Park Tool Internal Seat Tube Clamp - ISC-4 The new ISC-4 securely holds most bikes without clamping on any frame tube. To use, simply insert the ISC-4 into the frames seat tube, tighten the knob, and clamp into any Park Tool Repair Stand. The ISC-4s hex-shaped shaft resists twisting while clamped in the stand. Works with a huge range of seat tube sizesanything with an inside diameter of 24mm to 32mm.