Re: 315s main rod

The rectacngular rod end houses a split bushing and an adjusting wedge. As the clearance between the bushing bore and pin diameter increases due to wear on the bushing and the pin, the wedge can be adjusted to reduce the clearance back to an acceptable limit. This arrangement was also commonly used on the front end of the main rod--the K36's were originally fitted with split wrist pin bushings on the main rods, but most were replaced with the solid type by the Rio Grande. There may be one or two still in service.

The split bushing arrangement had a few disadvantages which resulted in their replacement with solid bushings over the years. As noted by loco340, the split bushing requires more machining than a solid round type. In time, due to the excessive forces on the main rod, the rectangular cavity for the bushing can become distorted, which results in more complex machining to restore, wheresas, restoring an oblong bore for a solid round bushing is a simple matter of re-boring. And, while the adjustable feature of the split type bushing is a quick and easy fix of a pounding rod bearing, it has limitations, as the bearing bore becomes badly oblong with continued wear and subsequent adjustments. Also, the center-to-center distance between big and small end bores changes with each adjustment. I suspect also that the round type bore results in a stronger rod end than the rectangular type bore.