Gasket for NEMA 4X Enclosures

Rule of thumb in tightning connectors, bring down to a snug fit, then add 1/4 turn to seat. Never crank down on any type of threaded device. If you find yourself doing this, you are now destroying the thread face and stripping the bottom section of the thread. Which in turn, once loosed, will not seal on the next time it needs to be fixed.

Maximo

I don't think you can use % of compression unless you know precisely the material. For example, O-Rings use 10% diameter reduction; meaning if an o-ring is on a shaft in a groove, 10% of it needs to stick out above the shaft edge so a 10 mm shaft will have an 11 mm OD ring, however, O-rings are essentially non-compressible so they just deform into the groove and rely on elasticity to force a seal in the "10%" regime. If there was no place to deform, the o-ring would not compress and trying to compress it would strip the bolts.

If you forced me to guess on closed cell foam, I'd say no more than 20% reduction and likely 15%, but it is really a wild guess uneducated and based on me thinking how the trapped gas bubbles compress. It also depends on how wide the gasket surface is to allow more or less side to side deformation.

Tell us what is the real issue at hand? You want to seal a box that isn't sealing, change gaskets, or prove to the manufacturer that they screwed up their crush or material selection ?

Many NEMA 4X enclosures are designed so that the threaded fasteners are torqued to the ordinary value for a fully-tightened fastener, with metal-to-metal contact (or fiberglass-to-fiberglass contact) occurring in a boss typically encircling the fastener. Thus the gasket is placed under controlled compression, filing a designed-in space. That space depends upon the stiffness of the gasket, but for foams, it is often about 50% of the uncompressed dimension (I'd say by casual observation).