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Placed between the camshaft nose and timing chain cover to prevent the camshaft from "walking" forward in the block.Pit Stop USA offers Manley Roller Thrust Button - SB Chevy w/ Early (Up to 78) Timing Covers & All BB Chevy 42111 at low everyday prices. Be sure to check out all of our products from Manley Performance including Manley Performance Cam Buttons.