Perfect Plinkin' Pistol, PPP, Mods stage I

Perfect Plinkin' Pistol, PPP, Mods stage I

Joined: July 3rd, 2010, 8:17 pm

May 22nd, 2011, 4:35 am #1

This topic is also posted on the Crosman AirPistol Owners Forum.

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After lots of googling....
After lots of searching 'n studying....
Even more pondering......
Then posting some questions....
More study...etc...etc........

It all comes down to eventually you gotta get out the tools and do sum-thin.

Bought a new 1377 at local discount chain, Bi-Mart. 2010 model with three screw plastic coverplate.
Chrony'd it first day....and again after 350 shots.. The numbers were consistant from new to broke in by 2-3 fps.
Kind of surprised me how consistant it was from the beginning. (results posted below)

My goal was to improve simple plinking power at 3-5 pumps. Hoping to gain one pump worth more power, over stock.

Started doing the 'poor man's' mods that I found, shown in these threads;

http://www.network54.com/Forum/275684/m ... piston+rod

http://www.network54.com/Forum/275684/t ... %28Pics%29

Hindsight, I should have started with the valve tip shaving first, but did it later in my sequence, so some informational insight was lost.
Just finished and here are some numbers for ya'll.

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RWS Basic pellets, weighed 7.1 to 7.2gr.
F-1 Chrony, indoor, with stickup 3 bulb LED closet lights set on screens. End of muzzle to 1st screen = 18"... NO errors recorded while doing any of these tests.
--------------------------------------------------------------------------------------------------------

Column 1, stock 2010 1377, Brown, consistant day to day fps results from 1-350 shots. 5-5 to 5-13-2011
Column 2, piston rod stiffening, Filed valve tip to improve valve/pump cup fit to reduce headspace. Added .031" thick washer shim 'tween piston & pump cup to make up loss of valve length and readjust pump stroke length. Put in a lighter valve spring. Consistant day to day fps numbers, from 360- 575 shots. 5-14 to 5-20-2011
Column 3, added small 'o'ring(A012) shim/spacer between piston cup holder and pump cup, to take up the space created at the piston cup holder joint & the piston cup. First 50 shots 5-21-2011.

Pumps - 1 - - 2 --- - 3 --- Total FPS gained
2 ----- 296 - 299 - 324 = 28
3 ----- 350 - 366 - 397 = 47
4 ----- 391 - 413 - 449 = 58
5 ----- 424 - 444 - 488 = 64
6 ----- 455 - 471 - 519 = 64
7 ----- 470 - 496 - 541 = 71
8 ----- 491 - 509 - 559 = 68
10 ---- 518 - 541 - 588 = 70

Extreme spread on todays tests, for each 5 shot string were within 2.1 (5 pumps) and 6.3 (6 pumps) most in the 3-4 range.

(used my 2009 1377 with over 2500 shots as a Chrony test constant to verify each days tests were within 3-5 fps of this pistol's known consistant preformance. )

Without getting to my stage II mod, "reduce valve volume", I have more than exceeded my realistic goal.
It is also interesting to note the gain per pump increase through the middle of the pump count range. From 4 - 8 pumps gain from each additional pump was amazingly similar.


Many many thanks to all who have posted thier insights, methods, thoughts and just plain chewed on these ideas on the forums before me. Nothing here is new, just rediscoverd !
THANK-YOU !!!


See threads linked above for picture clairifcations, and a couple on this forum;
http://www.crosman-air-pistol-owners-fo ... pic=3688.0
Last edited by Daysailer on May 22nd, 2011, 5:10 am, edited 1 time in total.
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Joined: January 16th, 2006, 1:17 am

May 22nd, 2011, 7:56 am #2

What an interesting post, it was a lot to comprehend since I'm not familiar with pump guns. I will have to read this post over again to knock my brain into the right place...hehe, so cool when you make the gun do what you want it to.
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Joined: July 3rd, 2010, 8:17 pm

May 23rd, 2011, 12:58 am #3

That should add a bit more, now that I have a more effiecient pump.
At my target 3-5 pumps, a bit smaller resevoir should yield a gain,
like that seen at the higher pumps.

