Hi, David. Some people are more familiar with metric sizes, some not. Grungebuster Dots and washers have a jillion potential uses; in most cases, exact precision isn't needed and your best guess regarding size is appropriate.
Grungebuster Dots placed against the flat side of cones or under cone cups usually work very well at the same size, or just slightly undersize in diameter for neat appearance. Either 18mm or slightly larger 3/4" will do fine; I think appearance would be the only significant factor between them. The Dots would not have to precisely fill the entire recess, but there would be no harm in that, either.
Though not every size is listed, Extra-Thick grungebuster Dots can be made in any of the sizes listed for Thin and regular grungebuster Dots, so you wouldn't necessarily need to stack two thinner Dots to achieve a desired thickness. By stacking, however, you have additional leverage with auditioning--you're likely to find that a single thinner Dot is sufficient or ideal and end up with some extra Dots for use elsewhere in your system.
When using grungebuster washers in a configuration where a nut or spike end will be tightened against the washer, you'll want a matching Teflon washer so the nut can be turned against the material and tightened without pinching the grungebuster material.
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