CRL's 6" Vacuum Cups With Threaded Studs have a rugged accessory mount with a 1/4-20 x 3/8 inch long threaded stud for attaching equipment. Attaches to flat or curved, smooth, non-porous surfaces. The Red-line indicator warns user of any vacuum loss and the check valve allows re-pumping without loss of remaining vacuum. Lift tabs on pad edges permit instant release. Each cup is supplied with a vacuum pad protector.