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If you're working with used heads, it's critical that they be cleaned and crack-tested before rebuilding-even if you're the one who used them. It's also critical to check the head surface, and Alvarez suggests doing so even with new heads. If you have a 0.003-inch gap between a straightedge and the head surface, you should have the head surfaced. If the gap is 0.004 inch, the head is bad, and you must surface it, according to Alvarez.