Step 1: Piston Cap Alternative
Step 2: Trying the End Cap
Step 3: Trying the Foam
This type of foam starts curing when exposed to the moisture in the air, so the outer layer hardens quickly, but the parts that are buried take a lot longer. When I tried it out I got a couple good shots, then water was able to make it down past the cured foam to the part that was still uncured. The uncured foam worked its way out and fouled up everything resulting in a total loss. Sigh...
This clearly didn't work out like I expected. The idea still has merit, but I'd use a two-part expanding foam if I did it again, so that the cure is not dependent on exposure to air. On the other hand it works okay without the foam, so try it that way first.
Step 4: New Inner Pipe Design
First picture shows the end cap and the tool I fashioned to hold it. The tool is made from a couple of discs cut from 3/4" with a hole saw. The discs are threaded onto a 1/4" x 20 bolt, and a nut is used to tighten them down. Next step was to wrap the discs with masking tape until the end cap fit snugly.
Second picture shows the end cap and tool assembled.
Third picture shows tool mounted on drill press, ready to turn the groove. Doing the groove this way was a little easier than the original approach, not sure if you could pull it off with a hand drill though.
Fourth picture shows the fit test with the finished groove. The pointy tool in the foreground is a dental pick from a surplus store. It works great for picking o-rings out of grooves.
Fifth picture shows modified end cap glued onto pipe. Note that with this approach we've done away with the wooden plug (though we still had to use them to make the tool).
Sixth picture shows the fit test of the new inner pipe. Looks good!
Step 5: New Mortar Design
The handle is a 2" PVC cross, with a couple short 2" pieces. The cross is attached to the base of the outer pipe. The inner pipe runs through the cross and into the outer pipe.
Step 6: Trying It!
It's impressive looking.
It's unwieldy for little guys, the original design is more practical for them.
The handles help, but getting maximum pressure on them while staying on target is a challenge.
I'm going to try shortening the outer pipe to bring the handles up higher and see if that makes it easier for an adult to control, maybe an under-the-arm thing...