BoaterTalk FAQ: Trimming Gaskets
Trimming Gasketsby Richard L. Hartman (rlhartman / RLH)
Last updated 21 Sep 2002
Uh oh, another religious argument. Others say "stretch", but I'll give you some solid reasons for trimming.
Reason #1: Material Fatigue
Compliant (i.e. stretchy or elastic) materials are designed to have "give", but like anything else the more you push their limits the more you stress them. Stress shortens their useful life - and their reliability. Think of it this way: Is a rubber band more likely to break if it's the right size and just snug, or if you have to stretch the daylights out of it to make it fit? It is far, far better to use high-tech products as close to their intended operational envelope as possible. That gasket was molded to be a certain size, and forcing it so far out of its operational envelope that it actually takes on a permanent deformation is NOT what the latex manufacturer intended.
Reason #2: More is not always Better
Gaskets are meant to be stretched only slightly - just enough to provide a good seal. And you can get a good seal with only a little pressure. You do not need crushing pressure on your neck or wrists. This is not a case of "tighter is better". Once it's watertight (and probably airtight too), additional pressure does not make it MORE watertight or airtight. It only stresses the material and makes you uncomfortable.
Reason #3: They're Meant to be Trimmed
Gasket manufacturers intend you to trim them, not stretch them. That's why they have those raised ribs running around their circumference. They're certainly not there as "rip stops", because once a rip gets going in a latex gasket those little ribs don't impede it at all (I've been there). The gasket manufacturer included the ribs as trimming guides. Take their recommendation and use them.
My recommended way to trim a neck gasket is to turn the garment inside out so the ribs are on the "outside" of the gasket. Get a ~2 quart stainless saucepan, deeper/taller is better, and wrap the gasket around the pan so the handle is on the outside (this makes it easier to position the pan as you work). I recommend an old saucepan, perhaps one you use for camping, since the trimming operation is likely to leave a scratch in the surface and most wives are not understanding about such things [grin].
The gasket should be snug on the pan, stretched a little but not too much. Too tight and the gasket will be under extreme stress while cutting (dangerous); too loose and the gasket won't cut as cleanly. The edge of the gasket should be a nice straight line all the way around the pan.
Using a brand-new razor knife blade held bare in your fingertips (buy a new pack of five for a buck or two, that's a $50 gasket you're trimming), press the blade tip through the gasket centered between the first two ribs so that you're cutting away one, and only one, thin ring of gasket material. The blade tip must be held against the metal of the pan. Slowly and carefully start sliding the blade smoothly along the metal surface, cutting the gasket as you go.
Do NOT pull the ring of waste material away from the blade, just let it sit there snug against the pan. You want to minimize the stress on that cutting point where the blade touches the gasket because until that ring is completely trimmed away, it's the most vulnerable point on the entire gasket.
Rotate the pan as necessary to keep the blade moving around the gasket. It really helps to have two people when doing this, with one person moving pan and fabric as necessary to keep the cutter's access easy.
While not absolutely necessary, it's much safer to do the trimming operation as a single continuous cut all the way around. If you must stop and restart, pull the ring away from the cut VERY SLIGHTLY so you can place the cutting edge back in the same place it stopped.
Some people prefer to center the cut between two ribs. Others try to cut right up against a rib. (In these photos I'm doing the latter.) Whichever you prefer, keep the cut straight. When you get all the way around, slow down, look ahead, and aim for the start of the cut well in advance so you can correct any positional differences over a large distance rather than a sudden angle. Finally, draw the blade through the last bit of latex to separate the waste ring from the rest of the gasket.
Put the blade aside safely, then CAREFULLY pull the waste ring away bit by bit around the pan. It's possible that you may not have trimmed completely through the latex in one or more places. If so, use the blade to CAREFULLY, CAREFULLY finish the job in those places. Do everything very slowly, and think about it twice before you do it.
Try on the newly trimmed gasket, and repeat the above sequence as necessary to achieve a fit which is just a little tighter than is comfortable. Once in use, the latex will stretch a bit and you can always remove one more ring if you decide that is necessary. Give it a few sessions on the river before you do, though - it's easy to remove material, impossible to put it back.