How to measure thru hull

Claude L.-Auger · Apr 21, 2015

njlarry said:
What is the correct way to measure for thru hull diameters? I want to replace the thru hull under the galley sink. The hose coming of the seacock is labeled 1 1/2 inches but the outside of the mushroom going to the seacock measures 1 7/8 with my cheap plastic calipers. The apallo seacock has "500" marked on its body.

Diameter of hoses & thru-hulls is always given for inside. So what you are looking at is probably 1 1/2".

woodster · Apr 21, 2015

measure the outside of the threads with you calipers and choose a hole saw measuring the same way and if you have doubts drill a 1/4 inch piece of plywood scrap and see how the through hull fitting fits in your test hole

Claude L.-Auger · Apr 22, 2015

woodster said:
measure the outside of the threads with you calipers and choose a hole saw measuring the same way and if you have doubts drill a 1/4 inch piece of plywood scrap and see how the through hull fitting fits in your test hole

Good idea as the hose installed could be larger than needed and simply compressed on the thru-hull. Just remove it from the thru-hull and proceed with Woodster's suggestion.

py26129 · Apr 22, 2015

I agree with Claude's first comment. I replaced all my thru hulls and always used the nominal hose diameter when ordering thru hulls. Never had an issue.

Cheers

Matt

Scott T-Bird · Apr 22, 2015

So I imagine you are intending to remove the thru hole and replace it without changing the hole size? I would start by removing the hose so that you can measure the outside of the threads as woodster suggests. Then you will buy a new, like-sized fitting. It's a pretty good bet that the existing hose matches the diameter of the fitting, but it doesn't hurt to verify.

It doesn't make sense (to me) to attempt to change the hole size for this purpose. Drilling a larger hole where a hole already exists is a tricky business, as I'm sure you know, and I wouldn't do it on my boat below waterline unless absolutely necessary. (Actually, I had the yard do it above water line when I had to change my exhaust port from 1.5" to 2" - they seemed to do it well).

Claude L.-Auger · May 2, 2015

njlarry said:
..., pipe measurements are based on the inside diameter. So a 1 7/8 outside diameter is likely a 1 3/4 inside diameter and therefore it is called a 1 3/4 thru hull....

Same info as you received previously. Considering the hose on it says 1 1/2 and considering your own statement "measured with my cheap plastic caliper", I would still remove the hose and measure the I.D. of the thru-hull before ordering a new one ! Good luck

woodster · May 2, 2015

may be this will help all pipe thread sizes are set to a universal standard and you can look this up on the internet but bear in mind that there are different pipe sizes for the same pipe sizes ....example schedule 40 will be larger id than schedule 80...the difference will be in the wall thickness...40 being thinner walled than the 80....on most through hull fittings they will be schedule 40...the reason for this is the greater the internal pressure the the thicker the wall thickness...but sense the pressure we are working with is minimal it in most cases will be schedule 40.....example.... https://www.google.com/search?q=pip...iv&sa=X&ei=9s1EVaK1FcWrNtvcgIgN&ved=0CB0QsAQ#

mitiempo · May 2, 2015

njlarry said:
To help all future readers. I talked to John at Groco today and he was immensely helpful and patient.
Take home is this, pipe measurements are based on the inside diameter. So a 1 7/8 outside diameter is likely a 1 3/4 inside diameter and therefore it is called a 1 3/4 thru hull.
Now I understand.

I disagree.

1. The wall of a seacock in a larger size will not have a 1/16" wall thickness - too thin with threads cut into it. Probably over 1/8" thick wall.

2. Apollo Conbraco does not make a 1 3/4" seacock.

3. Neither does Groco.

It is 1 1/2" - the wall thickness will be just under 3/16"