To date, the only split sleeve bearings I've made were the main bearings done earlier for this engine. For those, I took a whimsical departure from common practice and went down an involved but fun path that included rolling out pure silver plate and forming shell bearings using my own shop-made press dies. The final results with their pressed-in oil grooves turned out better than expected, and the experience was certainly worth the time and effort put into it. But, having been there and done that, I wanted to try something different and, hopefully, much simpler for the rod bearings.
While researching piston ring fabrication techniques several years ago, I came across a method being used by a model builder who was making his own cast iron rings. His technique was to measure the diameter of his cylinder bore from which he calculated a circumference, and to this circumference he added the width of the slitting saw that would be used to cut the ring's gap. This new circumference was then divided by pi and used to calculate the o.d. of the blank from which the rings were parted. Although the builder claimed his rings fit his cylinders perfectly, it seemed to me that most of the contour change needed to even get one of these rings inside its cylinder would occur at its highest stressed point directly across from its gap. His fits were evidently good enough to get his engines started, but I doubt they were perfect. I was more impressed by G. Trimble's quantitative arguments and adopted his method for making my own rings. I felt, however, that someday I might apply this builder's technique to making split sleeve bearings. With two individual bearing halves there are no concentrated areas of stress. Only one of two saw kerfs would have to be absorbed in each split bearing's contour adjustment when inserted into its rod. Since the absorption areas are much wider, a more uniform contour adjustment is possible.
I decided to finally adapt this ring-making technique to the machining of the Quarter Scale's rod bearings. To calculate the o.d. of the bearing blank, I started with the diameter of a rod's big-end bore which I multiplied by pi to obtain its circumference. To this I added twice the kerf width of the saw that will be used to slit the blank plus an additional thousandth for 'crush'.
In order to establish the bearing's thickness, I performed a similar calculation for the blank's i.d.. To the diameter of the rod journal I added .003" for a running clearance before calculating a circumference. To this circumference I added twice the width of the saw kerf and then divided the result by pi to obtain the i.d. of the bearing blank. The running clearance would have been closer to .0015" if my rod journals had ended up perfectly circular. Despite my best efforts during the crankshaft machining to prevent the Merlin's ten inch long 1144 alloy crankshaft from flexing while the journals were being turned, they wound up with circularity errors. On such a long crankshaft, grinding the journals would probably have given a better result.
The end of the rod blank was turned/bored for only one bearing at a time since I had decided to machine a custom size bearing for each journal. After machining its end to the calculated inner and outer diameters, the blank was moved to the mill and carefully slit on its exact center using a .010" slitting saw. In order to accurately locate the rather unwieldy thin bade, I marked the top of the bearing with a Sharpie and then carefully lowered the backwards spinning saw blade until it just rubbed the mark. The saw was then lowered half the diameter of the blank plus half the thickness of the blade before reversing the spindle and splitting the bearing.
After engraving the end of the bearing with an identifying number on each side of the slit, the blank was moved back to the lathe. A close-fitting Delrin rod, supported in a rotating tailstock chuck, was inserted into the end of the blank. The slit bearing was clamped around the rod using a simple shop-made Delrin clamp so everything would be nicely held together during the parting operation. After parting, but before removing the clamp, both ends of the bearing were manually chamfered with a large 90 degree countersink so the inner edges of the finished bearing would clear the journal's tiny inside corner fillets.
SAE 660, also known as 932 bronze, is probably the 'gold standard' for tin-leaded bearing bronzes, and it was originally specified for the Quarter Scale's rod bearings. This spec was later changed to a somewhat less common 936 bearing bronze which is about 10% softer than 932. I suspect this was done to reduce wear on the blade rods which were specified to be machined from 6061 aluminum. Since I used a harder 7075 alloy for my rods, this probably wouldn't have been an issue. However, I didn't have either alloy on hand in a large enough diameter, and so I ordered a $hort cored length of one inch diameter 936. I'd have wasted a lot less material with a 3/4" diameter workpiece, but no one seemed to have that diameter in stock when I placed my order.
Before launching the entire lot of bearings, I measured and recorded the diameters of all the rod journals as well as the bores of all the fork and blade rods. Using these measurements I pre-selected a best-fit rod pair for each journal and then machined a single trial bearing in order to test out the theory.
When placed side-by-side, the finished trial bearing halves don't appear to form a perfectly round circle. But, just like the commercial automotive bearings I've used, they literally snap into place inside each rod half. Under a microscope I checked the fit of the bearing's contours to the bores of the fork rod and cap before torquing the two together, and both sides appeared to match perfectly. The fact that the bearing halves are retained by the rod halves turned out to be a real convenience while installing the rods within the tight space inside the crankcase. The gap between the installed shell halves closed up to zero when the cap bolts were tigtened; and, as far as I could tell, the crush height came out as expected. I measured the inner and outer diameters of the bearing while installed in its fork rod and both agreed closely with the calculations.
When finally comfortable with the test bearing, I spent the next few days working as a Rolls fitter but without the skill and efficiency of those who performed the same task 75 years ago. After machining and checking the fit of each bearing in its rod pair while installed on the crankshaft inside the crankcase, I recorded the i.d. numbers and the orientations of all the components along with their journal location. Since the first few sets of parts involved installing and removing the rod bolts several times, I temporarily switched to a shorter set of bolts so I wouldn't add unnecessary wear the deeper threads that will be filled later by the actual rod bolts. I chose to not leave the rods in place after performing these preliminary fits. When the wrist pins are completed, I'll recheck the fits with the pistons installed and running temporarily in a pair of opposing cylinders. Checking these pairs systematically, one at a time, will make it much easier to locate any binds that could be caused by crank machining errors. Once the rods and bearings pass this test they can be finally installed on the crank. - Terry
