arms.

what about tanks and arms on bigger MCs?

I’ve been having good success with 2mmx2mm magnets. I did work some with 3mm, but they were a little large IMHO.

3mm diameter is perfect for backpacks. They fit exactly into the hole in the backpack. Clip off the knob on the torso and place the other 3mm magnet there.

For wrists: 2mm diameter.
For shoulder: you will need 3x1 or maybe 2x2. 2x1 is too weak. 3x1 also works for terminators.

I use 3x1 for arms and weapons as well, so that I would not need to get two different sizes of magnets. Thy are a little bigger but not that noticeable after painting.

I googled "40k magnet guide" and came up with a miniwargaming magnet guide.

Covers what size magnets you need for different units, though it uses yank measurements.