For the pie crust:
In a food processor pulse 3 1/2 C flour, 1/4 C sugar, 1 tsp. salt. Add 1 stick of cold butter, pulse three times until combined. Add 2 more diced sticks of butter, pulse three times, or until the mixture resembles coarse meal. Add 1 T cider vinegar. Gradually pour in 1/3 C ice water, pulsing about 4 times until combined. Turn out onto a clean surface and press into a disk, wrap with plastic wrap. refrigerate for 1 hour before rolling out. When ready divide into 2 equal parts.
In a bowl combine: 4 1/2 C cut up rhubarb, 1 can crushed pineapple , drained 1 1/3 C sugar, 6 T flour.
Place 1 portion of the rolled dough into a 10" pie plate. Pour in the filling. Dot the top with butter ( about 2 T). Place top crust on. Make a few holes on the top of the crust for venting. Brush the top lightly with milk and sprinkle with 1/2 T sugar. Bake for 1 hour at 350 degrees or until golden and bubbly.