Every gasket spec sheet has the same four numbers buried in it somewhere, and every leak we get called out to investigate traces back to one of them being wrong, ignored, or guessed at. Not exotic failures — the basics. So before you place another order off a part number alone, here’s what to actually check.

Start With Thickness & Tolerance

Thickness sounds like the easiest spec on the sheet, and it’s also the one people get wrong the most, usually by trusting a nominal number instead of a tolerance range. A sheet listed as 1/16″ can run anywhere from 0.058″ to 0.070″ depending on the manufacturer’s tolerance band — and on a tight bolt-load flange, that difference changes how much crush you actually get.

When you’re cutting a ring gasket, tolerance matters even more, because compressibility is what makes the seal. A material that’s too far outside spec either won’t compress enough to seal, or compresses too much and extrudes out the ID over time.

Temperature & Pressure Ratings Aren’t Optional Reading

Most spec sheets list a maximum continuous temperature and a separate maximum intermittent temperature — and they are not the same number for a reason. Running a gasket at its intermittent rating as a steady-state condition is one of the fastest ways to cook a binder out of a compressed sheet material and turn a seal into a leak path.

A gasket rated for 400°F in a 600°F line isn’t a corner you cut once. It’s the seal that decides whether your unit stays up.

Pressure class works the same way, and it’s tied directly to the flange rating, not just the line pressure. A Class 300 spiral wound gasket in a Class 150 flange isn’t “extra safe” — it’s the wrong part, and it usually shows up as an uneven crush across the ring.

Where People Actually Get Burned

Almost every temperature-related failure we’ve seen didn’t come from a bad batch of material. It came from someone reading the “max temperature” line on a data sheet without checking whether that number applied to steam, to a dry gas, or to a cyclic process — because chemical compatibility and temperature rating are almost always tested together, not separately.

Chemical Compatibility: Match the Media, Not the Marketing

“Chemically resistant” on a spec sheet is doing a lot of work. It usually means resistant to a specific list of media at a specific concentration and temperature — not resistant to whatever is actually flowing through your line. This is where PTFE and filled compressed sheet materials genuinely differ: PTFE’s resistance is close to universal, while a compressed sheet’s binder can be attacked by exactly the acid or solvent you’re sealing against, even if the fiber itself would hold up fine.

If your line runs anything outside water, steam, oil, or mild gas, it’s worth five minutes on the phone before you order — not because the spec sheet is wrong, but because it’s usually written for the general case, not your specific one.

Stock vs. Custom: Reading Between the Lines

The last line most people skip is stock status. A material can meet every spec you need on paper and still add two to four weeks to your turnaround if it has to be cut to a custom pattern or sourced special. Standard sizes in stock ship the same day; anything outside that gets built to order, and the spec sheet won’t tell you which situation you’re in — only your supplier will.

• Confirm actual measured thickness against tolerance, not just the nominal size
• Separate continuous vs. intermittent temperature rating for your real operating condition
• Match pressure class to the flange rating, not just the line pressure
• Check chemical compatibility against your specific media and concentration
• Ask about stock vs. custom lead time before it becomes a turnaround surprise

None of this requires an engineering degree — it requires reading past the part number. Send us your spec sheet or your old gasket and we’ll tell you, straight, whether it’s the right call or not.