threading

[polecat]

morning all i have a pcp air reciever i need to make a plug to seal the ends the id is 20.5mm it has a 1.25mm thread inside the tube what size do i turn the plug before threading
i dont know new to this
thanks polecat

[polecat]

am i right in thinking the thread stub should be 21.75mm dia so if i cut a 1.25pitch thread it should fit the id of 20.50 of the tube which is already threaded
thanks polecat

[polecat]

anybody if you know tell me please am i right
polecat

[RobWilson]

Hi Polecat

What pressure will this be operating  @  ?    where was it manufactured ?


If it was 22mm x 1.25   , I would expect the bore to be 20.75,,,,,,,,,,,,,,,, 1.25 mm pitch is VERY close to 20TPI   and 22mm is very close to 7/8  ,,,,,,,,, and 7/8 20 is used in high pressure fittings .


Rob 

[polecat]

rob it is a 19" falcon air rifle tube all pressure tested so no worries will check your suggestions
thanks polecat

[petertheterrible]

Will the plug be made of brass?
If your modding a pressure vessel, it won't be pressure certified afterwards, potentially a big bomb.
I have made a couple of plugs for CZ200's out off brass, sometimes even replacing the original pressure gauge with one's used on fire extinguishers although they tend to be a bit weak and even blowing out.

Remember that the vessel will swell when charged in different ways resulting in a blow out with the tube swelling more than the plug.  You should compensate for this in your design.  On the CZ's the threads are cut as a tight fit, compensating for the lesser swelling of the plug against that of the tube that is steel.

Best advice is that you provide more information as to the rifle and the vessel fitted, what gas the vessel is intended for, at what charge you are going to charge it and if you will be charging the vessel through a regulator.  Better be save than going to hospital if it fails catastrophically shooting a hole in the side of your pickup.  

[polecat]

pressure tube is off a falcon pcp air rifle normaly charged to 2700 psi from a divers bottle so no worries about it blowing up
polecat

[lordedmond]

pressure tube is off a falcon pcp air rifle normaly charged to 2700 psi from a divers bottle so no worries about it blowing up
polecat

you are joking I hope  evan at 100 psi a vessel can rupture

Stuart

[mcostello]

An aerosol can is way less than 100 psi. and it took out an neighbors eye.

[RobWilson]

Hi Lads

I think Polecats air receiver will be fine its made for the job , BUT an ill fitting threaded plug is another thing all together with roughly ,1400Lb (+/- a little bit )  of force  behind it     


Rob   

[John Rudd]

 

Hi Lads

I think Polecats air receiver will be fine its made for the job , BUT an ill fitting threaded plug is another thing all together with roughly ,1400Lb (+/- a little bit )  of force  behind it     


Rob

So in order to prove the integrity of his newly made plug, he ought to pressure test the receiver with water before filling with any compressed gas.....?

[RobWilson]

Hi John

Aye a hydraulic test to twice working pressure of the  vessel  would be the safe way to test the integrity of the plug . 


Rob 

[Jonny]

Wise words Rob, although max operating pressure is 2700psi it should withstand min 4050psi. Theres no laws governed like scuba its not a removable cylinder. I know, don't ask same results remember that Ginb 'tatting' about with a titanium cylinder that went up serves him right, had it been ok then it could have happened to the new owner or anyone else in the vacinity.

Titans later Falcons with metric threads were never tested, just the prototype.
Steel end plug decent fit I wouldn't doubt it would take 8000psi.
The weakest link is irrelevant of any reg Peter its the end plugs and or the tube. Certain English and Korean made wouldn't get through a test centres door seen rooks blow from 235bar recommended pressure 200bar then they started fitting burst discs to mask the incompetence.

Original end plugs are brass for the Titan/Falcon, 20yrs passed and fairly sure its M22X1.25 with o ring at back of thread so exposed.

Closer could test to 10000psi hydraulically no one in right mind would pump a gas in to test, fill with water and pump oil in. My worry on this replacement end plug one was the naff brass HW100 fill probe seen at 300bar registered on hydraulic unit.

Always pays to max out available thread with a better tolerance fit. Tip aluminium keep oiled or it will pick up and lock up solid, more you rotate the more it locks up. M30X0.75 top original rated at 200bar, factory initial prototype test 300bar German rated, below exceeds that.

[polecat]

thank you everybody i will proceed carefully
polecat

[David Jupp]

Unlikely to be an issue on something of such modest diameter, but be aware that lots of pressure equipment is never tested to as high as 2xMWP these days - it all depends on the standards applied.

If not intended to be used at elevated temperature, original pressure test might be as low as 1.43 x MWP, or it may never have been pressure tested at all.

Always arrange any pressure test based on the assumption that failure may occur.

[polecat]

7/8" x 20 tpi is the thread size
thanks again
polecat

[petertheterrible]

With steel tube receivers, which I prefer for target rifles as I like a weighty bull-barreled rifle, I like to use brass hex stock for the end caps.  It saves me to mill spanner flats or drill holes.  Another feature is to make it idiot-proof by fitting a burst disk.  Don't know about you guys but for extra comfort I charge through a regulator. 

I use industrial mixed-oxygen bottles for charging, much easier to come by than scuba bottles and more convenient than the compressor.  Just please don't use the purer oxygen or O3 as spontaneous combustion may occur, Co2 is more stable for consistent shooting but haven't seen such a remarkable difference in using air.

[Jonny]

Polecat if its a Falcon its metric assured, the earlier Titan were imperial.

[vtsteam]

Just a reminder. In a test, yield needs to be checked, too. Catastrophic failure isn't the only consideration.

[RobWilson]

7/8" x 20 tpi is the thread size
thanks again
polecat

Good to hear that you now have the correct thread size Polecat 


Rob

[Jonny]

He don't Rob as said its metric since 94 or 95 when became Falcon.
If it actually is imperial its not a Falcon, its a Titan pre 94 both look very very similar. Titan breach loading area is angled, Falcon horizontal flat.

The M22X 1.25 end plugs are undersize which would amount to the 7/8" measured.
A mate had the metric tap given to him for remedial work by a Falcon director now at AA.

Check the thread again.


Peter why would you need to crank the end plug up, finger tight ample!
CO2 is for low pressure else it will freeze above 120bar well below operating pressure of air. Scares me the seriously thin walled tubes used (burst pressure circa 150bar) for CO2 and idiots running them in excess of 200bar air.
Most decent target rifles are aluminium tubes with 10yr life span as a removable cylinder.
Choice of 6 scuba centres within 15 mile, 5 of which top up to 300bar.

[airstream]

 Titan and Falcon threads are 7/8 x 20 tpi. not metric 22x1.25. close but no cigar

[Arbalist]

I'd let someone experienced do it. Not sure where you are but a guy up north did some work on one of my airgun cylinders, if I remember right his name was Mick Tromans. Did a great job and only cost £50. There was also a machinist that used to shoot at my club who made a new piece for the rifle end of the cylinder from stainless steel and only charged me £25 so there are folks out there.