A method and apparatus for forming a seal over a hole in a structure. The apparatus comprises a washer, a sleeve, and a fastener. The washer has a first channel. The sleeve has a second channel and an expandable section. The sleeve is positioned inside the first channel in the washer. The fastener has an elongate portion positioned inside the second channel of the sleeve to cause at least a portion of the expandable section of the sleeve to form a first interference fit with the washer.