Instead of trying to form a good seal on a surface within the valve body I am making the valve seat out of stainless and it will sit in the body, but can be removed/replaced if needed. You will see in the photo an FX Bobcat valve stem and seat, and next to it my valve seat with O ring groove cut into it and the valve body behind.

Next stage is the valve stem with sealing surface.

My plan is to make the pin out of a screw driver shaft and then use valve grinding paste to bed them in with each other.

That's the plan.

I could make the whole pin and seat out of stainless, but I wasn't sure how well it will handle being struck by the hammer, where the screwdriver shaft is hardened.

If you need to machine the valve then a steel than can be annealed and hardened is good.

I've a used Class 10.9 or was it 12.9 bolt as they are strong but still machinable with HSS tooling.

The screwdriver shaft is not hardened only the tips are hardened steel. and you would find it very difficult to machine the hardened steel anyway. you will need grind it.

If it was me doing it i would be looking into 316 stainless for the valve and bronze alloy or brass for the seat. I have both here if you want to play with that. I have 10 and 20mm round 316 bar here. can easily make what ever bronze alloy you want.

Just a thought.

Of course, hadn't thought of brass or bronze. That would make sense.

With the screw driver shaft I don't need to change the diameter at all. I just cut to length then harden it. Giving it a polish afterwards.

I have about 1% metallurgy knowledge, but heat to cherry red or non ferrous state and quench in oil.

I have some 8 and 10 mm 316 round bar, but might take you up on the bronze alloy. Would only need a few small pieces.

In the past, I have used delrin on the shaft which has worked well, but I was thinking of keeping the HP side of the valve stem to a minimum using metal which means the metal head can be made smaller and more streamlined.

"Therefore you can reduce both major closing forces on the valve:"

1) reducing the static pressure on the valve stem

2) reducing the dynamic pressure on the valve head

I am trying to make as much as I can, but will use a Crosman trigger frame and barrel. I have a steel breech which will save a lot of time, but would like to make a custom breech with a straight pull bolt that has a locking action rather than drop into a slot. very similar to my last build. That way I could get a little creative with open sights maybe.

It'll be .22. I do have a .25 cal FX barrel off an old Bobcat, but of course it'll need chopping down, but had thought about using that for a rifle build.

nunga wrote: If it was me doing it i would be looking into 316 stainless for the valve and bronze alloy or brass for the seat. I have both here if you want to play with that. I have 10 and 20mm round 316 bar here. can easily make what ever bronze alloy you want.

Hi, I don't think 316 would be hard enough for the valve stem/shaft, well that would depend of course on how hard you need to hit it and the diameter. FX valve stems (among others) have been known break

A thing about dissimilar metal corrosion is it depends on the ratio of surface areas of each metal.

A stainless seat in an aluminium airgun could have no issues but an aluminium seat in a stainless airgun

would be at high risk.

Look up the galvanic series and make sure that the most reactive metal makes up the vast majority.

I find 385 brass to be very good for seats. It machines and polishes well. Bronzes can be more tricky.

Thats because 385 brass as a lot of lead (around 3%) compared with other types of brass which makes it very good for machining it is also know as free machining brass. But it is a lot softer as well so i dont know how it would handle been used on HPA valve seat.

If you have had good results using it BC_Clark then i would be happy to mix up a batch for you paul, like i said i have everything here and i will be smelting other stuff this weekend anyway.

Some of my most powerful and highly stressed valves have been with 385 brass seats

so I can say that normal HPA usage is no challenge.

Doing pest control I was always up in ceilings and sparkies often forget stuff and they do like to have expensive screwdrivers which are decenyly hardened. So I have a couple I can wreck. So this will be my valve stem.

NZ Post has botched up Nunga's foundry delivery, so valve on hold. I bought a piece of 3.9 mm piano wire today. Pretty hard, and hopefully harden up, but I might try that for the valve stem.

I also decided to use the .25 cal FX barrel I had. It was off a Bobcat. I cut it down and reduced the breech end to fit into what will be the breech block for this build. Fits nice and snug with no movement.

