Unlock Ring
A non-sparking drift is preferred, but with some water still on the tank, the chance of fire is low. Carefully tap each of the ring's drive tabs CCW working around the ring frequently.
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The pump mounting plate rests on the big rubber O-ring, which rests on the tank lip. The lock ring is between the pump plate & the tabs, which are welded to the tank. So the O-ring is the "spring" that holds the ring tight up against the tabs. If the ring won't rotate due to dirt &/or rust, use a hammer & drift to knock the ring & plate straight down so the rust & grit are released from the tabs & plate, but NOT enough to bend anything. Just make it vibrate. Wet the ring & tabs with motor oil to work in & rinse some of the grit out. Once all the tabs have "broken" free, the ring should rotate, using a hammer & drift. Use a shallower angle for more rotation (torque), or a steeper angle to release the ring from the tabs.
Don't do ANY of this until you have a replacement O-ring in-hand.
