3 Liter Pop Bottle Mini-Kegs
C.D. Pritchard  rev. 2/04
The "tap cap" gizmo below screws on top of a 3L plastic pop bottle thereby turning it into a mini-keg.  Here's a drawing of the thing:

New 3L Pop Bottle Cap Drawing

New 3L Pop Bottle Cap Photo
Photo of tap cap- the thumbscrew core depressor is on the right

Cap Disassembled Photo
Photo of disassembled tap cap

It's much simpler to make than the old design and allows the use of a really gas-tight fitting.  The old design used an metal air stem (it's normally used on tires) and I could never find an air fitting/chuck  to connect to it to my CO2 rig that didn't leak if left connected.  If you are only going to use a $9.95 tire inflator gadget/12 gramm co2 cartridges (details/source below),  it does not have a leakage problem and the air stem is a better choice.  The inflator uses the high pressure of the CO2 cartridge to force open the Schrader valve in the air stem.   The downnside is that the cartridges are a bit expensive.  The rig is real handy for traveling- especially if your CO2 source is otherwise a 20# CO2 cylinder!

1.  Don't plan on storing brew in the mini-kegs for much longer than a month or two assuming you keep them cold. YMMV with delicate brews and warmer temps.   Long term storage is said to cause the brew to become oxidized since the PET plastic the bottles are made of is not a good O2 barrier. Yes, the CO2 pressure in the bottle after filling is much greater than atomspheric pressure, but, as counter-intutive as it seems, oxygen will still pass through the plastic.
2.  I've only used plastic bottle caps. Aluminum bottle caps should work tho'.

The CO2 Fittings
The fitting mounted to the bottle cap is intended for use in refridgeration systems and is generally called an "access fitting" by refridgeration folks.  They have a male business (no sexism intended :-) end that accepts a 1/4" female flare fitting and they have an internal Schrader valve which functions like those in a tire air stem.   The other end has both tubing (for soldering) and a 1/8" MPT male end.  The connector that attaches to it is the cool thing.   $5 gets you a very  omminious sounding "thumbscrew core depressor".  Here's a drawing of the thing:
Thumbscrew Core Depressor
This fits via hose to your valved CO2 source via a male flare and then screws onto the end of the access valve.  After connecting to the access fitting atop the keg, turning the thumbscrew depresses the stem of the Schrader valve then opening the CO2 valve allows gas to pass thru the valve with no leakage like tire air stems.  The ones I use were made by C&D Valve Mfg. Co. and are available at refridgeration supply houses- I got mine at the local Johnstone Supply store (thy are a chain- they have a web site, but it's atrocious...):
Universal Line Service Valve C&D Valve Mfg. model# CD3600, Johhnstone # B10-734, $1.50 each.
Thumbscrew Core Depressor C&D Valve Mfg. model# CD5050, Johhnstone # H24-849, $5.00 each.

The only problem I've had with the depressors is the small o-ring in the end that attaches to the access fitting will somethimes fall out after a bit of use.  I just keep a male flare fitting handy and screw it into the end when it's detached from the access fitting- this retains the o-ring..

Making the Top
The top is easily made using a 7/32" and 1/4" drill, hacksaw, files and propane torch. The drawing at the top of this page and the following poop should make it self-explanatory but, here's some poop that might help:
  1. Use one of the "blocky" type street elbows- they afford more surface area for soldering in the beer tube.  A street elbow has both a male and female outlet while a regular elbow has two female outlets.  If you can't find one, use a regular elbow and solder a close nipple into a female outlet.   The downside is that the nut won't run up tight against the washer/cap/gasket.   To remedy this, either 1) chase the thread(s)  (i.e. a threading die turned "backwards") on the close nipple or 2) use additional washers to take up slack between the nut/washer and the cap.
  2. Firtst drill a 7/32" hole completely through the elbow from the 1/8" MPT end then enlarge with a 1/4" drill BUT don't drill completely through the elbow!   This is shown in the drawing above.  A drill press/vise is good for doing this since the bit will tend to grab and hence increase the chance of drilling to deep.
  3. The 7/32" OD brass tubing is avaiable in 1' lenghts at any decent hobby store and at some hardware stores.  Clean the joint area of the tubing before soldering and use lead-free solder just to be safe.
  4. Saw off any tubing that's on the 1/8" MPT end of the access fitting and screw into the elbow using teflon tape.
  5. If the nut doesn't run up far enough on the male end of the street elbow, either  1) chase the male threads (i.e. a threading die turned "backwards") on the exposed part of the close nipple or 2) use multiple  washers to take up slack between the nut/washer and the cap.  Don't try to enlarge the threads on the nut while holding it in a vise- it'll likely deform during the process.   Maybe holding it in a socket rather than a vise would prevent this....
  6. The 3/16" ID vinyl dip tubing in the bottle is whacked off via  two 45 deg. cuts-  the end is shaped like ">".  A single-cut end may come to rest against the bottle and cause dispensing problems.
  7. To prevent the 3/16" ID vinyl dispensing tubing outside the keg from getting detached from the cap (results in a beer geyser!), secure it with a couple of nylon tie wraps or or wrap wire over it.
  8. Don't sweat finding a washer with the precise ID hole-  shoot for "too small" and enlarge the hole with a rat-tail saw.  A smaller OD washers work better than a larger one since the lesser surface area can compress the gasket easier for a good seal.
  9. Cut your own gasket rather than trying to find/buy one the correct size.   Ace Hardware sells  in 6"x6" pieces of 1/16" red rubber gasketing material.
Surface Lead Removal
If you are concerned about getting lead in you brew from the brass tubing, remove any lead on the surface of the fitting by following John Palmer's lead removal method:
A solution of two parts white vinegar to one part hydrogen peroxide (common 3% solution) will remove tarnish and surface lead from brass  parts when they are soaked for 5-10 minutes at room temperature. The brass will turn a buttery yellow color as it is cleaned. If the solution starts to turn green and the brass darkens, then the parts have been soaking too long and the copper in the brass is beginning to dissolve, exposing more lead. The solution has become contaminated and the part should be re-cleaned in a fresh solution.
FWIW, I don't de-lead since only a short piece of brass tubing is in contact with the brew and usually for a short period of time.  An expensive alternative is to use stainless steel.  Worse still, it's much more difficult to machine and solder than brass.

