Flat Top Pistons? How And Why?



Author Topic: Flat Top Pistons? How And Why?  (Read 1895 times)

Offline Nail Gun

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Flat Top Pistons? How And Why?
« on: January 11, 2013, 09:39:52 PM »
As the proud owner of a 760, 1377 and 2100 I've done a lot of reading.  Everything suggests I need to acquire a flat-top piston/valve to maximize velocity.  On all three I have done every mod imaginable short of the above. 

What I'm curious to know is how or why exactly does a flat-top setup provide more velocity than a factory setup?  If they do produce better performance than a factory setup, why doesn't Crosman simply adopt that platform?

Those are just a couple of nagging questions that keep me awake at night...
  • Slidell, LA
My Nutter Killers, 2013-14 Hunting Season
Beeman RS2-.22 - 2 Fox Squirrels, 8 Gray Squirrels, 2 Rabbits, 1 Incredibly Stupid Possum

Daisy 880 - 4 Gray Squirrels

Daisy 922 - 1 Gray Squirrel

Offline breakfastchef

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Re: Flat Top Pistons? How And Why?
« Reply #1 on: January 11, 2013, 09:50:51 PM »
The cone shaped bit on the top of the valve represents air space that could be pumped into the valve. By removing that cone and installing an adjustable flat top piston, you get to pump in that small bit of air. So, for example, you could pump 5 times and it might create a pellet velocity that you could obtain with the stock parts with 8 pumps. Just an example, not hard data. Some folks don't like to pump much and use the flat top setup to get higher velocities with fewer pumps.

On the power side, I flat topped three Crosman pumpers and gained an average @100 additional fps at 10 pumps in .177 caliber Every gun responded the same. Adding longer barrels is another good way to up the fps. You can figure a gain of 5-10 fps per additional inch of barrel over the stock length.
« Last Edit: January 11, 2013, 09:52:31 PM by breakfastchef »
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Offline Nail Gun

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Re: Flat Top Pistons? How And Why?
« Reply #2 on: January 11, 2013, 10:09:10 PM »
Ah, so if I'm willing to crank away at what I have there is little benefit.  If that's not feasible, such as hunting situations, there is potentially more benefit?  Correct?  I'm still scratching my head over why a company such as Crosman should be aware of a "better design" but be unwilling to do anything about it???
  • Slidell, LA
My Nutter Killers, 2013-14 Hunting Season
Beeman RS2-.22 - 2 Fox Squirrels, 8 Gray Squirrels, 2 Rabbits, 1 Incredibly Stupid Possum

Daisy 880 - 4 Gray Squirrels

Daisy 922 - 1 Gray Squirrel

Offline breakfastchef

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Re: Flat Top Pistons? How And Why?
« Reply #3 on: January 11, 2013, 10:21:19 PM »
Ah, so if I'm willing to crank away at what I have there is little benefit.  If that's not feasible, such as hunting situations, there is potentially more benefit?  Correct?

The flat top setup provides lots more power. Or, you can pump a lot more to get your compression higher. 

I'm still scratching my head over why a company such as Crosman should be aware of a "better design" but be unwilling to do anything about it???

I am sure for both legal and economic reasons.
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Offline Cal

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Re: Flat Top Pistons? How And Why?
« Reply #4 on: January 11, 2013, 11:11:16 PM »
Flat top offers no particular advantage in reducing unusable volumes or increased compression ratios.....

Other than, it is a very easy geometry to fit or duplicate on the valve body head.

Form fitting the trapezoid or domed  piston face to the mating surface would take some careful work. And still,flex and distortion will play devil with the efforts as the seal and piston are both rather "soft" in light of  1000 psi forces.

There is no equivalent number of pumps that will over come dead volumes in the pump face to valve head combination.  It would be like pumping against a spring...an air spring!
The compression RATIO defines the limits.  The swept volume of the piston is one term of the ratio, and the combined volumes of the valve chamber and any dead volume are part of the other.

Reduce the dead volume for every stroke to zero, and the ultimate pressure is increased to the ratio of the pump volume compared to the valve chamber volume.

 Any way you can do this is equivalent. 


