I put a new drain assembly in a kitchen sink. It has the typical metal threaded nut to attach a plastic or metal tailpiece with a flat flange. It came with a clear plastic "washer" that is the same as the flange on the tailpiece.

If I put the washer inside the tailpiece (PVC), I can't get the metal nut to bite on the drain threads, the flange of the tailpiece and washer are too thick together. But without the washer, I have a very small leak if I fill the sink and let it drain.

Are the washer and flanged tailpiece meant to be used together, or is the washer only used with different tailpiece or something?