Hydro Test
After assembly, do a hydro test on the assemblied keg. Don't skip the "hydro" part and use just air or co2- it's a safety thing. If something ruptures during the test with water filling the bottle, less energy will be imparted to targets in the blast zone since water is practically imcompressible- OTHO, there's lots of energy in a compressed gas.  Fill the bottle with water and secure the top to the bottle. Pressurize with air or co2.    I use 30 psig but once took a plastic 3L bottle up to my water tap pressure (55 psig) without any problem. You'll note that the bottle expands a bit and there is a bit of air or co2 atop the water in the bottle. This violates my advice on hydro testing but it's only a little gas space and it's actually a good thing since air or co2 will escape through any leaks more readily than water would. Assess leakage by applying a soap solution to the tap cap.  Assessing leakage by filling with CO2 and noting if the pressure drops over time is not a really good idea- the water will absorb some co2 and lower the pressure.  Air may work tho...

Using the "Kegs"
I used to fill the bottles from a corny keg with a counter-pressure filler.    A caution tho': if your experience with counter-pressure filling is limited to regular glass bottles, be prepared to use ~3 times more force to hold the filler on the larger diameter 3L bottle tops when co2 pressure is applied!    Preserving the brew's carbonation during the filling operation isn't as critical as it is when filling bottles since the carbonation can be "topped-off" by applying co2 via the mini-keg cap.  When I'm filling more mini-kegs than I have keg tops, I cap the bottles with unmodified plastic caps then replace this cap with a keg top when I'm ready to tap it.  Flat brew or pop can also be force carbonated in 3L using either the tap cap or a cap fitted with a metal air stem.  .

There are two faster and easier ways than counter-pressure filling to get brew from a cornie keg into a mini-keg.  Both are ideal when the brew will be consumed without much delay:

FAST/EASY:  Attach the snout of the cobra tap on a sanitized 3 liter mini-keg to a co2 line using an "adapter" made from 3/8" ID vinyl tubing and a hose barb, latch the cobra tap open, open the co2 valve at the regulator and loosen the cap on the mini-keg to allow the co2 to purge the bottle via the "beer out" dip tube for 30 seconds or so,  tighten the cap on the mini-keg to pressurize the bottle, close the cobra tap and the co2  valve to trap the pressure, attach the snout of the cobra tap to the snout of the cornie keg cobra tap using another "adapter" of made from vinyl tubing, latch open both of the the taps,  barely loosen the cap on the mini-keg to allow the brew to flow into the mini-keg- control the flow by adjusting how much the cap is loosened and fine-tune by pressing down on the cap, when just about full (leave some head space if you use a tire inflator gizmo for dispensing!), tighten the cap on the mini-keg to stop the flow of brew, close both cobra taps (this is very important- don't ask how I know! :-)   and finally disconnect the taps.   This is MUCH faster and easier to do than sanitizing and connecting up the counter-pressure filler.  In fact, I can fill a mini-keg about as quickly as it took me to write up the procedure!

FASTEST/EASIST-  Where the brew will be consumed without much delay, simply fit a 1' or so length of 3/8" ID vinyl tubing to the snout of the cobra tap from the cornie keg, place a 3 liter bottle at a lower elevation that the cornie keg, reduce the co2 pressure a bit (including in keg by cracking open the pressure relief valve), open the cobra tap fully to fill while adusting the co2 pressure regulator to adjust the flow of brew into the bottle.