The gain from stock, per pump, after the 4th is consistantly around 60fps per pump until #8.
I think that if the valve volume is a little smaller, that it should
charge up with the same increase per pump in the 3-5 range.
I expect that the gains at the other end, 7-10, will suffer the smaller
valve volume, but that is not how I will best enjoy the gun.

I have another 1377, as yet unmodified that will also get this treatment.
Are you interested in seeing it done ?

Darren in Salem also renewed his invitations for us to shoot at his place
some Saturday.

DS
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Joined: February 9th, 2006, 10:35 pm

May 23rd, 2011, 1:58 pm #4

...You installed a washer (phase 2) and an O-ring (phase 3)between the piston and the pump cup? Or did you remove the washer before installing the O-ring?

Very good information Daysailer.

I plink, therefore I am.
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Joined: January 16th, 2006, 1:17 am

May 23rd, 2011, 8:11 pm #5

That should add a bit more, now that I have a more effiecient pump.
At my target 3-5 pumps, a bit smaller resevoir should yield a gain,
like that seen at the higher pumps.

The gain from stock, per pump, after the 4th is consistantly around 60fps per pump until #8.
I think that if the valve volume is a little smaller, that it should
charge up with the same increase per pump in the 3-5 range.
I expect that the gains at the other end, 7-10, will suffer the smaller
valve volume, but that is not how I will best enjoy the gun.

I have another 1377, as yet unmodified that will also get this treatment.
Are you interested in seeing it done ?

Darren in Salem also renewed his invitations for us to shoot at his place
some Saturday.

DS
We should take up his offer...
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Joined: July 3rd, 2010, 8:17 pm

May 24th, 2011, 1:37 am #6

...You installed a washer (phase 2) and an O-ring (phase 3)between the piston and the pump cup? Or did you remove the washer before installing the O-ring?

Very good information Daysailer.

I plink, therefore I am.
I got the modest gains shown in column 2 and was pretty happy.

But then remembered one of the posts I had studied, mentioned the
o-ring... Thought about the space between the piston cup holder, and
the piston cup, created by the shim/washer, which was needed to adjust the
piston length, because of the valve being slighly ~.040" shorter.
(the hip bone is connected.....)
The washer shim is still in it + the o-ring.

Sooo, I put the puny looking o-ring between the cup and piston to take up
that area. Picture shown in this linked thread. (easier to post pics there)
http://www.crosman-air-pistol-owners-fo ... pic=3688.0

WHA-LA ! (magic) that small puny o-ring must have let everything else seat
completely, granting me my wish of "mo-powa" at 3-5 Pumps, and even more power
at more pumps. Amazing how much that space was giving away, even after stiffening
the piston with the steel rods.

If I do this to another 1377, Or someone else ?, I would like to try the sequence
of mods in a different order....
1. Get good reliable Chrony numbers for baseline.
2. Change out valve spring for a lighter one... Chrony change.
3. File/mill valve face to mate with pump cup, shim adjust piston length loss....Chrony change.
4. Add o-ring to cup/piston....Chrony change.
5. Put rod/shims in piston to simulate solid piston...Chrony change.

Based on my numbers, If you arent trying to get to 700fps on 10-?? pumps,
these relatively easy and cheap mods will improve the 1377 to be an even better Backyard plinker.

BTW...after about 100 shots, re-chrony'd today at 3 pumps. 399-398-401-404-402..
Last edited by Daysailer on May 24th, 2011, 1:46 am, edited 1 time in total.
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Joined: July 3rd, 2010, 8:17 pm

May 24th, 2011, 1:49 am #7

We should take up his offer...
maybe after it arrives would be good.

I'll have a proper hunting gun to maybe accompany you on one of your
hunts someday...

(is it ok to hy-jack my own thread ??)
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