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 Initially, all modern 4-8-4's in the 1940 era that were equipped with roller bearing rods using the so-called tandem rod connection between #2 and #3 axles. In addition, because of this configuration, all of them had relatively long crankpins on #4 axle.  Because all railroads used the same method (tandem rods), this was likely the industry standard for handling the forces that had to be distributed through the running gear, particularly between axles #2 and #3 of a 4-axle engine set.

N&W revised the Class J rods to do away with the tandem rod setup between #2 and #3 driving axle on four locomotives  from about 1952 to 1956.  From a side view, the change can be seen in a much larger rod hub on #3 axle, because one rod bearing must carry the same load that two smaller bearings carried before.  Also the hub on #4 axle is much shorter (the rod is closer to the driver face) which reduced the stresses that were present in the longer crankpin
Dave Stephenson



    On Friday, October 14, 2022 at 10:41:35 PM EDT, NW Mailing List <nw-mailing-list at nwhs.org> wrote:  
 
  
Dave,
 
Pardon my ignorance, but what is the modification of rods on these four Js?
 Gordon Hamilton

  On 10/14/2022 10:29 AM, NW Mailing List via NW-Mailing-List wrote:
  
 
 The four J's that I have for modified rods were 600, 605, 610, 611.  So with your photo, we've narrowed that to two.  Really does look like 611 in the negative enlargement, but the top part of the second "1" does look a little different  from the top part of the first "1"............  Thanks for posting!! 
  Dave Stephenson 
 
  
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