If you want to use use one of those co2 powered pocket tire inflator gizmos for dispensing, you'll need an adapter.   Remove the rubber from a tire air stem,  That'll leave a brass tube exposed.  Solder it onto a flared  piece of copper tubing with female flare nut attached.  The later attaches to the thumbscrew core depressor.  Going further, I've spliced a hacked 0-60 psig tire pressure gauge between the inflator and the depressor so the mini-keg pressure can be determined.  WARNING- if you use a co2 inflator gizmo to pressurize the keg via a thumbscrew core depressor/access valve without depressing the core of the access valve, the pressure gauge will be pegged and ruined.   Even worst, the  the gauge could explode.  Including a quick acting pressure releif "weak link" in the connection helps avoid this vinyl tubing attached to in the line attach d a I used between the inflator/pressure gauge and the  core depressor/access valve below off a 1/4" tube I'd used as a barb.  The gauge was ruined but

Dispensing with an co2 tire inflator without a pressure gauge is pretty easy with a bit of practice.  What I do is add the co2 only as I'm actually dispensing brew- i.e. when brew flows too slowly, give it a brief blast of co2.  repeat if/as needed.   You''ll know when you've over-pressureized by the brew foaming too much as it's being dispensed.

I use about 10' of 3/16" ID vinyl tubing for the dispensing line and 10-12 psig of co2 pressure for dispensing ales.

An insulated jacket for mini-kegs
It's made primairly of aluminum-faced bubble wrap.  It will keep brew cool for  3 hours or more and is very easy and cheap to make.
Insulated Jacket
1.  I used aluminun tape to make the jacket.  It costs a bit more than fabric duct type but works and looks much better!
The jacket extends 2-3" past the top of the keg.  That allows the material to be gathered and tied shut with a piece of string or whatever for transport.
2.  DO NOT stick the cobra tap inside the jacket- it's lever can be easily depressed by the jacket which will dispense brew into the jacket!
3.  If you want to use the cap with a co2 powered tire inflator gizmo, cut a slit in the upper portion of the jacket so you can access the co2-in fitting easily.
4.  Coiling the brew line and placing it atop the keg keeps it cool and reduces foaming while dispensing.

A cooler for mini-kegs
Photos of the thing:
Rubbermain minikeg cooler assembled picRubbermain minikeg cooler detail pic
It's made from a Rubbermaid water cooler I got at Target.  I forgot the size, but, it's capacity is 2-3 gallons.  The only hacking required is to remove a chuck from it's lid since the mini-keg is taller than the depth inside the cooler.  A circular piece of 3/4" styrofoam with aluminum faced bubble wrap glued to its top side closes the hole in the lid.  Well, nearly so- there's a hole in the foam to allow the bottle top to protrude through.  Note in the second photo above that the brew line is coiled in the annular space between the cooler and mini-keg.  This keeps it cool and greatlyy reduces foaming during dispensing.   It will keep a pre-chilled ale cool for several hours without adding ice.  When the brew starts to get too warm, just add some ice.  Filled with ice, it'll keep the brew cold for more than a day at room   temperature.  To increase the insulation, wrap the cooler in aluminum faced bubble wrap.  This really hepled on a cornie keg cooler I made from a hacked 5 gallon Rubbermaid cooler.

Going Further

If you want a tap cap that allows the use of both an access valve and a tire air stem, use a 1/8" NPT  tee and  close nipple instead of the street elbow.  You'll have to either chase the threads (i.e. a threading die turned "backwards") on the exposed part of the close nipple or use multiple  washers to take up slack between the nut/washer and the cap.

Make a giant 6 liter keg using a 6 liter plastic bottle.   It accepts caps from 3 liter bottles so you can use your tap cap.  The only downside is they have a round bottom hence will not stand up-right unaided.   Details on this and related stuff:
Photo Description Model No, '02 Price & Source
6 L Keg pic. Reusable 6-liter (1.5gallon) blue bottle. The bottles are 14" from end to end, and 6.25" in diameter with a 38mm (Standard 3-liter bottle) finish. Depending on what is put in them and how they are cleaned to how long they will last. We estimate around 8-10 refills for the average user KEG930 
$6.95 (from Beer Beer & More Beer)
CO2 Inflator pic. CO2 Injector Only Does not include ball lock or adaptor hardware. Use to flush kegs or carboys prior to transfer or to flush the headspace in wine fermenters. Uses 12g cartridges KEG954 
$9.95 from Beer Beer & More Beer)
(no picture) 12 gram CO2 cartridges- pack of 12 KEG955A
$7.80 from Beer Beer & More Beer)

John Schnupp's TapCap. (or at his page)
My old 3 liter mini-keg which uses a metal tire air stem.  It's more difficult to make than the one on this page and I've not found an air chuck to attach to the air stem that didn't leak if left connected.

Comments or questions are usually welcomed: cdp@chattanooga.net
Revision log:  New, 2/2002,  Added photos 12/02  Updated/revised and added photos of new keg cooler 2/04.