FAA Powerplant Written Practice Test

What method can be used to check the fit between a valve seat and valve face?

1. Using a pressure gauge

2. Prussian blue or lapping the valve

3. Measuring with calipers

4. Visual inspection only

The correct answer is: Prussian blue or lapping the valve

Using Prussian blue or lapping the valve is a well-established method for checking the fit between a valve seat and valve face. This technique ensures that there is proper contact between the two surfaces, which is essential for maintaining engine efficiency and preventing leaks. Prussian blue is a dye that can be applied to the valve face. When the valve is pressed against the seat, the transfer of the dye will reveal areas of contact. This allows the technician to see if the entire face of the valve is making adequate contact with the seat, indicating a good seal. If there are gaps or areas where the blue dye is not present, it shows that there may be an issue with alignment or surface wear. Lapping the valve involves grinding the two surfaces together with a fine abrasive compound. This process further refines the fit and ensures that any imperfections on either surface are smoothed out, creating a better seal. Lapping provides not only a visual indication of contact but also promotes better surface conformity. In contrast, using a pressure gauge focuses on measuring the pressure within a system and does not directly assess the physical fit of the valve and seat. Measuring with calipers could help in determining dimensions, but it would not necessarily indicate the quality of the contact between the valve