Hamboards Risers 1/8 (3.1mm) Rubber Riser Pad Set

3mm

These will reduce vibration between the trucks and deck and alleviate "loosening" attributed to variability in moisture content of the natural wood.  Dry air will lower moisture content of wood causing the deck and block risers to "shrink" slightly and that could make the bolts seem "loose".  Compressed rubber risers will "absorb" the variability.