I just cleaned a tank out for a bike I am restoring.
I dumped in a whole mess of BB's and stuck the tank
in the trunk of the car for about a week. I also set
the tank in a different position every other day so a
different side of the tank would get cleaned.
Then rinsed with gas.
Thing looks like new inside.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project