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When you replace the factory MDS lifters with non-MDS lifters, the oil passages that fed the MDS solenoids are left open. These passages MUST be sealed or you'll lose oil pressure — the oil that should be lubricating your bearings and cam will be dumping out through open passages instead. These simple aluminum plugs press into the lifter valley to seal those passages. It's a $30 part that prevents thousands in engine damage. Four plugs are needed per engine (one for each MDS solenoid bank). Automatic transmission HEMIs only — manual trans models don't have MDS.

The HEMI's VVT (Variable Valve Timing) system uses a cam phaser to advance and retard intake cam timing for efficiency. Aftermarket cams aren't designed for this phaser movement — it can cause erratic timing, poor idle, and potentially dangerous valve-to-piston clearance issues. The Comp Cams VVT limiter restricts the phaser to a fixed position, effectively locking the cam timing. This is mandatory when installing any aftermarket camshaft on a VVT-equipped HEMI (2009+). Simple part, critical function.