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For stems that are too tight, submerge the cap in warm water for a minute, pat dry and mount on your keyboard immediately. It should fit nicely after.

For stems that are too loose, submerge the cap in warm water for a minute, remove from water and pinch the stem gently with a tweezer. Allow the cap to cool down before mounting. It should fit nicely after. Alternatively, you can follow the steps in this video.