photo sharing and upload picture albums photo forums search pictures popular photos photography help login
Compass Marine How To | all galleries >> Welcome To >> Replacing Thru-Hulls and Seacocks > Dry Fitting The Seacock
previous | next
07-JUN-2007 ©Hale Kai Inc.

Dry Fitting The Seacock

Now that your backing plate is installed and cured the first thing to do is to drill your hole for the thru-hull. This is made easier because you have already centered it over the old hole and already have a pilot hole in the center of the backing plate left over from the large hole saw you made the backing plate with. Now you simply choose a hole saw, to match your thru-hull's diameter, and using the existing pilot hole, drill your new mushroom-head / thru-hull hole.

Once that is done you'll need to thread the thru-hull up into the flange and measure it for length. In most cases the thru-hull will be too long and need to be cut to size. Cutting the thru-hull stem is easy.

1- With the thru-hull bottomed out into the flange measure from the outside of the hull to the mating side of the mushroom-head and add 1/4 of an inch.

2- Insert the thru-hull into a vice carefully clamping the threads that need to be cut off NOT the threads your keeping.

3- Using a hack saw cut the thru-hull to its required length.

4-Clean the threads you just cut with your drill and brass brush.

Now that you have the proper length thru-hull, re-insert it into the flange and tighten it down by hand. Climb back into the boat and adjust your seacock to align it with it's hose for proper orientation. Once you have everything aligned, and situated where you want it, drill your flange bolts holes, one at a time, inserting a bronze bolt after drilling each hole. Inserting a bolt, after drilling each hole, guarantees proper hole alignment by the time you get to the last one! Doing it by eye, without bolts, leaves room for the seacock to twist and a potential fit failure. There is NO room for error with fiberglass like there is with wooden backing plates and all bolts must be 90 degrees to the hull and perfectly centered and aligned for a proper fit.

The next step is to countersink the machine screw heads into the hulls exterior. Again having the proper tool, in this case a countersink, is imperative. Not only must you use a countersink but it needs to be the right size for the silicon bronze slot head machine screws! Using a regular drill bit for countersinking is a HUGE NO, NO in this situation! In the picture the holes have been properly countersunk and the through-hull hole, in the hull, is over an inch thick of solid fiberglass.

Nikon D70s
1/250s f/4.5 at 50.0mm full exif

other sizes: small medium large auto
Mike 26-Apr-2015 11:47
The aviation industry has tools that assure the countersink is perpendicular to the surface but beware that the angle of the heads on countersunk bolts is different in the aviation industry. Great website.
edward 09-Jan-2014 10:13
Hi what do you do if your existing seacocks are wider than the new ones and you do not wish to widen your own existing hull? Cheers Ed
Guest 01-Jul-2013 21:40
How do you know if the countersink you select is the right size for the machine screw heads? How do you select the right size?
Tomas 14-May-2012 13:25
It is paramount to have bolt holes at exactly 90 degrees and in parallel to the fitting hole but how on Earth can you do that? What tool should I use to make sure it will be in parallel and at the right angle? When doing this by hand I cannot guarantee that.
I'm just doing that, have the backing plate fixed and with the centre hole but the only side I can do it is from the outside which is angled, so I have no reference to the 90 deg.
How would you approach such?
Fred 05-Mar-2009 22:36
Thanks this is a great website. Whatever you do, you must be good at it.
I just bought a book on ship maintenance, but this is a lot better!
Dan 09-Jul-2008 21:10
Shoot - I didn't see the 2nd page where you describe how the bolts are sealed. Sorry. But I still wonder what you think about screws instead of thru-bolts. Obviously, not as strong, but strong enough? -Thanks
Dan 09-Jul-2008 20:24
How are the flange bolts sealed? Is it acceptable in your opinion to use screws which thread into the backing plate and are inserted from the inside of the hull? That way there are three less holes in the hull. -Thanks