2.5 LITER. A Spring Bolt, which is a specially designed shouldered Bolt with matching Spring and nut. It is designed to bottom out against the flange, which preserves the Spring's force (acting as a locking measure) without over-compressing it, which can lead to failures during the high temperature cycles found in Exhaust. These Bolts always mount through the flange, with a nut on the backside. Exhaust Manifold Bolt and Spring.