They forgot to drop in a 645...

This was done in 1955 in the 567 era.

Extra 2200 South in Issue 130 says "EMD tried very hard to convince the Santa Fe they should re-engine their PA1/PB1 fleet with General Motors engines, and the railroad tried it out by having the 51, 51A and 63 re-engined with 567C engines. The Santa Fe found out all too quickly that they now had three 1750 HP locomotives rather that three 2000 HP locomotives, and the reduction in horsepower had a significant impact on their performance. At 5250 HP the 51-set could not maintain the schedules with the same trains. As a result no more PA1/PB1 locomotives were re-engined. The only way around the problem was to try and restrict the three units in the 51 set to lighter trains, or not run them together so the loss of horsepower had less effect. They spent a lot of time in California on Los Angeles - San Diego trains."

The three units kept their GE 746 traction motors and the GE 5GT566 main generator.

The 51C (ex 63) was retired in 1966 after a wreck. The other two were retired in June 1968 about the same time as most other PA's.