It's a 4-bolt main engine, all others were 2-bolt. There were some RA III engines that were 4-bolt main but not all. Also the valvetrain in a RA IV was completely different. Longer swirl-polished & tuliped valves & springs, retainers & rocker studs were used. Because the cam was so radical for it's time it required a beefier valvetrain. Also the pushrods were longer & were 11/32" diameter as opposed to the standard 3/8" diameter in all other engines. Almost nothing is interchangeable with any other Pontiac engines. Rocker arms were 1.65 ratio as opposed to the 1.5's in everything else. Rods & crank were not forged, however. It did not need a forged crank, but probably could have used better rods, nonetheless it was a monster engine & the peak of performance in the 400 engine.
