Why Use a Small Base Circle Cam in a Small Block Chevy?

The base circle of a camshaft lobe is a term used for the imaginary circle of a cam that does not have a lobe. It is the portion that the lifter "rides" on when the valve is in the closed position.

Cam/Rod Interference

Interference between the connecting rod and one or more cam lobes can occur when longer stroke crankshafts and/or oversize connecting rods are used. When the crank/rod assembly rotates near top-dead-center (TDC) of the cycle, a shoulder one or more of the rods can contact a cam lobe.

Connecting Rod Modifications

To alleviate interference, a possible solution is to modify the rod by grinding the area of the rod that is interfering with the cam lobe until sufficient clearance is gained. Ideally, at least 0.035-inch clearance is desired---preferably a bit more.

Small Base Circle Cams

An alternative is to have a camshaft manufactured with a smaller base circle. In doing so, this allows the same lobe profile to be ground onto the cam, but in a position which is closer to the camshaft centerline. This can result in additional clearance---often enough to provide adequate clearance through the complete cycle of the rotating components.

References

• Crane Cams: Technical Information--"Small Base Circle Cams"
• Circle Track: Engine Camshaft Lobe Diagram
• Hot Rod: Connecting Rods Modified for Cam Clearance

Resources

• Steve's Nova Site: Discussion--"How do I determine if I need a small base circle cam?"