A system for an engine defined about an axial centerline includes an inner case that includes a bushing, a duct case radially outward of the inner case with respect to the axial centerline, and a seal located on the duct case. The system further includes a borescope plug assembly that traverses the seal and the bushing in mounting to the inner case. The borescope plug assembly includes a spring, a housing sleeve, and a washer sleeve at least partially nested within the housing sleeve. The spring is isolated from the seal by the housing sleeve and the washer sleeve such that the spring is contact-free with respect to the seal.