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Engine and clutch (stock)/Top dead center
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Overview
Top dead center (TDC) is the point at which the piston is positioned at its maximum height in the cylinder. This cannot be seen on the assembled engine, so the crankshaft pulley is marked at the corresponding point. The position is identified by aligning the mark on the pulley with the split centerline of the crankcase behind the pulley. These markings are an essential part of engine timing adjustments and valve adjustments.
All pistons do not reach TDC at the same time. For air-cooled Volkswagen engines, the term TDC is typically used to describe the position of the #1 cylinder (located on the passenger side, toward front of car). In this engine, cylinder #3 is always in the same position as #1; cylinders #2 and #4 are exactly opposite, so they will reach the top position when the engine has rotated 180 degrees past TDC.
If you aren't certain what each mark represents on the pulley in your car, it will be necessary to identify them using this guide. If your pulley doesn't have any markings at all, buy a new stock VW Beetle pulley. With the exception of positive stop, methods for determining TDC with a completely unmarked pulley are generally inaccurate.
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