If the left over part of the screws will turn you might be able to use a pair of 90 deg plyers to work them out. If they broke off because they were rusted in about the only option is 51Jimmy's advise. If you have a sharp bit & keen eye you can drill them out without having to oversize the hole. If you don't have a tap the right size you'll have to buy or borrow one. You will still have to clean the shavings out of the tank when finished. Good luck.

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Name your dog Naked so you can walk Naked in the park.