Spacer bearing makes up for difference in crank journal diameters.
Clevite's H-Series was developed primarily for use in NASCAR type racing, but they are suitable for all types of competition engines. These sets are specifically for 1969-Up 302-305-327-350 S/B Chevy "Large Journal" engine (with 2.100" rod journals).
Not to be confused with standard "passenger" car bearings, Clevite's P-Series is their oldest line of premium high performance bearings. These sets are specifically for 265-283-302-327 S/B Chevy "small journal" engines built from 1955-1969 (with 2.00" rod journal).
P-Series Features (Link)
Choose ARP S/B Chevy Main Stud Kits for the ultimate in strength and reliability. These studs fit 2-bolt main blocks and accept windage trays (windage tray not included).
Place a piece of this thread on a rod or main journal, torque cap in place, remove cap and by using the included chart, you know what your bearing clearance is. You should have one of each when assembling your engine.