Why did they have such heavy connecting rods? That's something that always puzzled me about the J's.
The J-1s made during WWII had those heavyweight side rods because making the lighter ones required a more exotic mix of lightweight metals, used Timken roller bearings, and in general made use of "strategic" war materials. The pre- and post-war locomotives all used a piston, piston rod, crosshead, side-and-main-rod assembly that was a special light-weight design by the Timken Roller Bearing Company, part of the effort to achieve the most perfect counter-balancing possible. According to vibration calculations, the balancing theoretically would have allowed speeds of up to 140 mph without the rail damage that could have occurred with conventional designs.
According to
Wikipedia, " In 1943, 605–610 were delivered without shrouding and lightweight side rods, due to the limitations on the use of certain materials during the war; due to these distinctions, they were classified J1. But, when N&W showed the War Production Board the reduced availability numbers because of this, the Board allowed the J1s to be re-fitted as Js with the lightweight rods and shrouding in 1944."
#610 was one of the J-1 "war babies", and after rebuilding with lightweight rods and steamlining, the PRR tested #610 at 110 mph, near Crestline, OH, in 1945.
