1720t mod question “4 o-ring bStaley” mod is?

[Red-Feathers]

Hello all, I am new to AGN, I’m an old guy, retired, Vietnam Vet,( member of the “Tonkin Gulf Yaht Club”). I have several springers, but am interested in trying a PCP pistol. I would like a 1720t pistol at about 13 fpe. I have seen “bStaley” mod mentioned several places but do not know what that is. If someone could explain it or better, a pic or video, I would appreciate it. I’m looking for a back yard friendly pistol, mostly plinking but also bird feeder “protection.” 

I understand a larger TP of around 0.09 in. and the bStaley mod should get it to about 12/13 fpe.

Thanks in advance for any help and advice given.

 

[marflow]

https://www.airgunnation.com/topic/tuning-the-crosman-1720t/

maybe this will help a little

 

[nervoustrig]

Here's a simple diagram of how the O-rings form a bumper for the hammer:



It is most effective when seeking to tune to a somewhat lower velocity in exchange for a longer shot count and a higher overall (total) energy delivered.

Here's an example of the results with a .177 PCP I tuned for a friend:



The upper curve in blue was the first attempt but the stack was too short to do anything useful. As you can see, the velocity peaked over 1000fps which is too fast and the shot count was terrible...only 12 shots inside of a 4% ES. I ended up using the arrangement represented by the red curve which gave 30 shots at an average of 867fps (14.2fpe), and the total energy delivered almost doubled.

BTW in the thread marflow linked, there is a post from me that details a 1720T tune that is almost identical to what you're planning.

 

[Red-Feathers]

Thanks guys, that is what I was looking for, a diagram. So if I go with a 0.09 or 0.12 TP and the hammer bumper, the 1720t should give me around 12/13 fpe? At a descent shot curve.?

 

[nervoustrig]

Take care not to go too large or it can cost you shot count. To use your example, please note a 0.125" port will flow almost 100% more air than a 0.090" port.

Seeing as how my 11.4fpe (avg) tune used a 0.094" port, I recommend trying a 0.100" port for your 13fpe goal. That would be a #39 or #38 drill bit. If all you have is a 1/64" drill index, you'll be limited to choosing between 0.94" or 0.109" (3/32" or 7/64"), in which case I would use the larger of the two. 

If you drill it too large and need to back it down, it's no big deal. Just fill it in with JB Weld and re-drill.

 

[Guest]

Just use a stock PRod TP.

 

[Red-Feathers]

Thanks again, all I need to do now is find a reasonably priced used 1720t.

 

[AirNGasman]

RF, first thank you for your service Bud.

I'm attaching an excerpt form my latest book, Custom Classic and Otherwise AWESOME AIR PISTOLS! detailing the performance of my 1720T from purchased used (and stock) to enlarging the TP, and finally reducing the power back to the field-target competition limit of 12 foot-pounds. Oh... and also some shameless marketing propaganda!

View attachment 1560400122_12349618695d01d0fab8bc39.05610219_1720T Excerpt pdf.pdf

View attachment 1560400156_13198250135d01d11c8336b2.79064888_AAP AGN ad PDF.pdf



BTW, many thanks to N-N-NervousT-T-Trig for his excellent posts on this subject!

 

[Red-Feathers]

Will these mods work on a Prod? Which is quieter in stock mode, Prod or 1720t? Looking for back yard friendly. I like the .177 caliber, I like the LW barrel of the 1720t, but maybe a stock Prod would be all I need with no mods to get my 13 or so fpe.

 

[Red-Feathers]

What is the effect of using a lighter hammer and stiffer (heavier) spring. Does it give a more consistent velocity?

 

[nervoustrig]

Stock velocities are similar for them both and since the .22 is responsible for accelerating a heavier pellet (produces more energy), it uses more air and therefore is louder. However it isn't what I would consider loud and will be be quieter with tbe bstaley buffer because hammer bounce will be reduced.

A light hammer and heavier spring can be used to reduce valve dwell (how long the valve is open). It's particularly benefical to efficiency and shot count when converting an unregulated gun to regulated. However on an unregulated gun like the P-rod or 1720T, it can work either for or against you. That's because the self-regulating behavior that produces a bell curve is dependent on the lift and dwell _varying_ as the reservoir pressure falls. It's a balancing act. Here's a markup that perhaps will help illustrate it.



So for example if you go super light with the hammer, it may not be able to produce enough dwell at low pressure which will limit your useful pressure range. Whereas if you go with a modest weight reduction, there may be a benefit. It's just something that you'd need to experiment with.

 

[AirNGasman]

There goes N-N-NervousT-T-Trig again... making all kinds of sense, and providing MOST-EXCELLENT graphics for greater insight(s)! Wish I'd had that bell-curve chart when trying to explain (in writing) PCP power-bands, bell-curves, sweet-spot charge pressures, etc. 

Awesome job, NT!

 

[nervoustrig]

Ron, much respect to you sir. I count you among the great ambassadors of the sport so that means a lot coming from you.

 

[Gwelo]

Thanks again, all I need to do now is find a reasonably priced used 1720t.

Don't forget that Crosman offers a 25% discount on their website by using AGNATION (all caps) in the coupon code box. I think you can get free shipping depending on what day you order. That would be $374.25 for a $499 1720T or $307.49 for the $409.99 marauder pistol.

Academy has the 1720T for 375.99 with free shipping also. 

Good luck!

 

[Red-Feathers]

Thanks NT, Thanks Gwelo. I’ll look into the Crossman shop.

 

[Red-Feathers]

Thanks guys for all the input. I ended up going with a TalonSS. It was between the Talon and the Prod. I think the Talon has a longer shot count in the sweet spot, also easier to change calibers. I’m hoping the only mod I will need to make will be a top hat shim to get it shooting in the velocity range I’m looking for.

Since I’ll be hand pumping I will only pump to the top of the sweet spot and count about 30 shots and refill. This should keep the shot spread relatively close without the need for a regulator or moderator. At 600 fps it will be fairly quiet with (hopefully) no need for a moderator.