Is this an older 'square taper' crank? check the taper hole on the arm it maybe rounding out, cranks that repeatedlt come loose is sure sign cause of the hole rounding out.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk