12mm irrigation piping: 16mm Top Hat Grommet works beautifully. 12mm Ring Grommet fit will be a very tight fit, while a 15mm Ring Grommet’s fit could be a little loose (This could be countered by using an insert fitting (eg. straight connector/T-piece/elbow) inside the piping at the point where it goes through the grommet, to flare out the piping and create the waterproof seal)
25mm white PVC electrical conduit: 25mm Ring Grommet
Whole Saw Sizes for Grommets – Please note: these hole saw sizes are for flat surfaces, not for curved surfaces. A slightly smaller hole can be tried on curved surfaces to prevent possible leakage due to the curving of the grommet on the curved surface. (Drilling test holes and testing if seals are watertight is always advised)
6mm Ring Grommet: 9mm drill bit
12mm Ring Grommet: 19 mm Hole Saw
15mm Ring Grommet: 19mm Hole Saw
20mm Ring Grommet: 25mm Hole Saw
25mm Ring Grommet: 31mm Hole Saw
6mm Top Hat Grommet: 9mm drill bit (very tight fit), or 10mm drill bit
16mm Top Hat Grommet: 19mm Hole Saw
19mm Top Hat Grommet: 21mm Hole Saw
It is not recommended that Petroleum Jelly is used as a lubricant. A silicon lubricant, although more expensive, is better. Olive oil can also be used.
Please note if grommet is fitted and then taken out a number of times, it may stretch and warp.