No free lunch,  just preference and choices.
« Last Edit: January 11, 2013, 11:19:40 PM by Cal »
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Offline Bentong

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Re: Flat Top Pistons? How And Why?
« Reply #5 on: January 11, 2013, 11:28:24 PM »
Ah, so if I'm willing to crank away at what I have there is little benefit.  If that's not feasible, such as hunting situations, there is potentially more benefit?  Correct?  I'm still scratching my head over why a company such as Crosman should be aware of a "better design" but be unwilling to do anything about it???
Better design = $$$. You can compare the $$ of a stock 2240 to the one from their custom shop in which you can have the better design.
I will be a one happy camper if they sell the 1377 with a metal breech instead of the mediocre plastic one and a 2289 pump arm for the current price.

Offline rsterne

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Re: Flat Top Pistons? How And Why?
« Reply #6 on: January 11, 2013, 11:42:29 PM »
In a word, headspace, or rather the lack of it.... Any volume that remains between the piston and the valve reduces the compression ratio, and ultimately limits the amount of air pushed into the valve per stroke.... This makes little difference at 3 or 4 strokes.... but a HUGE difference after 20.... The standard cone shaped cup is not only a very accurate fit against the cone on the valve, but represents a flexible material that can compress as the pressure increases with each stroke.... In addition most of the pistons are made of plastic which will also distort.... All this compression adds even more headspace.... Why does Crosman continue using this system?.... It's a fail-safe design.... It prevent the buildup of huge pressures which stress everything in the gun, from valve retaining screws or lugs, to the seal on the poppet to the pivot pins on the linkage and the linkage itself.... If you replace the piston with rigid parts, and get rid of virtually all the headspace, you can quickly overstress all the other parts of the gun.... It wears out, and Crosman would get a bunch of warranty returns they would have to argue "but you pumped it too many times".... The way they make them, it's just not an issue, because pumping 50 times won't build the pressure you can in a properly adjusted F-T-P setup in 15.... The fact than a good F-T-P setup will retain air in the valve, but a stock 1377 won't do that even at 50 pumps is the proof....

Bob
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Airsonal:
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"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

Offline Nail Gun

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Re: Flat Top Pistons? How And Why?
« Reply #7 on: January 12, 2013, 10:44:28 AM »
In short, the flat top assembly allows for more efficient transfer of the compressed air into the valve. The factory setup allows the piston to compress at high pressures, therefore not transferring the compressed air into the valve. Correct?

That makes a lot of sense. I've always wondered why a few pumps create so much velocity while higher pump volumes (say 20 or so) only add a few FPS per pump.

Thanks folks!
  • Slidell, LA
My Nutter Killers, 2013-14 Hunting Season
Beeman RS2-.22 - 2 Fox Squirrels, 8 Gray Squirrels, 2 Rabbits, 1 Incredibly Stupid Possum

Daisy 880 - 4 Gray Squirrels

Daisy 922 - 1 Gray Squirrel

Offline Daysailer

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Re: Flat Top Pistons? How And Why?
« Reply #8 on: January 12, 2013, 02:52:49 PM »
There have been many many discussions and threads and experiments, and great ideas
on these forums about Crosman pumper mods.

I have a 766(original 2100), and 3 1377's and one 1389. 
(I never pump more than 5 for my general shooting, 1377's getting 505-510 fps at 5 pumps)

Flat top piston and valve will give some power improvement.  Easy installation.  $$$
Look up the actual results of those who choose to go this way..   Pay attention to # of pumps,
when interpreting that data.  ..( do you really want to pump 15+ times for each shot ? )

Very near the same improvments can be realized by optimizing the factory parts for less than $10.
If you are handy and like to "do it your own way",  its kind of fun and you can learn more about
how these guns work.

If you go Crosman Air Pistol Owners Forum and the Crosman Airgun fourm, you can do a search and find a weekends worth of reading about the subject.
Search topics:  Stuffed Piston, shaved valve tip, reduced valve volume.

Here are a couple of links to threads about experiments i have done and documented
(you many have to register (free) and join the forum to read them.

http://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,3688.msg35888.html#msg35888

http://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,4088.msg40044.html#msg40044

http://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,9762.0.html

Following links to links in those links, there is wealth of past knowledge and experiences that have been shared
by those who went before me.
Also using the search function, you can see what others have accomplished with similar tweaking.

