screw thread questions

I recently picked up some CO2 cylinders which I want to use as the power source for small combat robots. Think flipper, hammer...

Couple pics here:

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I make the external thread out to be 3/4-27. That's something I've never heard of! Proprietary?

The things are made by the Walter Kidde Company Limited and, although unused, say "Made in England 1958", "Part No. WKA25533"

Can't find anything on the web.

Any idea what kind of fitting this is?

Tanks, DOC

Small pun intended! :-)

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Loading thread data ...

There is a 3/4-27 UNS thread listed.

Reply to
Michael Rainey

That'll be 3/4 Gas thread

Mark Rand RTFM

Reply to
Mark Rand

I'm guessing pipe thread, possiblly NPT. 3/4" pipe thread isn't 3/4" in diameter, though. Look on a thread chart that has the actual major and minor diameters.

Reply to

Reply to

Thanks to everyone who helped out on this.

The short answer seems to be that there IS a 3/4-27 thread.

The longer answer goes like this...

I took a harder look at "Machinery's Handbook" and there are more threads than you'll ever want to deal with.

They show a 3/4-27 UNS thread and suggest that UNS not be used unless other threads "do not meet requirements". I don't know what's so special about this CO2 cylinder but...

It isn't metric. The closest metric pitch is .09 and 27 TPI comes out as .094 ( 25.4/27) And 27 is the correct # (Using thread gauge.)

It isn't NTP. While NTP does have a 27 TPI, it's down at the 1/16" size.

Now all I have to do is find or make a 3/4-27 tap! :-)


Reply to

DOC wrote: ...


Do a good job: the vapor pressure of CO2 at room temp is about 800 psi, IIRC. You don't want your home made fitting popping off! Bob

Reply to
Bob Engelhardt

Forgive me for sounding like a spoilsport, DOC, but high pressure gasses can become serious dangers.

I've probably told this tale here before, but it seems appropriate to tell it to you. I lost a SCUBA diving buddy back around 1960 when he used a cast iron 3/4" NPT to 1/2" NPT reducing bushing to adapt a SCUBA valve onto a CO2 fire extinguisher bottle. He likely would have been OK with a brass or steel bushing, but cast iron wasn't up to the task and the threads sheared off while he was filling the tank. The 1800 psi air blew the valve up through his chin and into his brain.

Whatever you do, If those 47 year old tanks haven't been recently tested, they, and whatever adaptors you end up with to mate with those threads, should damn well be recertified by a qualified place before you fill them with CO2. That should include hydrostatic testing in a vessel which will measure the "expansion and contraction" of the tank. Fire extinguisher shops have that kind of equipment, that's where we used to take out SCUBA tanks.

Just my .02, but if it was me I'd consider retiring those antique tanks and buying some new paintball gun cylinders for your combat robot, the fittings for those are readily available, and they're made to stand up to the rigors of being bumped and dropped by the paintball players.



Reply to
Jeff Wisnia

They look like inflators for life vests or rafts. That's about the only thing I can think of with that small a capacity. The thread is probably British, but might be something proprietary if it's ex-military.

You might want to rethink your power source. Figure out how many cubic feet of gas you're going to get out of the volume of liquid CO2 that's contained in those cylinders. It probably isn't as much as you'd like.

Stan Stan

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Maybe the printing on them saying, "Cylinder CO2 Life Jacket", gave you some hint of that.

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Hey, givem a break! I bet 9 out of 10 folks here who looked at the photo missed seeing those words there until you pointed it out to us. I know I did.


Reply to
Jeff Wisnia

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