Now rotate the engine counterclockwise until you hit the positive stop again, and note the second measurement. In this case, we have come up with 43 degrees. Since the numbers do not match this means we do not have true TDC determined. To correct the degree wheel, divide the discrepancy in measurements by two. So, we have a 12-degree difference, meaning our correction is 6 degrees, or 37 degrees total (you add the correction to the small number and subtract it from the large number).