Running around in a Beeman Chief circle...

[Ribbonstone]

Valve retention screw change on the Chief:

Would have thought it was simple.

All I wanted to do was swap out the 3 Phillips head screws on the Chief's valve tetention for something of known quality.  NO INTENTIONS of boosting pressure, so didn't counter sink the screw heads into the valve body.

Bob's recomendation of these screws was correct.  Right length. head size a good tight sug fit to the existing 7mm holes in the tube.  Fit really well, removed the little bit of "slcak" in the issue screws, and while the issue Phillips head scres COULD HAVE been of a great toughness, I'd rather have screws that are of a known toughness.



Anyway,,..three screws, one at the bottom...and two on the sides...and as I was NOT counter sinking them, the heads stuck up higher.


Degass the rifle.

Loosen all the screws that hold the valve i place.

Pry the screw holes to visual center of the tube holes.

Replace screws.


I did think of lathing the heads down, getting them flush (even if I had to slot them).  But I'm not really in love with the Chief stock.,,,know it will need a total refnish by summer time.

So I just nothced the stock into a "Frankenstien" (Yeah...leave room for the neck bolts).




Yep...previously moved the barrel band to inside the stock (can search out why, but basically it kept the band and POI/POA from shifting when hit a moderate lick).


Was just shooting it STOCKLESS, from bags, over a chronograph.  Stock was NOT IN PLAY...just wanted the chronograph numbers.

All should be good right?


NOPE..it most certaily wasn't.


First test was a diasater.




Rather than tear into it again, I THOUGHT ABOUT IT FOR AWHILE (day...day and a half).

(Thought process)

What changed?

Well it could have just been unstocking/degassing/loosening-replacing screws.

Or it could have been that minor shift forward of the valve with the bigger heads  better filled the tube holes.

Sersioulsy? That just seems too small a difference. Phillips head screws ran about .265" at the head.  New swers fill the holes nearly prefectly, and run .275".

So at best, that would have shifted the balve forward by oly .005 (OK..due to other factors, by .007").

That's not "JACK"...the striker woukld just move .007" farther forward when fired.


OH..that's not quite right.


There is a steel ring between the valve and the striker (it takes the stcok screw from the bottom and the forward breech screw from the top).


SOooo...wanting to test this theory....I took it aprt and got that steeel rin in my nads, then filed/stoned that ring down by .008-.009" (it's not quite dead flat).


Tried it again.



Which doesn't really proove anything.

Could just as easily have been the radom nature of tolernces stackig up one way or another in a normal take down.  PCP's are not hand fitted/uniquie bits of tha airgunsmith's art...are built with tolerances (slack) in the systems, and that "slack" can stack up one way or the other.


But I'll take it. 


Puttig it back into the stock for accuracy tests, had to re-inlet.  Not just the side "frankenstien" bolts, but that bottom bolt.  Located it and drilled a 9mm "hole" for it to live in (large enough that it wouldn't contact wood).





Not a "cold" guy...live in the deep south for several reasons (New Orleans/Cajun/Creole food is certainly on that list...but being warm is right up there).   The only coat I own that's warm enough has duck blood staining the pockets.

REASONABLE OVER FILL:

It's like 74-76F in the house.  It's like 34-36F outside.  Thats like a 40F difference, which works out to something close to 200 PSI.   So I filled inside (75f) to about 2150-2200psi....when it had rested outside, was more like  1990-2010 psi when the shooting started.


OK...over all...it's NOT LIKELY to be worth the doing.

Does make me feeel better, just becasue the screws are of a known value, and while the threaded scrction is not a great safety ratio...at least I now the current screw's worth.

I had to readjust/retune just for a screw swap.

Becasue of the screws, also had to make some beddig chnages to get it to shoot "right".


BOTTOM LINE:

I'd not do it.

Not unless youy recess the screw heads into the valve body and intend on higher than 2K pressure.  MAYBE the isse screws are &^^&, and maybe they will show signs of problems....but until they do, I'd not bother.


Basically....I ran around in a circle.

[canadian_shooter]

Maybe, but now you know. And now I know too. Thanks. 

[CraigH]

I had planned a topic on this but it is now started.   (all which follows assumes 2000 psi fill pressure)

I have two questions:

1. Notwithstanding the poor design of the factory screws on the Chief, what is their strength?

2. McMaster-Carr has two options:
       - 4mm/0.7mm x 5mm   - this is full height - 4mm - head which will stand proud of the air tube or could be sunk deeper into the valve body for even more strength (presumes 1.3mm removed from the threaded end.   These are 170,000 psi tensile.
       - 4mm/0.7mm x 6mm   - these are low head and could be mounted flush by removing 1mm from the threaded end.    These are 110,000 psi tensile.

