Piston to wall clearance is the space between the piston and the cylinder wall in an engine. It is important to check piston to wall clearance regularly to ensure that the engine is running properly. If the clearance is too small, the piston can seize up and damage the engine. If the clearance is too large, the piston can move around too much and cause the engine to lose power.
There are a few different ways to check piston to wall clearance. One way is to use a feeler gauge. Another way is to use a dial bore gauge. The specific method that you use will depend on the type of engine that you have.