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The 1957 vacuum advance mechanism contains a spring, a stop, and washers. Washers can be removed to advance the timing or added to decrease the timing.The stop P/N 12202 is .25" i.d.-.36" o.d.-.95" long.The stop was increased to 1.00" long with dual 4/b carb and the later distributor. The stop restricts vacuum timing. The importance of the internal components of the vacuum advance mechanism and its proper adjustment should not be overlooked. If any of the components are missing it will lead to erratic timing and can contribute to an over heat condition. The threaded cap can be removed with a 3/4 wrench to make the necessary washer adjustments. When the cap is reinstalled insure the brass washer is in place which seal the vacuum chamber.