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@Drench: What is the ideal pipe ID for the grommets? As you know, PVC is made to very precise OD specifications, but the ID can vary wildly. This is why I have trouble seeing PVC as a good, consistent pump shaft material.



^ In this image, both pipes are essentially identical, 1/2" Charlotte "True Fit" sch 40 PVC, both sourced from Home Depots in Connecticut (not the same ones but in the same county). You can clearly tell that one of the IDs is vastly larger than the other. 1/2" copper, aluminum, and at least one size of brass will nest in it, which makes the larger ID pipe an incredible material for barrel sheaths in Nerf hoppers. However, it also makes one of the pipes fail against a homemade water gun pump seal.

The only way to really avoid this problem is to know the exact ID that is best for those 5/8" grommets and measure the pipe you see in the store before buying it.

OD of grommets differs from OD of O-rings, with O-rings being slightly higher. I haven't tried grommets yet (they work in the older pipe I have but not the newer), but O-rings have been working for me. I should point out that the MK II in the Op failed at the inside, as it leaks through the rod itself, not around it, so the outside of the O-rings works fine.

I recommend direct experimentation in all cases. Test if the seal you're using fits in the pipe you're using before you buy, even if following my guide.

Drenchenator's design looks to be very good and is what inspired mine. I'm not exactly keen on using a wooden dowel, but it should make things easier to build and work with. I may need to find 11/16" for the rubber grommet OD for the pipe I have though.

Regarding pump diameter, DX and I had this discussion earlier. We agreed that larger diameters, such as 3/4", may not only be effective, but may be more so than 1/2". Keeping in mind that the main purpose of building the homemade is for combat, the higher pump volume is great for APH's and how much pumping they require. High pump diameters (around 3/4") have been proven in the XP 150 and SS 300.

Uh the 300 has a 1" diameter pump, I measured. Still need to fix the pump handle on mine

I've only measured the 150's, which is around (just over) 3/4".

Duxburian wrote:@Drench: What is the ideal pipe ID for the grommets? As you know, PVC is made to very precise OD specifications, but the ID can vary wildly. This is why I have trouble seeing PVC as a good, consistent pump shaft material.

I've used rubber grommets as pump seals in both Riptide and Cloudburst, and both had slightly different IDs despite being nominally the same size (1/2 inch schedule 40 PVC). If I remember correctly, Riptide's ID is larger than Cloudburst's ID. But this doesn't really matter all that much, because you can squish the rubber grommets enough to make the seal thicker, if you need to.

What do I mean by squish them together? Well, the grommets are just about 5/8 inch in diameter just out of the package (when not subjected to any force). But when installed, the screw squishes them in the direction of the motion, which makes them expand radially. This radial expansion means that even large ID 1/2 inch schedule 40 pipes will still seal well. I remember having no trouble inserting the pump rod into Riptide, but I had more trouble with Cloudburst (which had slightly thicker pipe), so I just dialed in the screw a little to adjust the tightness and used a little more lubricant.

So, to be brief, I think this design works well despite the variations in pipe size.