FX only rifled the last 2 or 3 inches of these barrels and you can see that they make the muzzle smaller diameter to do this, then thread on a piece to match. You can see the marks on the outside where they "squeeze" the rifling in giving it a "smooth" twist.

After it was fastened I finished off the seat and polished it so the it was concentric with the shaft. I am quite pleased with how it turned out, but of course being pretty doesn't mean it works.

I also polished the valve seat. I have some of the 3M polishing cloths, 800 up to 15000 grit. It is satisfying working through them and seeing the finish improve.

Silly question, You are going to lap the seat in right? That brass looks nice but is it just the picture or are there a lot of machine marks still on the face of that? hope it seals.

Nice work! I will get the other brass sent out again for you anyway and you can use it on another project if you dont need it for this now.

Nunga, this is kind of a test run. I don't know how well this will work and so the good thing is that I can take the whole valve system out and replace it, so I will use your piece of brass because it was made for it. It means something to me to use the piece cast especially for it. I can make the mistakes on this first one.

So I got the barrel fitted then milled out the loading breech. then drilled and tapped two retaining grub screws for the barrel. Next I chamfered the corners on the top. I'm not totally sure what I am going to do here, but have been toying with keeping it with clean lines and mount a small red dot. I could also fasten a weaver rail to it The result emerged from the aluminium mess.`

Now that the barrel is fitted I was able to start on the bolt. This turned out really well and confirmed that the barrel and bolt are nicely lined up.Alu had an 8.5mm bore and I reduced the stainless bolt to 8.3 mm at the back and 6.03 mm for the probe part and it slide into place beautifully, so I am very pleased with that.

I had also made a preliminary mockup for a fill valve to include a gauge. This is not the final piece, but I know what I want to do. Gauge is mounted into a boss that I plan to have a barrel support fitted to. Fill probe fits nicely underneath it. I just need to get a 10mm x 1.0mm thread tap for the gauge.

Also see the finished firing valve body, poppet and seat. They came up really well and I am excitied to see how well it works. Looks good for now anyway. Note the custom Nunga forged brass wrapped in my hori custom made spring.

Unfortunately the wood bit isn't an exact shape with a flat spot making the resulting cut a bit like a flattened dome, so I cut a chanel out with the end mill so there's good contact. I will use a bit of fine sandpaper on a tube to take the sharp edges off the cut later.

So you can see what look like corrugations in the cut face. This is probably from my small mill picking up vibrations, but for this purpose it's fine and the light makes it look worse than what it is.

I got myself a proper drill and tap set. Amazing what a difference it makes having the correct drill size, especially when tapping stainless.

I remade a new fill valve with protection for the gauge. The fill valve is a simple but effective system. I milled off some threads from the screw to let air past and the O ring stops it getting out.

Breech has been drilled and holes tapped to secure it to the tank/body.

Cap screws sized and retaining holes drilled, tapped and counter bored for them to hold in the fill and main valve body.

Basically I took off the knurling so there was an exact fit.

Here you see the stainless probe being tapped. I learnt that with the harder material you need the correct drill bit size.

A std drill set has a 4mm drill bit, but it is most likely 3.9 mm, but a 4.2 mm drill bit is required for an M 5 tap. How many taps did I break learning this?

New tap set has 3 taps per size. Tapered, intermediate and bottoming. So you need to run all three through to get the final size hole. Fortunately I am enjoying all this learning.

The bolt handle will be a straight pull, being retained by a sprung clip. This way you can also close the bolt by the handle, or by pushing bolt with your thumb. Bolt just temporary for now.

I plan to feature a bit of nice wood on this too. I have some walnut for it. This is a mockup with a grip I had made ages ago, but I don't really like it. I will put some wood under the tank too. That should hide the industrialousness look of the tank. 

Excuse the mess. 

I found this video on youtube, might be of some use.

https://www.youtube.com/watch?v=cWCf2Tp1Pic" rel="nofollow - https://www.youtube.com/watch?v=cWCf2Tp1Pic

I would be keep to give the threading a go as opposed to pinning the valves. I am cautious with all that pressure, so until I have seen the correct calculations from a competent person, I wouldn't try it.

yeah you need to be very careful with the threads. Pinning is the safer way to go for sure.

