Heavy Duty Technician Engine Level 2 Practice Test

Piston ring end gap is measured by placing the ring into the cylinder bore and checking the clearance between the ends while the ring is at the bottom of the bore. This location gives a stable, representative gap that the ring will have during operation, away from the crown of the piston and other top-deck features that can skew the reading. The bottom of the cylinder is where the ring sits when the piston is near bottom dead center, and measuring there avoids the influences of the piston crown and top land that could make the gap read too small or too large. If the gap is too small, the ends can butt together and bind as the ring expands with heat; if too large, compression and sealing suffer. End-gap measurement inside the ring groove or at the top of the bore doesn’t reflect the actual end clearance the ring will have in service, which is why the bottom-of-bore location is used.