GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: Motorhead on March 17, 2012, 03:45:38 PM
-
Just picked up a NIP Crosman 1377b pistol. Did some reading on DIY mods and getting home last night tore into the gun dismantling it completely.
Systematically started Reworking / Modifying parts as they got reassembled. Sequence went like this ;D
*Valve dismantled, Transfer discharge passage funneled into backside of seat.
Stem of valve under turned in diameter behind poppet increasing flow around it. Poppet tapered slightly along it's side increasing area for air to flow past when opened, Stem smoothed up / polished with Crocus cloth
*Hammer contact surfaces with tube polished in the lathe, Sear catching area polished also.
*Trigger group taken apart, Sear flattened, edges stoned and polished. Thrust washers put in place taking up the side/side slop on sear & trigger. Delrin spring guide made up and spring shortened for less sear tension.
*Bolt probe drilled on end and a 1/16" wire extension installed. ( Now seated pellets skirt clears transfer passage hole in barrel when bolt closed )
* Barrel JB paste polished extensively followed by Flitz polish. Cratex polished breach lead in and intersection on rifling and breech free bore. Rework and clean muzzle crown.
Reassembled and lubed up with a light synthetic oil and moly on trigger pivot areas. WOW WOW WOW so much smoother operating and shooting !!
Just had time this AM before leaving for work to shoot @ 10 shots over the Chrony.
All were at 10 pumps and DID NOT check if valve was dumping all the air, I doubt it was due to velocity readings I got ?? ........... all were +/- 4 at 498 fps average with 7 gr hobby wad cutters.
Was hoping for better, but unknown if valve dumping 100%, so not done tweaking just yet :o
Just sharing,
Scott
-
have fun!
you stepped into the rabbit hole!
-
I think you will find that modding the valve to "improve" the flow does not pay benefits in .177 cal, in fact it may very well hurt the performance.... In .22 cal it's a totally different matter, you need the flow.... In .177, however, the theoretical benefit to improving the flow is outweighed (in my experience) by the fact that you are increasing the volume between the valve seat and the pellet.... That reduces the pressure of the initial pulse which in the smaller caliber is extremely important....
When I am working on .177 pumpers (or CO2), I do NOT hog out the valve exhaust port, the transfer port, or even the barrel port unless I'm building a gun capable of in excess of 800 fps.... Every time I've tried, I've lost performance, particularly in guns under 700 fps....
I would seriously doubt you were NOT dumping all the air at just 10 pumps.... Most 1377s can be pumped ~14 times with an F-T-P without retaining air.... and with the stock piston it's almost impossible to develop enough pressure to retain air unless you have put in a stronger valve spring and/or a lighter hammer spring (ie the opposite of what most people do).... If you are down on velocity compared with before, my guess is your mods to the valve are the problem....
Bob
-
Good thing these guns are CHEAP ... Following some other 1377 threads found today about actual pump efficiency issues such as pump piston compression ( adding support struts ) .... Mismatch of valve tapered nose and face of piston cup getting corrected, back shimming cup to decrease head space, as well softer spring within valve to be fooled with before I really will revisit your statement a Free flowing valve assm is costing performance ? ( Not disagreeing ... )
Tinker Tinker .... What else to do on a rainy winters day
-
Instead of fiddling around with the tapered valve head.... just purchase a Flat-Topped Piston and Valve kit.... It completely eliminates the head space (because it's adjustable) and therefore raises the compression ratio.... Not only will it give you more velocity with each pump, it can result in pressures that will force the valve to eventually retain air (with the stock springs).... You can either accept that as the limit, or if you really enjoy pumping and want a bit more, you can replace the hammer spring with one from a Disco which will allow much higher pressures to be dumped....
Replacing the spring inside the valve with a weaker one will give you slightly increased velocities at 10+ pumps, and will allow a couple more pumps before retaining air in the valve.... I use the one from the sear spring.... Collapse it fully to "set" it, and then simply use it inside the valve.... Unless I'm building a maxed out gun, I just use an F-T-P, a lighter valve spring, and leave the hammer spring stock (although sometimes I will fit an RVA).... After playing around with pumpers that have produced over 1000 fps in .177 cal (I actually had one that would break Mach with Hobbies) and 900 fps in .22 cal.... I now limit myself to what I can achieve with 17-18 pumps (ie before retaining air with a stock hammer spring).... That works out to about 650 fps in .22 cal (14" barrel) and 830 fps in .177 cal (24" barrel).... It takes twice that many pumps (hard pumps) to get another 100 fps....
Bob
-
Success !!! ... without spending mo money ;D ( Stock at 10 pumps with 7 grains @ 500 fps )
Flattened tip of valve matching it's tip size to piston cups shape.
Placed a thick washer under piston cup and piston, and a small o-ring around O.D. of cups stub supporting outside edges to piston interface.
* Tried a few thickness's of washers until feeling ZERO headspace when snapping pump closed and stuck with that.
** Using a round file increased the smile size then polished smooth air inlet of air tube due too the spaced forward piston position when pump was fully retracted.
Took a couple 10 pump shots followed by No pellet checking to see if any air was still in valve at 10 pumps ... NOPE ;D
So at least at 10 pumps hammer springs strong enough to open valve and get a 100% dump.
:o :o Next 10 pumps and a 7 grain hobby over the chronograph ..... 578-582 range for 5 shots !!
That's a 80 FPS increase just from increasing pump efficiency by taking piston cup / valve head space to near zero !!
Tried 15 pumps getting 631 FPS and little burp of air left in valve. Wow pretty hot out of a stock length barrel.
Now where talking near advertized velocity and worth packin as a side arm on AG hunts :o
-
I think you will find that modding the valve to "improve" the flow does not pay benefits in .177 cal, in fact it may very well hurt the performance.... In .22 cal it's a totally different matter, you need the flow.... In .177, however, the theoretical benefit to improving the flow is outweighed (in my experience) by the fact that you are increasing the volume between the valve seat and the pellet.... That reduces the pressure of the initial pulse which in the smaller caliber is extremely important....
When I am working on .177 pumpers (or CO2), I do NOT hog out the valve exhaust port, the transfer port, or even the barrel port unless I'm building a gun capable of in excess of 800 fps.... Every time I've tried, I've lost performance, particularly in guns under 700 fps....
I would seriously doubt you were NOT dumping all the air at just 10 pumps.... Most 1377s can be pumped ~14 times with an F-T-P without retaining air.... and with the stock piston it's almost impossible to develop enough pressure to retain air unless you have put in a stronger valve spring and/or a lighter hammer spring (ie the opposite of what most people do).... If you are down on velocity compared with before, my guess is your mods to the valve are the problem....
Bob
I remember reading one of your threads on your 1377 experiments, don't remember the link and sorta need it cause I am currently thinking about modding my 1377. What did you do to get yours to get to 1000fps? I already have a 24" barrel and a steel breech on mine and its doin 630fps with cphp with no other mods other than the barrel.