Speedo cap

Is there some sort of knack to get the speedo cap to seat properly?
I can get the bottom bit flush but then I tap top bit an bottom comes out
I tried putting a socket on and tapping that but that doesn't work either

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Not really. Should sit flush.

 

Just stretch and pull it on

 

Check the cap is round and its not just rocking on the wide bit where somebody trod on it.

 

I like to pick the S of mine