Why was the Indianapolis 500 race stopped after 255 miles in 1976?

The 60th 500 Mile International Sweepstakes was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 30, 1976. Polesitter Johnny Rutherford took the lead on lap 80, and was leading when rain halted the race on lap 103. Two hours later, the race was about to be resumed, but rain fell again. USAC officials called the race at that point, reverted the scoring back to the completion of lap 102, and Johnny Rutherford was declared the winner. Rutherford famously walked to Victory Lane, his second career Indy 500 triumph, having completed only 255 miles (410 km), the shortest official race on record.

Rutherford's victory would be the final win at Indy for the venerable Offenhauser engine. It was the beginning of the end of an era which had seen 27 Indy 500 victories for Offenhauser.