A simple fixture was made up to support the rod caps while their rear ends were finish machined. All went well, but then I discovered the diameters of the previously machined big end bores had all come out different. After retracing my steps I found the bores had been growing by a thousandth or so with each succeeding operation. My first suspicion was built-up edge on the tool, but the surface finishes were great, and the cutting edge looked clean under a microscope. I made another test cut and it too was oversize. Since the grub screws in both the dovetail and toolholder seemed snug, I have some more detective work to do. Although the Offy's rod journals ended up within a thousandth of their target diameter, these bore variations will result in a unique rod on each journal after all.
The bearings were machined from a 932 bronze. After turning an appropriate blank, each was split using a .008" slitting saw so both halves could be used. A zero thickness kerf would produce perfectly round bearings for truly round journals, but my rod journals are out of round by a thousandth, and so I went for a next best solution.
The blank's i.d. was calculated by dividing the sum of the journal's circumference (including a running clearance) plus twice the saw's kerf thickness by pi. The blank's o.d. was then calculated by dividing the sum of the circumference of the rod's bore plus twice the saw kerf by pi.
I've included a scale drawing comparing one of my bearings with a perfectly circular equivalent. There's a .002" difference between the two which for all practical purposes vanishes when the bearing shells are deformed by snapping them into place in the rod halves. With the bearing installed and the rod tightened around a gage pin with zero running clearance, I could see no light passing between the two. The fits are probably as good as anything I could have achieved using more conventional soldered or wasted half fabrication techniques.
The rod bearings were fitted to the crankshaft with a .0015" running clearance. Although I was prepared for a slight bind as an out-of round journal rotated inside one of these compromise bearings, the fits were all uniformly smooth for their entire revolution.
The rods were finished up by drilling an oil passage connecting the bearings on the big and small ends. A fixture designed to reduce risk to the rod was used for this final machining step. A drop of Loctite 290 (wicking grade thread locker) was added behind each cap-side bearing in order to reduce the chances of a spun bearing later on. The Loctite was allowed to cure overnight with the finished rods tightened snugly around gage pins. - Terry
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