This is a 60mm (ish) pulley for 12mm rope. I use these a lot on my sailboat and they break when you least expect it, I thought I would see if I could print one and see how long it lasts!
This design uses an M8 35mm through bolt, 22mm 608zz Stainless bearings (skateboard bearings) and a 6mm wide jaw shackle. I used stainless as they last forever on a boat.
I wouldn't warrant this is as good as a Harken block! But it takes my weight fine and seems pretty good. Early tests showed weakness in the wheel on extreme weight (which would split) I have tried a nylon one (which worked fine) and also tried surface treating the wheel with thin superglue, which also seems to work fine, but ultimately this is a light use guide pulley only.
Plenty of cooling needed for the wheel or it will sag on the top lip
The two cheeks and the block are best acetone welded together. Sand the top of the block to get rid of any printing marks, dip in in acetone and place it on the cheek lining it up nicely. I then do the other side and bolt it together, leaving it for half an hour to weld. sand it smooth over the weld and with the shackle it seems as strong as it needs to be.
The block responds well to sanding and an acetone wipe and becomes a lovely thing to simply hold!
The wheel is designed to be very slightly oversized, such that you sand the faces on a flat surface (particularly the top which will have some roughness to it) then once the bearings are in fit it and rotate it in the block, sanding the outside edge, removing any roughness and imperfections in the print, once done you get a really nice fit to the block.
22mm bearings press into the wheel, one either side. Use a vice and a piece of card to protect the print. I've also done a plastic bearing to use whilst waiting for the Stainless ones, it doesn't last long in use, but it does to line things up.
Before and after sanding