Jialing Motorcycle 125 thimble machine, when adjusting the valve, the end point is in the small hole on the left cover of the machine. Why is f not t?

The distance between point t and point f is very close. When the piston is at the top dead center, there seems to be little difference between point T and point F, but the valve timing requirement is very accurate and almost impossible. In addition, the relative position between the magneto and the crankshaft is fixed by a semicircular key. Unless the semi-circular key falls off, the top dead center will not be at point F. Check whether the position of the semi-circular key of the piston and crankshaft is normal at point T. If there is no problem, the valve must be adjusted at point T.