We use small blocks on our downhaul and medium becket blocks on our snotters for many of our Cat Ketch rigged designs. The block pictured is a Ronstan 20mm single block (RF-20101). The small end of the s-hook must be opened up to accept the block and then reclosed. The strap of the block must be squeezed slightly in order to rotate smothly in the closed hook.
What you will need:
-Bench vise or C-clamp and scrap wood (pictured)
-Pair of vice grips or the corner of a bench vise.
-A medium to large size adjustable wrench
Clamp the S hook with the long side down in the vise or between two blocks of scrap wood as show.
Use an adjustable wrench to twist the top of the round part of the S-hook to the side which will open the hook.
Use a pair of vise grips or the corner of a bench vise to squeeze the top strap of the block as shown. This will narrow the straps inner radius. The radius of the strap should be smaller than the radius of the inside of the s-hook for it to rotate smoothly.
The block on the left is unmodified while the strap of the block on the right has been squeezed.
Test fit the block on the s-hook.
The plastic body of the block may rub on the s-hook. If this is the case, squeeze the strap the other way to increase the height of the strap a tiny bit.
Once the block rotates freely on the s-hook, clamp the hook back into the vise. Almost finished.
Use a wrench to twist the s-hook back into a closed position.
All finished. The block should rotate smoothly on the s-hook.