So 3000PSI equals 210.9KG of force.

I also bought myself some decent metal cutting paste too.

I also started on the grip, doing a trial run using some birch ply glued together. I plan on making the grip from some walnut I have. So I'll update that once I get into it properly.

I got RTD metal cutting compound. Its a paste that sticks to the bit as opposed to flooding it. 

Appreciate the help guys.

What a relief, I was starting to hate stainless.

Looking good Pauly. Pity about the radius cutter not being as advertised.

The inlet screw does not really need the flat down the side and it causes more stress on the o-ring

to force it around a non-round shape. The air flows past a regular screw anyway

but if you must fill faster you can get a similar effect by oversizing the tap drill to 4.5 for M5 and 

leave the screw standard.

When you have stainless parts with a lot of machining time in them and the fastener

isn't working close to its capacity, it is advised to use larger tap drills to avoid broken taps.

Just adding 0.1mm can go from from walking on ice, to a walk in the park.

Anyone not having fun with stainless should try 303 grade for machining.

Not only stronger than 304 and 316 but much nicer to machine.

The main thing with drilling stainless is high feed and lower speed.

It is not the heat that hardens stainless. Regular 304 and 316 are non heat-hardenable.

They only become softer with heat. But they work-harden very readily (316 moreso).

So applying too little pressure to the drill causes it to skim along the surface

without starting a chip. This work-hardens the surface you're trying to drill.

The problem with heat is that it blunts the cutting edge of the drill

which leads to work-hardening of the stainless, accelerating the issue.

HSS with cobalt can handle higher temperature.

Here's  photo and it's first chrony reading. there's still a lot to do, but it's nice actually firing it now. 

Just over 15 ft/lb at around 1200 psi in 25 cal. 

I had been struggling trying to find a leak. At first I thought it was a line on the inside of the tube from production, so I honed it with 300 grit on a dowel in the lathe. It wasn't this, so I took a serious look at my O ring grooves and I had left a small ridge on the sealing side of it. removing this  seems to have done the job.

I reassembled the pistol and fired a shot over the chrony, and very interestingly it only read about 4 fpe. I figured this was due to it being too close because the next shot, from further away was better than I expected at a healthy 19.77 fpe. I still have to make the lockup device for the bolt/probe, so I am holding the probe in to fire. On the next shot I got 20.98 fpe.

 I have learnt that making sure there are no leaks in the transfer port area has a big impact on the power output and you can loose a lot of energy through leaks.

I had cut a tapered lead into the barrel and I need to get a slightly thicker probe O ring too as it gets blown out of it's groove, so once I get that and the lock up sorted I am likely to see over 20 fpe for this .25 cal  puppy.

I don't know how accurate it will be, but I am very happy with the results so far.  26.23 grn pellet @ 600 fps

I have made a start on what I call the cocking plate. This is part of the bolt lockup very similar to what I have done on another build.

Bolt will be straight pull, but be spring loaded and will lock into and locate into place being held captive when loaded.

It just a piece of mild steel, but I squared it up in the mill and countersunk the 4 screws that will hold it in place.

I fired about 6 shots over the chrony and got a fairly consistant 21 - 22 fpe from a 26 grn pellet. Next I am looking for a compact red dot/holograph sight for it. I won't make rails, but will attach the sight directly to the breech.

Here's a link to a short video.  https://youtube.com/shorts/naeNdHIvOVo?si=UgD87Ts6zoEL3e7s

But, I did beef up the hammer length by adding an adjustable striking surface. I filled it to around 1500 psi and it pushed a 25 grn pellet at 706 fps, which equates to just over 28 ft/lb. I don't think I will get many usable shots at that, but I am really pleased with that as a result, as it means I have made the valve breath well enough to do that.

I have a small red dot to mount on it now and accuracy testing to come next.

I will load photos when I can.