Non-adjustable Flat top Piston !

[Davio]

Start throwing rocks, guys! Just kidding about the rocks.

Give me your initial reaction to the concept.

Why do you think the originals were adjustable? (I'm sorry if I ask questions I know the answer to).

[BillK]

Manufacturing tolerances were "by guess & by golly" on inexpensive air guns.  Adjustable pistons were able to correct the head space.
Just my guess???

You don't sound too "sorry"...... lol

[mudduck48]

Adjustable flat top piston? Sounded like a good idea, at the time.LOL 

[bgmcgee]

Got to agree with Bill.

[JMJinNC]

Agree with Bill. Nobody flat-tops the valve to the same length, and nobody has exactly the same lock-up force on the pump arm. A worn pivot hole will add more variability.

These variations can be measured in millimeters, so you need adjustment somewhere to take up the tolerance stack-up.

JMJ

[Davio]

As far as I can tell, the adjustable piston rightfully died with the Crosman 1300 in 1976. It became the 1322 and never had an adjustable piston again. The 760 shed its adjustable piston the previous year in July of 1975.

Since then, there have been no adjustable pistons in the 2100, the 766, the Benjamin 392/397 or C9, etc.

The reason they were adjustable was because the design and available materials of the piston face was such that at some point it would have to be replaced (without replacing the entire metal piston body. To make it replaceable (at that time) it had to be removed, replaced and re-adjusted. So "adjustable" was not a positive feature as much as it was a necessity at the time.

My opinion of the felt oil wiper, the pressed (or pounded on) brass ring end around the center metal shaft, and the not-so-long lasting plastic cup (black, then white, then clear) was and is inferior. Another thing is the high percentage of people that do not set them correctly. I see that.

As soon as design and available materials improved, then the adjustable piston was tossed to the curb like a warm flat six pack of Billy Beer. Only thing was that millions had sold and were / are still out there.

Agreed that DIY flat toppers may not be consistent, but they certainly should be. The first clean pass across the flat means stop. If people are using sand paper or grinders, then shame on them. There's not a ton of meat there (the flat after the cone is gone). Just faced is just faced.

If a pivot hole gets egg shaped, then it should be replaced. I've never had a problem there.

Not here so much, but the populace seems to think flat top equates to more power, yet flat top is only about a tiny bit more pump efficiency... and nothing to do with the power potential of the valve, transfer, hammer / spring etc.

... and that's the way it really is. 

[Underdog]

Since you're giving out answers on the subject...

How does it increase pumping efficiency? 

[Davio]

Since you're giving out answers on the subject...

How does it increase pumping efficiency? 

The flattening (removal) of the face of the valve and then the use of a flat faced piston (no longer having the conical recess) adds that small amount of tube volume to the pump stroke. It's not very much at all as a percentage of the total tube ID x stroke length, but it is an additional tiny gulp of volume for the piston to force into the valve each pump. Maybe saves one pump out of 15? 20?  Not really significant.

We get very similar results with flat top and not flat top setups where both valves are ported on the exhaust end and a small increase in pressure chamber volume allows both to equally allow for more pumps netting higher fps (and fpe). 115 additional fps at 20 pumps is a good expected increase for both .177 and .22 calibers. It's there if you need it, but otherwise for plinking, targets, etc. just treat the gun as stock. 

Both the stock Crosman rubber cup for stock gun's pistons... and a well seated o-ring at the end of a flat top piston can do a better job of compressing that volume of air than the old plastic setup from the 60's and 70's. Cheaper and easier to replace, too if one gets nicked or abused from lack of lubrication.

If nothing else, I am trying to open a discussion that will possibly prove helpful in the end. Please correct me if I am wrong in my observations.

[Underdog]

Forgive me if I state what is obvious to the rest of you. I'm starting to get a handle on what the mods are, and what they do...

So if I understand this correctly, piston/valve mods that remove the conical sections from each, only incrementally increase the pump volume so you get to a certain pressure just a bit faster.

The valve modifications that increase valve volume will give you more air to dump when the valve is opened, provided you pump enough times to get there. (I assume cutting off a section of the threaded cap increases volume. What other mods will increase valve volume?)

Valve modifications that open up the air flow route (valve stem, valve exhaust, transfer port, barrel port, and bolt/probe) allow that volume of air to dump faster, hopefully increasing FPS.

Now is where I get a bit fuzzy on what's done to the hammer and spring... What's the purpose of these mods?

I think I get the longer barrel idea. If you have a short barrel, the pellet only gets up to a certain speed before it gets out the end, and the rest of the volume of air gets dumped...

Am I understanding all this correctly?

(Oh and trigger mods are usually just to smooth out the raspy trigger, yes?)

[Davio]

Everything you said indicates that you understand correctly.

The stronger hammer spring provides additional kinetic energy for the hammer to strike the valve and keep it open slightly longer for release of a larger dump before the internal spring forces it closed again. Actually, you can do the same thing by putting a lighter spring in the valve. Easier to just get a stronger spring (or power adjuster if you're a tweaker speed demon).

[eric]

i think they went by the wayside due to cost --- look at their airguns of yesteryear and today ---THEN more real metal and real wood vs NOW plastic and pot metal ..... as well as trying to mass market and keep things at a price point for the average joe (sorry joe)

[Underdog]

So what other mods can be done to the valve to increase the volume?

Also at what point do you stop doing mods because of a stock length barrel? Or asked another way, how many mods would you do if you stuck with a stock length barrel?

[Davio]

So what other mods can be done to the valve to increase the volume?

Also at what point do you stop doing mods because of a stock length barrel? Or asked another way, how many mods would you do if you stuck with a stock length barrel?

What gun model? What caliber?

[7624452]

I've always had difficulty adjusting the flat top piston.  If there were a non- adjustable piston that was already the correct length, that would be very helpful. 

[eric]

I've always had difficulty adjusting the flat top piston.  If there were a non- adjustable piston that was already the correct length, that would be very helpful. 

the correct length would be subjectable --- the saving grace would be the cup as it is compressible --- those are close but not the same in the above picture