The 6.0 I'm looking at already has an aftermarket cam — what do I need to know about its valve springs?

Find out what cam and springs are in it. Springs have to match the cam's lift and duration: too weak causes valve float at rpm, and too little travel causes coil bind, which can damage the cam, lifters and retainers.

• Ask for the cam card. It lists the recommended spring rates.
• If nobody knows what springs are in it, plan on replacing them with springs matched to the cam.
• Installed height depends on any head machining and gasket thickness, so check it as assembled.

How to choose (Speedway):

• Start with the cam card. Cam makers list recommended spring rates for the lift and duration, even if they don't name a spring.
• Lift: more lift compresses the spring further. The spring has to handle it without coil bind (coils touching), which can damage the cam, lifters and retainers.
• Duration: longer duration keeps the spring compressed longer; higher-rate springs prevent valve float at rpm.
• Your heads as assembled: installed height changes with machine work (decking or surfacing) and head gasket thickness, so check it on your engine.
• Type: single springs suit stock and mild cams. Performance cams want dual springs or high-rate singles. Beehive and conical springs cut weight at the top for rpm without going to duals.