Fuel Canister Cap? Or No Cap?
Many hikers do not carry the plastic cap that comes with the fuel canister. As a result they save about 2 grams of weight. However, the canister cap serves a valuable purpose by protecting the threads of your canister and keeping dirt from entering the top of the fuel canister.
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Use the cap: it protects the canister threads and keeps dirt out!
A typical canister that carries 100 grams of fuel weighs around 200 grams. The larger canisters that carry 230 grams of fuel weigh around 360 grams. This means the 2 gram fuel canister cap makes up about 1% or less of the total weight.
Damaged threads can damage your stove threads when the canister is attached. Cross threading a stove is one of the most common reasons for stove replacement on the trail (which is both expensive and inconvenient)!
Clean Threads
Dirty Threads
Any dirt in the top threaded area of the canister will enter your stove when it is attached, which increases the chances of clogging the stove and reducing performance.
You save about 2 grams (the weight of about 2 paperclips) by leaving the cap behind, but the risk to damaging your stove is relatively high. Therefore, I recommend using the canister cap to protect your canister and stove from damage.