Metal washer confusion

[jim 77 done a lot]

The simple reason to NOT use a metal washer inside the tank is that a seal is required between the bolt head and the tank. A metal washer against the bolt head would leave a leakage path, albeit minor, to the bolt thread and along the threads, even via the nut on bottom. Compression of the rubber washer seals to bolt and tank.

 

[ImOld]

And...that is why many tanks now come with the bolts pre-installed.

Giving DIY's a bag full of hardware, even with pictorial instructions, is a lesson in human logic & ingenuity!