Are the factory screws stronger than the low-head screws immediately above?

Edit:   I suppose the head of the full height screw could be shortened instead.

[rsterne]

Ribbon, I have NO explanation for the strange, erratic string you got when you first installed the new screws.... However, I seriously doubt that the hammer is ever coming close to hitting that steel backing block that the receiver and stock screws are threaded into.... and if it WAS, it could only do that at the low pressure end of the string (when the valve opens further).... The valve stem protrudes about 0.150" from the back of that block (by my measurements).... PCP valves, in a properly tuned gun shooting a nice bell-curve.... do NOT open very far, about 1/4 the throat diameter at the start of the string (2K) and maybe twice that at half the pressure (and you aren't shooting down to 1K).... My guess is that the valve in a Chief is never opening even 0.100", even at 1K.... You remember what happens when you start cranking in more preload in a previously tuned PCP?.... They get louder and use more air, right?.... That is because the valve is opening further and for longer.... Pretty much proof positive that the hammer isn't hitting the back of the valve (in this case the steel block)....

You do bring up one point about the new McMaster Carr screws.... The have full height Allen heads.... The very best solution would be to machine a flat in the valve body so that the head was sitting completely down on the valve body, like I did for my 10-32 screws.... A 7mm end mill, touched off on the valve body, until you had a nice round flat, removing the minimum material, would be great.... if you have the equipment.... This would sink the head down just a whisker, so you would have to remove less wood from the stock.... HOWEVER, you might have to check the length of the screw and file a bit off the end to make sure they were actually sitting on that flat, and not bottoming in the threads....

Bottom line, I have no idea what caused your strange initial string.... grit, a wayward knat, or whatever.... but I'm 99.9% sure the hammer wasn't hitting the steel block.... The only way to be sure is some sort of telltale on the hammer, or high-speed photography inside, looking at the hammer cocking pin to assess the travel of the stem.... A 1/16" thick piece of plasticine (kid's playdough) on the back of the steel block might suffice?.... It would either be hit, or not affect the velocity.... or maybe a single 1/16" CS O-ring (like a bstaley mod).... No velocity change, hammer isn't hitting it....

Bob

[CraigH]

Thanks, Bob!   That was my thought on the full height screws.    It could be done with a drill press and a bit of care to locate the end mill.    (and no side loading of the end mill    )

[Ribbonstone]

Best guess is cold weather (although I attempted to get that 40F difference back to "even") and the build up in "slack" from production tolerances.  COULD have been either (keeping tot "slack" pushing to the "stop? everyting in one direction, the slight "bob" of the steel ring, or both.


Don't really care.



As it is...it's working.



OR>>>>at your demand....so you really thing it would be worth it to undo- everything, put the "slack" in the "wrong" direction, and try it again?




LEts put it another way...although I do like expeimentation...really think it'ws worth it to take ia apart once gain and then try to "F" up the fit of the "slack"parts just to see?








It's easier to see the holes from the back side (at least when the target paper isn't backed-up buy somthing "crsiper".

20 yards:
FRONT:


BACK:



40 yards:

Love my bother-in-law...but he also loves those big-arse "splatterburst" targets.  So He things, just becasue I shoot, that I also love them.  I don't...but I can cut some od them into reasonable airgun sized tartets.



Front:


BACK:

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OK now...if you really plan on long-long range shooting (past 50 yards) ..then Bless you, keep cranking it up in power,  and hope for the best.


BUt I will "tell you what"...I will un-do what has been done, lossen all the screws, letting all the sits/toerences build up in the OTHER driection, and then see what happens.  Betting right now it wouldn't be "good"...but hey!...it's data.

[rsterne]

When I had my Chief apart today I slipped a 7/8" OD x 0.070" CS O-ring in behind the steel block (bstaley style).... Basically you couldn't tell the difference, the velocity was essentially unchanged with the exception that at full max. preload I lost about 20 fps.... but that is at over 50 FPE.... Like I said, under normal operation, tuned for a bell-curve.... there is no way the hammer even comes close to the steel block....

Bob

[Ribbonstone]

Wonder what it was....likely something just wasn't fitting right one way or the other....but it got back on track and I won't be going back in until something leaks.