Hello:
Many here have been making oil tanks and the capacities vary and we wonder if adequate.
I took my K2 tank, which needed to be cleaned for my project anyway, plugged the holes and filled it with parts solvent to the top mark on the dip stick. Filled it with my "ratio rite" and it came to just a hair +/- of 3 quarts. There is about 2" of dead air above that where the breather/condensor is.
I have 2 manuals, one copyright 1972, one 1978. Both say total oil capacity is 3 quarts, no mention of filter change or no. Both say on a change to drain tank and engine, put 3 qts in the tank, run for 2 min, top up if necessary. So ther's just not much oil in the engine I guess.
If I were to build a tank, I would lean towards more capacity not less. But less will work.
JMHO