Vintage Double Hose

ScubaLawyer wrote:Ok you machinist types out there. I started looking for a bottoming tap to clear the threads on my SW elbow for the new burst disk.
I have confirmed it takes a 3/8-24 and have found many taps that size. However, it looks like some taps are UNC (course) and some are UNF (fine) both in the 3/8-24 size. Being the perfectionist I am, I'd like to use the correct tap. Here is my question: Can anyone point me to a source that will tell me if 3000psi 3/8-24 burst disk threads are UNC or UNF? maybe it doesn't make a difference? I've been researching taps until I'm blue in the butt and I figure someone here might know the answer. Thanks. Mark

Taps, Left to right: 3/8-16 (UNC), 3/8-24 (UNF), 3/8-24 homemade bottoming tap I should have added this photo to my previous post describing the thread differences.

Thanks Bill, but I'm still confused. Do I need a bottoming 3/8-24 UNC OR UNF for this job?


NEVER MIND - DIDN'T SEE YOUR EXCELLENT POST ON PRIOR PAGE. YOU ARE THE MASTER!

Marl

Here are some options.

From McMaster Carr (my first option)
https://www.mcmaster.com/#standard-taps/=182kfyp
You can get a thread repairing bottom tap for $3.82

Or you can get a 3/8”-24 UNF tap set for $20.14. It includes the bottom tap.
https://www.mcmaster.com/#standard-taps/=182kig0
https://www.mcmaster.com/#standard-taps/=182kfb1


If you have never ordered from McMaster Carr, her is an alternative:
https://www.amazon.com/Century-Drill-To ... bootom+tap

Just be careful. You are just trying to chase the existing threads, not cut new ones. And like BT said, you don’t want to scratch the bottom sealing surface.

I suppose it's too early to bring up Heli Coil PN 5528-6? In a few of my burst disc holes the soft brass threads cleaned up (a little) but the plug was sloppy in there. Just haven't gotten around to putting Heli Coils in there. I don't see any reason why they wouldn't work as a last resort just in case but there's something about a failed insert leading to rapid air loss so I just kind of shelved that idea for now.

I would be concern about corrosion with a Heli Coil. The Heli Coil would be exposed to salt water and I know they are stainless, but I don’t really like stainless in salt water. It may work OK if you install it using some sealing compound. I don’t know if that can be done.

I wouldn't attempt to use anything other than a flat end bottom tap. If a bolt will start into the thread so will a bottom tap. Just be sure to keep it straight.

https://www.mcmaster.com/#26955a94/=183erwr

Thanks Captain, Luis, BT et al....
I found a local machine shop/ hardware store that has a 3/8-24 bottoming for $6 so I'm off to pick it up this afternoon. Will report back. Mark

UPDATE: Got bottoming tap. The f'n thing actually worked !!!!! The burst discs screw in just fine now. No play in the threads at all. I love it when a plan comes together! Next step is assembling the twins. I'll keep updating until I'm told to sit down and be quiet.

What size o-ring goes on a Sportsways 1/2 inch straight valve? The one I have is too thick and extrudes when I tighten valve to tank. Thanks. Mark 

Ok, thanks to everyone who helped on this project. Special thanks to Ron B who had the seat in an old SW valve; Our own BT remembered that the 1/2" straight thread Sportsways valves took #908 Boss O-rings only made for tractor hydraulics. Thanks Bill! Found them on-line. Cost me $2 for 50 of the suckers. If anyone needs some let me know. Gotta make my money back somehow. As for the bottoming tap I found one at a local industrial tool supply company for $2.12 including tax. Worked great. Cleaned out the threads on the burst disk openings and the new burst discs installed without a hitch. Now that I have my 38's together I'll start a new thread to display them. Here is a pic of the tap doing its job.