GROCO Installation Tool Thru-Hull. Slide the thru-hull fitting flanged end first over the tool. (It will stop at the proper step.) Tie a length of monofilament to the eye and place it into the already-cut hole in the hull of the boat. Apply caulking to the flange of the fitting and pull the fitting through the hole and against the hull from inside the boat. Thread the mono-filament through the locknut or seacock. With the fitting pulled firmly against the hull the locknut or the seacock may be screwed onto the fitting.THT-530 fits GROCO Thru-hull Fitting Sizes from 1/2" through 3" Specifications for GROCO Thru-Hull Installation Tool: Manufacturer: GROCO Color: Black Package Contents: GROCO Thru-Hull Installation Tool GROCO THT-530: Installation Tool Thru-Hull