Author Topic: Got into an MRODAIR Precision PCP Pistol & went for it down to its bones !  (Read 3452 times)

Motorhead

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Linking over to the GTA  ... far too much info to transcribe  ???

See: http://www.gatewaytoairguns.org/GTA/index.php?topic=105627.msg1001737#msg1001737


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rocker1

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 Good stuff Scott. David

oldpro

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That a boy!!! Make that $400 gun a $800 gun!!

Monkeydad1969

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LMBO!!!!!
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Joe the P-Dawg Slayer!!!!!

Motorhead

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LMBO!!!!!

Exactly ...
all the bling, makeup and plastic surgery satisfies the owner but adds little if any value to others  ;)

Still fun to do  ;D
  • Old Hangtown ... California
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oldpro

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 It looks like a dam fine pistol for the money and appears to be easy to work on. Keep up the good work Scott.

int3man

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Linking over to the GTA  ... far too much info to transcribe  ???

Hi Scott,
               I had the same issue with those Brown Viton O-Rings.  I ended up using the Green ones.  They must be porous and the gas gets in them (impregnates) and then they expand exponentially when decompressed.  We see the same things in Hydraulic Seals but it is called explosive decompression.  Occurs when a cylinder is extended or retracted by a load and a void or vacuum is pulled in the opposite end.

Michael :- )
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WAR .22 PRod
WAR .25 Marauder
Broadening my Horizions
.22 Brocock Super 6 XL
.25 BSA Scorpion Huma Reg.
.25 Air Arms Extra FAC
.30 Kalibr Cricket
.22 Taipan Mutant Standard

WAR .22 Marauder, Huma, LW Hammer,  Rapid 70 Bull  Stolen
Flex .30  Stolen

Yes I still have an ICQ Number.  Raise your hand if you do.

Motorhead

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Linking over to the GTA  ... far too much info to transcribe  ???

Hi Scott,
               I had the same issue with those Brown Viton O-Rings.  I ended up using the Green ones.  They must be porous and the gas gets in them (impregnates) and then they expand exponentially when decompressed.  We see the same things in Hydraulic Seals but it is called explosive decompression.  Occurs when a cylinder is extended or retracted by a load and a void or vacuum is pulled in the opposite end.

Michael :- )

Green which in my experience are High Temp Buna/N or Nitrile rings ?
  • Old Hangtown ... California
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int3man

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Linking over to the GTA  ... far too much info to transcribe  ???

Hi Scott,
               I had the same issue with those Brown Viton O-Rings.  I ended up using the Green ones.  They must be porous and the gas gets in them (impregnates) and then they expand exponentially when decompressed.  We see the same things in Hydraulic Seals but it is called explosive decompression.  Occurs when a cylinder is extended or retracted by a load and a void or vacuum is pulled in the opposite end.

Michael :- )

Green which in my experience are High Temp Buna/N or Nitrile rings ?

Nope these are Viton.  Trust me.
  • Central California
WAR .22 PRod
WAR .25 Marauder
Broadening my Horizions
.22 Brocock Super 6 XL
.25 BSA Scorpion Huma Reg.
.25 Air Arms Extra FAC
.30 Kalibr Cricket
.22 Taipan Mutant Standard

WAR .22 Marauder, Huma, LW Hammer,  Rapid 70 Bull  Stolen
Flex .30  Stolen

Yes I still have an ICQ Number.  Raise your hand if you do.

Bob La Londe

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I've been reading up on Viton, and its pretty resistant.  Something doesn't sound right. 

Quote
Along with temperature, other environmental conditions differentiate these two seals. An exceptionally broad range of chemical resistances make Viton seals a perfect option for most applications involving oils, fuels, and mineral acids, and these seals also boast excellent resistances to oxidation, ozone, UV exposure, weather, fungus, and mold. While also boasting some chemical resistances, Buna is much less universally resistant than its Viton counterpart suffering degradation from weather and ozone exposure. For most circumstances, however, this is not an issue, and Buna seals also offer the benefit of superior abrasion and tear resistance making them more suitable for more heavy duty industrial applications. - See more at: http://www.humphrey-products.com/content/viton-vs-buna-which-seal-for-your-applications#sthash.zWWQbs1p.nNW6da0k.dpuf

No, that is not the only resource I've read. 

