This is a followup to my Build a water mortar instructable. It shows some design alternatives tried this weekend.

Step 1: Piston cap alternative

I had some great feedback on the original instructable and wanted try out a suggestion by thinkahead for a different way to construct the piston. The suggestion was to get rid of the wooden plug at the upper end of the inner pipe, substituting a cap at the lower end instead. I also wanted to try filling the inner pipe with expandable foam. Here are the ingredients.
:0Wow! How high can it go!
Thanks for putting this information together, particularly your specifics the O-ring size and suppliers. I use water guns similar to the water mortar on rafting trips. They're handy not only for squirting each other, but for rinsing off sandy or muddy boat surfaces. I made some changes to the fabrication and design that work quite well - better than the commercial water guns I own. First, I used a 1 5/8" hole saw wrapped with tape until it's a snug fit into one end of the 1 1/4" pipe coupling. Put the hole saw into a drill press and you have a very easy lathe setup to cut the O-ring groove into the coupling. Once the groove is cut and tested, glue a 1 1/4" plug into the grooved end of the pipe coupling. You now have a piston to glue onto the inside tube. Take a 2" cap, mark and punch the center, then cut a 1 3/4" hole with a hole saw. This is your retainer that will keep the inside tube straight and prevent the gun from coming apart. Slide this retainer over the inside tube with the dome facing the handle end of the tube. Install a cap on the inside tube. Grease the ring and install this assembly on 2" pipe. Use screws or set screws that are just long enough to secure the retainer to the 2" tube. The retainer can then be removed to later service the O-ring as needed.
for aiming why not have a hose or tube attached to the nozzle, and have one kid pumping whilst the other holds the hose and aims?
This may work better than the wood plug and tape...
I like it. We need to identify a readily available source of rubber donuts, any suggestions?
Big rig and auto stores carry some for shock absorbers. You may have to grind to size.
I'm picturing rubber bushings? I replaced some of those in the front end suspension of my car, had to do some grinding/sanding to get the aftermarket parts to fit, not fun. I think if we manage to find exactly the right thing from an auto parts store it's going to be more than we want to spend. All the guys out there with lathes are laughing at us right now... :)
lol, your right!
Not really when you consider the cost of a lathe.
Also you may find in the alternative that layers of sheet rubber from cut up pieces of inner tube will work and if not custom molded donuts from polyurethane liquid rubber marine sealant.
The challenge with all these ideas is that we want something that's going to fit pretty close to the part we want to turn, and we want the threaded rod going through it to be in the exact center. I like the idea of casting something up, we don't have much volume to fill, so a few dollars of material ought to do it. I can't come up with a good way to center the rod during casting, though. I have this nagging feeling that there's something readily available that would be perfect for this, we just haven't put our finger on it yet!
Casting are no different than sheet rubber when it comes to centering. Its a simple matter of using a lathe.
Okay, got it: a rubber stopper! You can get them from home improvement stores and drill your own hole, or scientific supply houses have them with a hole already in them.
I've used &quot;piston cups&quot; from McMaster-Carr (http://www.mcmaster.com/) before. They wouldn't be good for this though because they seal too tight. The friction reduces performance noticeably when the force applied is relatively low (as in arm powered water guns).<br/><br/>Check McMaster-Carr out for parts if you haven't before. They have almost anything you can think of. You might be able to find some real &quot;rubber donuts&quot; on there.<br/><br/>Here's something you might want to consider though: <a rel="nofollow" href="http://forums.sscentral.org/t4814/">http://forums.sscentral.org/t4814/</a> . This is a pretty easy to build piston water gun.<br/>
Thanks for the pointer to McMaster-Carr. Now, if only I knew what to search for... :) Nice design, I'm going to borrow some of the ideas and try scaling it up.
The product you illustrated here is available in the plumbing section of Home Depot. I bought a few 3 inch plugs like this when I was replacing the floors in my home. When you remove a toilet, you have this big stinking pipe that lead to the septic tank. This plug stops the smell. I will recommend against using channel-locks/pliers to tighten the wing-nut that comes with it. They break. Go ahead and spend $0.10 to buy regular nuts while you are there. (This illustration here does not show wing-nuts.)
When I saw your first posting I thought about using "Great Stuff" foam instead of the rubber gaskets. (I hate special order stuff when I can avoid it.) My thought was: 1. Take a section of the larger pipe about 6 to 8 inches long and put the end cap on it. 2. Line the inside with a garbage bag. 3. Fill about 3/4 of the way full of foam. 4. Insert the smaller inner pipe into the foam. 5. Let it cure 24 hours. I know... It's torture. I end up eating 1/2 my beef jerky before it finishes drying too. We're guys. We hate waiting. 6. Remove the inner pipe to find your perfectly fitted piston.
I want to see the kid use the one with the cross-bar like a pogo stick! You may have to drill the hole in the cap off center to keep him from shooting his own face though.
DO you really need NSF-PW (Pressure rated) stuff on this? Its just a water shooter, not a Pneumatic Spud Cannon
Good observation. I started from a table of PVC pipe sizes and looked for combinations that fit close enough for an o-ring seal. This is the first combination I hit that was readily available in the PVC aisle, I'm sure there are others.
I run a website about water guns and have a section on building them. I've added a link to your gun here: <a rel="nofollow" href="http://www.sscentral.org/homemade/">http://www.sscentral.org/homemade/</a> . Us who build water guns call these &quot;piston water guns.&quot;<br/><br/>If you want a bunch more ideas be sure to check my website out.<br/>
Thanks for passing on the info about your site, looks like a good place to do some research!
It would be easier for the young man if the handles were up higher, if you plug the ends of the handles then he could still get the full benefit of the water gun. Have you considered temporary plugs that use the tightening of a bolt to squeeze it into place. that could also be used for the drill press, lathe centering/securing. good idea I'm working on something like this to blow twisting balloons up
You can't tell from this picture, but the handles start out about has high as he can reach. This is near the end of the shot. Needs to be cut down some to make it more manageable. I think shorter is better for the handles, gives you something to grip, but close to the barrel so there's not as much leverage to pull it off target. Just putting a stub of pipe there to slide an end cap on would be perfect. The cross slides onto the outside of the outer pipe, so the piston is too small to make use of the space inside the cross. I like that thinking though. I saw some temporary plugs that look like they would do what you describe, they were for pressure testing and cost about $10 each. If that's not what you have in mind, could you post a link?
That's probubly them, I had never priced them so I had no idea they cost so much.
Awesome job <em>once again</em>! Your Instructable(s) are great, this one is really cool, along with <em>awesome</em> photos. Great job. +1 rating.<br/>

About This Instructable




Bio: My name is Carl, I'm an engineer. I like to build things and solve problems. I like learning how other people build things and ... More »
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