The pistons are made from an aluminium alloy, designed to withstand high temperatures. An indentation is incorporated into the piston head to prevent the pistons from hitting the valves, should the timing belt break (this is not true of the later BEAMS - an acronym which stands for Breakthrough Engine with Advanced Mechanism System - motors). This is commonly referred to as a "non-interference" engine. Piston pins holding the pistons in place are locked by snap rings. The "Outer Shim Type System" allows for the replacement of the shims without the need to remove the camshaft. To adjust the valve clearance, adjust the shims above the valve lifters.