Have fun !
« Last Edit: January 12, 2013, 03:04:41 PM by Daysailer »
Walther LGR-U
FWB 124 Sport
HW30S Peep
Sheridan Blue Streaks, '71 & '75
Crosman 147 ('56-'62)
Crosman 766, 1980 (original owner)
1377's, '82,'09,'10  (all still 10" pistols...so far)
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Offline Nail Gun

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Re: Flat Top Pistons? How And Why?
« Reply #9 on: January 12, 2013, 10:32:54 PM »
I can't log into the Crosman Pistol Owners forum because registration is disabled  :o

However the stuffed piston is a fantastic idea that I was not aware of. 

This helps out especially because I had a huge face-palm moment when I understood the mechanics of head space.  On my 2100 the valve hole was too far forward of the tube's hole and wouldn't line up completely.  The stamped tab in the tube that stops the valve was stamped too far forward.  To remedy the situation I actually filed away some material from the back of the valve to allow it to seat further back.  DUH, I just created more head space...  Well, now I know not to do that again and how to fix it.

Thanks!
  • Slidell, LA
My Nutter Killers, 2013-14 Hunting Season
Beeman RS2-.22 - 2 Fox Squirrels, 8 Gray Squirrels, 2 Rabbits, 1 Incredibly Stupid Possum

Daisy 880 - 4 Gray Squirrels

Daisy 922 - 1 Gray Squirrel

Offline Daysailer

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Re: Flat Top Pistons? How And Why?
« Reply #10 on: January 12, 2013, 11:38:28 PM »
Sorry 'bout the  resgistration issue with the CAPOF,.. keep trying....in the mean time....

For your 2100 valve issue, you could have 'pinned the valve'  (google search Crosman Airgun Forum...Green )
My only 766(2100) mod has been trigger work to lighten pull with lighter spring and polishing.  It's still
shootin strong and holding air forever, with everything else factory stock....1980.!

Link to Green forum about cone valve/cup mod:
http://www.network54.com/Forum/275684/thread/1211140448/1211339438/some+interesting+valve+stuff+for+pumpers+%28Pics%29

I stuffed my valve with brass rod, instead of steel. 
Keep in mind this is still a Crosman 1377 and will not stand up to 50  pumps... Mr. Sterne is correct... tks Bob.

Click on pics for larger view
« Last Edit: January 12, 2013, 11:48:59 PM by Daysailer »
Walther LGR-U
FWB 124 Sport
HW30S Peep
Sheridan Blue Streaks, '71 & '75
Crosman 147 ('56-'62)
Crosman 766, 1980 (original owner)
1377's, '82,'09,'10  (all still 10" pistols...so far)
Daisy 717

Offline Daysailer

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Re: Flat Top Pistons? How And Why?
« Reply #11 on: January 12, 2013, 11:58:24 PM »
For CAPOF  registration....:

http://www.crosman-air-pistol-owners-forum.com/board/index.php

Click "For Starters"
Click "want to join"

follow directions...

Administrator is in a very different time zone...please be patient.

HTH
« Last Edit: January 13, 2013, 12:01:52 AM by Daysailer »
Walther LGR-U
FWB 124 Sport
HW30S Peep
Sheridan Blue Streaks, '71 & '75
Crosman 147 ('56-'62)
Crosman 766, 1980 (original owner)
1377's, '82,'09,'10  (all still 10" pistols...so far)
Daisy 717

Offline 1377x

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Re: Flat Top Pistons? How And Why?
« Reply #12 on: January 15, 2013, 05:41:55 PM »
the 2100 cant be flat topped by conventional means there are other ways to do it but takes machining as no readily available parts are out there
the 760 13XX and 2289 can all be done in the same manner
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Offline Nail Gun

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2100 Update
« Reply #13 on: January 28, 2013, 05:38:32 PM »
I went ahead and stuffed the piston and reinforced the piston arm.  Previously I sealed the barrel to the housing, added an o-ring to the bolt probe, enlarged the transfer port and re-sealed the transfer port. 

@ 15 pumps she is now sending CPHP's at 725 fps!   Out of the box she was struggling to hit 590 fps with 15 pumps and the CPHP's.  After the first round of mods she was averaging 680 fps with the same conditions. 

Those are some huge gains.  Thanks for the fantastic guidance folks!
  • Slidell, LA
My Nutter Killers, 2013-14 Hunting Season
Beeman RS2-.22 - 2 Fox Squirrels, 8 Gray Squirrels, 2 Rabbits, 1 Incredibly Stupid Possum

Daisy 880 - 4 Gray Squirrels

Daisy 922 - 1 Gray Squirrel

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