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Monday 18 December 2017 at 17:40
Quoted from boilerman:a 1/4" piece of a rubber band glued to the inner lip of the cap work great too
Have used this method several times. A little rubber cement, small piece of rubber band, 1/2 hour drying time, and your pop cap is secured.
Source: http://pinside.com/pinball/forum/topic/how-to-secure-loose-bumper-caps#post-4129645