If it´s too flat you will crush the center gum washer before putting any sealing pressure on the outer perimeter. Heres the fix in lieu of replacing the cap:Find a flat solid surface.. Find a piece of pipe or a pipe fitting with about a 3.5-3.9 ID so only the perimeter (outer edge) of the cap lays on the pipe or fitting. Lay the cap on it.
Plateflow heat exchangers provide more heat transfer in less space. The Plateflow design is compact and efficient. Plateflow models have higher surface area to volume ratios than conventional shell and tube heat exchangers. Plateflow offers superior heat transfer coefficients compared to shell and tube heat exchangers.
Sierra International 18-0389 Heat Exchanger End GasketMake sure this fits by entering your model number. The Sierra 18-0389 Heat Exchanger End