Drain plug seal, I have been using these for a while now, they are much nicer than the copper drain washers. These are a steel washer with a rubber seal built into it. The nice thing is these can be used over and over again, the copper crush washer are one time use. I recommend these for use with my sump plates and other aluminum plates that use the stock 14mm plug size.
.540 ID .880 OD
Price is for two washers.