Frag plug hole sizes, and why fit matters

A frag rack is only as good as its holes. Too small and the plug won’t go in. Too big and it rattles around until a snail knocks it out. The trouble is that two things move: the plug you bought and the hole your printer prints.

Common plug sizes

These are the starting sizes built into ReefCAD. They’re the stem diameters most plugs land near, not a standard every brand follows.

Brands differ, and plugs from one bag can differ a little too. Treat the table as a place to start and measure what you actually have.

Measure your plugs

1. Use digital calipers on the stem, just under the head, where the plug sits in the hole.
2. Measure three or four plugs from the same bag, not just one.
3. If the stem tapers, note the size near the top. That’s the part that has to fit.
4. Use the biggest number you got. A hole that fits the fattest plug fits the rest.

Mixing plug brands on one rack? Size the holes for the bigger stems, or make one rack per brand. The frag rack generator takes any hole size in 0.1 mm steps.

Why printed holes come out small

Print a 10.5 mm hole and measure it: on most filament printers it comes out a little under. A few things stack up.

• Circles become polygons. An STL describes a round hole with straight segments. Their flat sides sit just inside the true circle.
• The plastic squishes. Each line of plastic is pressed flat as it’s laid down, so the wall bulges slightly into the hole.
• Plastic shrinks as it cools. How much depends on the filament, which is why a new spool can change your fit.
• The first layers spread. The bottom of a part is squished hardest against the bed (“elephant’s foot”), so a hole can be tighter at the bottom than the top.

Some slicers can compensate: Bambu Studio and OrcaSlicer call it X-Y hole compensation, and most slicers have an elephant-foot setting. Either works. Just print your test with the same settings you’ll print the rack with.

Too tight, just right, too loose

You can feel the difference in a second. Drag the plug on the homepage to see all three.

• Too tight: it binds. The plug stops before its head sits down. Forcing it can chip a ceramic plug or crack a thin rack, and a plug jammed halfway sits crooked.
• Just right: it seats cleanly. It pushes in by hand, the head sits flat on the rack, and it stays put when you lift the rack out of the water.
• Too loose: it wobbles. It leans in the flow, and a bump from a fish, a snail or your hand tips it out, coral and all.

Find your fit with a coupon

Instead of guessing a correction, print a strip of test holes and let a real plug pick. The free Fit-Test Coupon steps from 10.0 to 11.0 mm in 0.1 mm steps by default, each size debossed next to its hole.

1. Print it with the same printer, nozzle, filament and settings you’ll use for the rack.
2. Push the same plug into each hole. Pick the snuggest one it still goes into by hand.
3. Tap that size in ReefCAD. The difference from the plug’s size becomes your hole offset, added to every plug hole you make after that.

Example: your standard 10.5 mm plugs fit the 10.7 mm hole best. Your offset is +0.2 mm, so a rack set to 10.5 mm gets 10.7 mm holes in the file, and your printer prints them to fit. Until you calibrate, ReefCAD uses +0.2 mm.

Print the coupon again when something changes: a new printer, nozzle or filament. Print the free coupon: you sign in with a free account, and it never uses a credit.

Lids and other friction fits

A coral jail’s lid and a plug stem that drops into a rack are friction fits: two printed parts that press together. They use a second number, lid clearance, the gap per side between them. It’s 0.25 mm until you calibrate.

The coupon’s peg and socket pairs measure it, starting at 0.10 mm and stepping by 0.05 mm. Pick the pair that presses together firmly and still pulls apart by hand. Next: print a custom frag rack with your new fit.