How to Replace a Coleman® Fuel Filler Cap Insert Gasket
You should always have a new replacement fuel filler cap on your lantern and stove when using it. The original cap looks nice for display, but it should not be used to hold pressure in your appliance.
Appliances prior to about 1963 have a 3-piece filler cap with a small hole in side that was designed to ventilate pressure while unscewing the cap. This small hole has a 
tendancy to spray a stream of fuel in whatever direction it happens to be pointing when depressurizing. THIS CAP IS EXTREMELY DANGEROUS. If you don't believe me, read the snippet off to the left and click the link.
That cap was replaced in 1963, with a 3-piece design with "slots" on the inside intended to re-direct pressure down and away from danger. This cap was called the Plamann cap (see patent). In 1970 an even safer cap was introduced, a one-piece unit, and that is the cap being sold to this day.
* Special thanks to Alain C. of St. Hyacinthe, QC Canada to point out that Coleman Canada made 1-piece filler caps with a side pressure release hole in the early 1960s, and also used a Plamann-style 1-piece cap as early as 1965.
If your lantern or stove has an original one-piece cap that is no longer able to hold
pressure, simply replace the cap. If appearance matters, go to the hardware store and find a paint to match.
If you have a 3-piece cap and want to replace the gasket, make sure you have a new one first. You can't just go to the hardware store and buy it so you'll need to order one. Use caution: some on-line sellers (mainly on eBay®) use cheap materials, don't have a clue on sizing, and will sell you an O-Ring rather than a gasket.
An O-ring is round stock and is not intended to sit against the flat surfaces of the filler cap insert or fount. It can roll and/or flex during compression, causing an unwanted release of pressure and/or fuel. DO NOT use an O-Ring in a filler cap.
Disassemble the filler cap. Tighten the filler cap down on your lantern or stove as tight as you can get it with your fingers. Take a large flathead screwdriver and turn
the large center screw counter-clockwise to remove it.
If the screw "slips" or turns too easily, spray it with penetrating fluid and allow it to sit. Apply a few gentle taps on the screwdriver with a wrench, re-tighten the cap and try again.
With the screw out, remove the filler cap. The filler cap insert will probably be stuck to the fount or tank. Pull it off, or gently pop it off with a screwdriver if necessary.
Remove the old gasket. Set the filler cap insert down on something you can safely apply flame to, like a stove brick or a large area of dirt. You NEED eye protection because it is going to spit at you when heated.
Take a propane torch and apply flame directly to the insert. It is made of brass so don't worry about damaging it. Allow the flame to make the insert glow a little bit.
Allow it to cool. Once you can touch it again, use a sharp object to remove the old
gasket. Most of it should come out quite easily. Clean the surfaces as best you can to ensure the new gasket has clean metal to rest against.
Install the new gasket. Set the new gasket on the insert and use a small flat screwdriver to gently press it down evenly. A properly fitting gasket will fit nicely in the groove. Press it down on the fount or tank to seat it.
Reassemble the filler cap. Place the insert on the fount or tank. Install the filler cap over the insert and turn it clockwise. Tighten just enough to where it starts to 
give you resistance. Next insert the center screw and turn it clockwise to install it in the insert. Once you feel the insert start to turn, tighten the cap down on the fount or tank as much as you can with your fingers. Re-tighten the center screw until tight. Keep in mind that the gap between the screw and the cap is intentional.
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