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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Valve stems, material, hardness, diameter?

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rkr:
Since my 2mm stem valve for .257 Evanix was a great success I started thinking of small diameter stems for other guns as well. Starting with the material and hardness, is HSS good material what HC hardness it should have? Aliexpress has lot's of HSS rods with different diameters and stated HC varies from 61 to 67. Then comes the stem diameter, 2mm seems to be fine for a 7.5mm valve tunnel. My .45 has 10mm tunnel so I guess it would need larger stem, 2.5mm or 3mm - what do you think? How about small diameter valve tunnel, could we use 1.5mm stem in a 6mm tunnel? I'm trying to figure out the limits of this small diameter stem approach, feel free to contribute.

rsterne:
I usually use O1 steel rod for my valve stems.... I have never bothered to harden it, and I have never seen it bend or peen over.... I don't know what the limits are on how small you can go.... You already found one problem, with your small stem penetrating the PEEK poppet head.... I often thread the end of my stems into the poppet to avoid that.... When you talk about using very small stems, remember that the resistance of the stem to bending changes with the 4th power of the diameter.... Half the diameter is 16 times easier to bend, for the same length....

Bob

Motorhead:
While there are some balancing ratios where the stem plays a part ... that alone does not dictate stem size in my tuning.
Driving the stem ( No matter its size ) threw the poppet head becomes an issue when poppet head over the throat increases and stem is kept small.  There is so much effort to open said bigger poppets the stem becomes a nail and can punch its way threw the poppet head.  With smaller throats and poppet heads, small stems can and do work just fine.  * Pressure being run at contributes greatly to the above paragraph.

Stem Hardness .... What are you striking it with ?  Peek face strikers can operate against milder / softer steels.  When hammer strike face is metal / steel, the stem needs to be harder to prevent mushrooming.

JMO .. sharing thoughts.

Scott S

sb327:
Is the smaller stem diameter wanted for more flow in throat or less closing force?

Some of the driving through large poppets issue can be resolved by making a steel cup that sets in poppet.

HSS would work but can be a pain if you want to turn it down. A simple cheap source for longer pieces is extended length drill bits. I’m not sure of the hardness but they do resist bending much better than most alloys. It is pretty tough and forgiving. I think it would work well.

Dave

rsterne:
A smaller stem reduces the closing force.... If the throat is marginal to allow full flow around the larger stem, then you could also get an increase in flow.... Using both ideas together, using a smaller stem allows a smaller throat and poppet, and hence also a reduction in opening force.... This can lead to a further reduction in closing force from reducing flow friction past the smaller poppet head....

Bob

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