Gate Valves Going - How to cut thru-hull?

[Tom-in-Annapolis]

I'm in the process of replacing my gate valves and thru-hulls. The gate valves looked good on the outside but the first removed showed that the valve had rotted through.

The thru hull is bottomed out with the seacock so I need to cut the thru hull down to size.

What is the best way to cut that thru hull to size evenly and without ruining the threads?

Thank you,

Tom

 

[Nodak7]

I'm in the process of replacing my gate valves and thru-hulls. The gate valves looked good on the outside but the first removed showed that the valve had rotted through.

The thru hull is bottomed out with the seacock so I need to cut the thru hull down to size.

What is the best way to cut that thru hull to size evenly and without ruining the threads?

Thank you,

Tom

Tom before you cut that gate valve out I would suggest you remove the bonnet and gate. When you do that you should be able to see the threads on the water side of the valve. Apply some penetrating oil (WD-40) to the threads and let it soak for awhile then try to turn the valve body off. Repeat the process a few times.

If Option 1 above does not work the next would be to cut the valve body in two being careful not to hit the threads. You can use a dremel tool or the like to do that.

IMHO the last resort would be to cut the thru hull out.

Good luck

 

[Tom-in-Annapolis]

Thru Hull is Out

I have the thru-hull out and have a new one ready to go in. I did have to cut the thru hull out after two hours of trying everything else. Took 2 minutes to cut it out with an angle grinder.

The thru hull needs to be shortened so it will not bottom out in the seacock.

What is the best way to cut that thru hull to size evenly and without ruining the threads?

Thank you,

Tom

 

[justsomeguy]

If you've got a spare nut for your through-hull, you can thread it down tight to the other nut, locking it in place just below your intended cut line.

Now you've got a perpendicular face to carefully cut along. When you remove the locking nut, it'll chase out the roughened threads somewhat, then you could bevel it a bit more with a file.

I think.

 

[Sailvayu]

You can cut them with a hack saw or cut off wheel, what ever you have available. I then use a small grinder or disc sander to clean up the cut and then bevel around the cut at about 45 degrees. Finally it helps to use a wire wheel to just smooth things up a bit. It does not have to be perfect and as you are working on them have a seacock handy to test the fit. If you have an old thru hull nut it can help to thread that on before cutting and after you have made your cut take it off, it will clear burrs as it comes off. Like I say it does not have to be perfect as long as it starts easily.

Capt. Wayne Canning, AMS
www.projectboatzen.com

 

[Allan12210]

I have a thread restorer file that is useful for chasing external threads that get chewed up. Bought mine at Enco Tools. You just need to know how many threads per inch so you can buy the right one. Each file has 8 different thread pitches.

http://www.use-enco.com/CGI/INSRIT?PMAKA=990-3074&PMPXNO=3011338&PARTPG=INLMK3

 

[Charles Erwin]

It is not clear what you mean by the through hull is "bottomed out" in the sea cock nor how shortening the through hull will improve things.

The seacock - in your case a gate valve - has internal threads cut NPT. That means the valve expects to screw onto a pipe stub that has tapered threads. The through hull has external threads cut NPS. That means the through hull threads are straight - not tapered. In short, you have a thread form mismatch which you cannot fix.

If you are interested in how to do this job - including how to avoid thread mismatches between these fittings - please read these explanations:

http://www.pbase.com/mainecruising/replacing_thruhulls
http://www.pbase.com/mainecruising/seacock_primer

If you do not want to make up your installation with correct thread matching - in other words you just want to screw a valve on a through hull - at least use a bronze ball valve and a through hull threaded with modified or "combination" threads shown at

http://www.groco.net/SVC-MAN-07/Sec4/PDF/IBV-FBV-TB.pdf

Charles

 

[Erieau]

I used a Dremel with cut-off disk. It was easy to make a reasonably true cut.
I had to do it twice, as the first cut was still three threads too long. I was overly cautious.
You've bought a ball-valve sea cock? A previous poster seems to be assuming you're replacing with a non-marine valve.

 

[DianaOfBurlington]

Seacocks word to the wise.

To answer this I would need to know which seacock/through-hull you are using. With Marelon ones we just cut them on the bandsaw. I have cut them with hacksaws as well. I've never threaded a nut on first-- never needed to with Marelon. I do what Wayne suggests and knock off the edges of the raw threads with sandpaper to ease the assembly.

Cutting a little short is better than cutting a little long, of course-- once you get out the 5200 you're sort of committed and it's a mess to return for more sawing. The Marelon OEM variety are meant to be seated against the hull and held there by the threads; so the more inside the better. They do not generally require Teflon tape but it can't hurt.

The Marelon aftermarket variety are made to use three bolts into ears in the flange. Most metal ones are as well. These are crucial too and should be matched to the material-- bronze on bronze. Do not torque them completely on the first day but allow the 5200 to set a few days. (This is why Forespar make OEM-spec seacocks, to expedite production.)

The best backing plates are of fiberglass, bonded with either 'glass or 5200. If using 5200 these can be installed during the same process as the seacock and through-hull. Make sure all fits work when dry first.

Avoid anything like a ball valve that screws onto a separate through-hull after the through-hull's been installed. I have a Marelon one like this and did use it; but though Forespar promise it's plenty strong enough it's not how I'd have preferred it. It's better to have the security of the seacock's base against the backing plate.

Also avoid ALL brass through-hulls, gate valves, and anything from Lowe's or Home Depot for anything that might be ever underwater (meaning everything on the outside of the boat). Your only sensible options are Marelon or marine bronze, properly installed.