What is a zero tolerance engine?

Answer 1

Also known as a interference engine, A zero tolerance engine is an engine that would damage the valves and likely the pistons without correct timing. This would occur because the valves and piston have the ability to come into contact with each other. With correct timing, this is not an issue because the piston travels away from the valves as they travel into the cylinder. When timing is lost, the piston could travel toward the valves as the valves enter the cylinder, resulting in severe damage to both. This is not an issue in a non interference engine because the piston and valves will not contact each other if timing was lost.