Twin Cam engines feature an oil pressure bypass passage within the cam support plate that is controlled by a spring-loaded plunger. The purpose of this passage is to allow excess oil pressure to be diverted back to the feed section of the oil system. Inconsistencies in 1999-2006 spring length and passage machining can cause the plunger to open prematurely and/or not fully close the passage, resulting in a loss of critical oil pressure and volume at lower RPMs. This shim assures proper spring pre-load, improving oil pressure and volume. Use with factory or aftermarket oil pumps and cam plates.