Irwin Record IWTM130 Clamp Heads, Black ,22.0 cm*6.0 cm*31.0 cm

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and the clamp head. Note the hard corners cause problems between the clamp head and the sash bar of alluminium.

Now you have some decent working clamps for about a third the price of most others. You will be amazed how these minor upgrades make the clamps feel in use. I cleaned the clamping surfaces with rubbing alcohol and then applied plywood pads with a high quality double sided tape, so the hold in place just fine. Step 3: Use a light lubrication like petroleum jelly to reduce friction between the head and shoe. I also use this to lubricate the threads of the head clamp and along the bar itself. This act alone makes everything slide perfectly.