Ok, they're Harken "Small Boat Blocks", but we use them as Paracord (or small diameter rope) Pulleys. 550 paracord usually runs around 4mm. Some 4.5mm. Max diameter cord/rope for the 16mm blocks = 5mm. Max for the 22mm blocks = 6mm.
We don't think the "masses" are out there searching for Paracord Pulleys, but if you find yourself in that camp, let us save you some time. a) You're probably a pretty resourceful person and b) these pulleys are among the smallest, strongest, lightest and best quality pulleys you'll find.
Our prices are set at the lowest prices Harken allows.
You can load these pulleys (see specs) and they are still smooth as butter. Super lightweight, small, durable, and functional.
Use two double blocks (with becket on one), gain a 4:1 mechanical advantage (MA). Lift 100 lbs with approximately 25 lbs of force. Flip it upside down, pull up on the "working end" and you will gain a 5:1 MA from the same system - all from a system you can carry around in your jacket pocket.
Work smarter, not harder.
(carabiner and paracord not included)