Although the crankshaft and block were machined long ago, the main bearings were not. A portion of the block's machining included recesses for seven 7/8" diameter bearings. These were machined using a single g-code routine in hopes of getting them all identical and in a straight line through the center of the crankcase. Ballbearings were installed in the two outside recesses, but the five inner ones have been awaiting two-piece bronze bearings.
Their machining began by turning work-holding spigots on the ends of a pair of SAE660 rounds. Each was sized for five bearing halves plus a spare, and their o.d.'s were turned to what will be the finished diameter of the bearings. One of these blanks, used for the halves that will be mounted inside the recesses, was marked for later return to the lathe in its same orientation.
After careful setup on the mill, half the diameter of each blank was machined away. The two workpieces were then clamped together with hose clamps while the hole pairs for the bearings' mounting bolts were drilled, counterbored, and temporarily tapped. With the workpiece halves joined together with temporary screws, the assembly was returned to the lathe where the through-hole for the bearings' i.d.'s was drilled and reamed.
When the crankshaft was machined, its two outer journals were turned just under .375" for their fits inside the ball bearings. The inner journals, although identical, wound up slightly elliptical with cross-measured diameters of .366"/.367" resulting from unavoidable flex during machining. To compensate for these errors and to provide adequate clearance in the engine's pump-less splash oil system, a .370" reamer was run through the assembly before the bearings were parted from it.
After parting, but before separation, each bearing half was engraved with locator numbers to avoid mix-ups during assembly. In addition, an oil collection slot was machined into each bearing's top half to aid lubrication. After reaming out the threaded holes for mounting bolt clearances, the bearings were installed and test fitted one-by-one.
I'm not a fan of brute-force spinning a tight crankshaft with an external power source until it 'loosens up". Instead, the fitting was done by bluing and scraping each bearing and/or machined recess as needed. This was a tedious process that required some dozen hours and something I'd put off as long as possible.
Rather than begin fitting using the actual crankshaft, I started with a .375" diameter test bar whose diameter between ends had been carefully turned down to .368". This diameter was chosen to arrive at a fit that would hopefully allow the crankshaft to be dropped in with minimal cleanup.
My initial plan was to fit the bronze bearings one-by-one to the test bar with it installed in the ballbearings. However, I discovered four of the five mounted bearings had actually come out as hoped and would require minimal work if the front ballbearing was instead lowered .0015" by scraping its recess. So, I ended up first fitting the five bronze bearings to the bar without the ball bearings and then fitting the front ballbearing to the bar.
With all the scraping/polishing completed, the crankshaft actually did drop in as hoped. When turned by hand without the ballbearings, the .001" errors in all five of the crankshaft's dry egg-shaped journals could be felt as tiny 'bumps' telling me the fits were as good as I could expect. Squirting a drop of oil into the oil collection slots made the bumps all but disappear. Installing the ballbearings totally eliminated them.
I've never found the fitting of a multi-cylinder crankshaft to be satisfying. Regardless of the steps taken to stiffen a long skinny crankshaft during its machining, there's always a bit of flex that leaves circularity errors in its lathe-turned journals. And these errors invariably limit the quality of the bearing fits. It could be argued that unless the journals are ground, some variation of the brute force fitting technique is good enough. I've spent a small fortune on tool post grinders though and still don't have a usable solution for crankshafts running on my lathe. - Terry
