Crankpin Location at Top Dead Center

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Jun 11, 2005
Hello to All!

I have a question about crankshaft rotation, specifically the reference point of the crankpin (timing/right side, #1 piston) when the piston is at top dead center.

As the barrel is not directly above the crank/case at 12 o' clock like earlier singles/twins, what is the crankpin location when the piston is at top dead center?

When piston #1 fires what is the location of the crankpin?

If the cylinder is canted 10 degrees from vertical, the crank pin will also be at 10 degrees when the piston is at the top of its stroke, or TDC.

Now, when the engine is running, the ignition system triggers the spark plug to fire at 28 degrees before the piston and its crank pin reach TDC. This advanced ignition gives the air/fuel mixture time to burn.

Or one could say (applies to any engine with straight connecting rods): When the centerpoint of the crankpin axis lines up with the line formed by the centerpoint of the crankshaft axis and the centerpoint of the wrist pin axis, that is the exact TDC.
Unless the pistons have offset pins, or the cylinder axis is offset from the crank center, or both.
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