Not sure if this would work but could use a little gun black or chalk or something to help fitting the cap. As Pharmboy said he could see where the cap was binding. Gun black does this when fitting a stock. You put it on the barrel and try the fit in the stock, everywhere that the barrel hits the stock some gun black rubs off so you know where to take wood off. Same idea could be used on the cap if careful not to get any in the tank.

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