I don't recollect reading of the need for clearance in the rod end bearings or it may not have been mentioned and it certainly came as a nasty surprise when my closely fitted and beautifully smooth running chassis jammed horribly as soon as any corner was depressed .  How to cure this?  And my total ignorance of locomotive matters immediately came up with the answer - self aligning bearings.



  The steel block carrying the big end bush is cut from a cylinder and the strap holding it to the rod is bored to accommodate it.  At each end of the block, fitting into recesses, are steel balls which are located by corresponding recesses in the strap and the rod end.  The bolt holding the strap is eccentric over part of its length so that by turning it before tightening the nut, the assembly can be lightly preloaded, leaving the block free to turn horizontally and follow any tilt in the crank pin.  The length of the block is sufficient to allow a cavity alongside the bush to act as an oil well and this is filled, by a hypodermic, through the hollow screw at the top.  This screws into the block and is reduced in the middle where it passes through the strap to permit the few degrees of movement required and the resultant of these is a few thous of sideways movement at the little end, easily accommodated by its normal clearance.
  Having read in some ancient book that the optimum proportions for an intermittently lubricated plain bearing are L=2D, I decided to take full advantage of  the self alignment and make the bush the full length of the pin with the coupling rod oscillating over its outside diameter.  This being, unavoidably, a very short bearing, a very small amount of play here was sufficient to keep the rods happy.