I would mentioned that even with "buna" rubber Chinese o-rings do not seem to last as well as ones purchased from a known good source like Parker Hydraulics.  I've experienced this with water, hydraulic, and air parts.  When I tear down a cheap Chinese hydraulic cylinder I never order the seal kit from the MFG.  I measure the seals and get quality ones from a reputable source.  Here in Yuma CSC is a Parker distributor and have almost every o-ring I need in stock on the shelf.  The only thing I've been disappointed on so far is the little stock square o-ring for the transfer port on a Marauder. 

Maybe Chinese Viton is no different Chinese Buna.  Not quite right yet.  I'ld also note there are a lot of factories in China making the same stuff.  Some are certainly better at it than others.  This is reflected in machine tools.  Why not in chemical products like o-rings too? 

FYI:  For those of you who hunt in sub freezing temps Buna might well be the better choice regardless due to its ability to remain soft and seal well at lower temperatures. 
CNC Molds N Stuff
www.cncmolds.com

Motorhead

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O-rings are cheap enough ... after doing a bit of research on AVAILABLE METRIC O-RINGS have come to realize finding polyurethane is not happening ... but CAN GET 90 duro Buna/N in metric sizing which would be my second choice.


On another note:
Am finding and also reading about others with BREECH o-ring blowby and the need to further snug up valve to move loading gate tighter to receiver  :P :P :P :P
Doing so am finding METAL ON METAL contact with Gate & Receiver right on the verge of happening ???

* Looking at SIZE of O-ring fitted on barrel thimble to seal the gate, its cross section is pretty damn minimal offering little compression potential.
GOING TO ADDRESS THIS by re-machining the thimble to take a fatter cross section o-ring & have it protrude out a tad more so the valve / gate setting can be more relaxed and a gap between gate & receiver maintained.

« Last Edit: March 12, 2016, 11:23:27 AM by Motorhead »
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oldpro

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Scott is this going to be and issue for production guns?

Motorhead

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Scott is this going to be and issue for production guns?

It already is becoming so by posts I'm reading on breech leaks.
That little bitty 1.5 mm cross section o-ring just lacks enough area and compression capacity to make a good seal used how it is  :P

Just retrofitted the OEM barrel thimble by recutting o-ring groove to take a 2.4 mm cross section ring. with a tad more area & height preload crush on the o-ring by loading gate seals it right up w/o having to run the gate up near snug to receiver.

*** While screwing around with barrels. 
The OEM 9" unchoked barrel had to go ... having a couple of the 15" FX LW barrels AOA blew out last year figured there was a good place to start.
Did away with the barrel thimble and mounted barrel threw receiver conventionally anchoring it threw top of receiver using two 10-32 grub screws. Reconfigured breech seal to use an HW rifle breech seal that is working so much better than o-rings !!

All done barrel came in at 14" shooting lights out one hole accurate at 20 yards. Velocity came up @ 35-40 fps as well.

Did find a power gain by drilling out the valves poppet spring cap to I.D. of the conical spring used.
Simply unstrangling air flow from plenum into valve woke it up more.
Currently as set up, taking hammer spring tension up where threads are flush with housing the pistol will just break 800 fps w/ 8.4's. As set for @ 725 fps @ 4 threads of cap are showing and preload of hammer against poppet stem is pretty soft. ( Certainly helping valve dwell w/o all the preload )




OEM breech sealing is this  :P :P


Scott
« Last Edit: March 13, 2016, 10:35:22 PM by Motorhead »
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match

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So are you going to provide upgrade services?
.40 Badger (arrived!)
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Motorhead

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So are you going to provide upgrade services?

If your seeking such work ... contact me, that simple really  ;)
  • Old Hangtown ... California
"Home of Motorheads Air Gun Tuning Service"
    ***** Airguns need love too !! *****