After reading some of the responses to my question about using silver bearings, I checked out Ron Colonna's 270 Offy and, indeed, he used .010" thick silver sheet liners for the crankshaft's three main bearings. He was able to manually form the thin silver sheet into the bearing webs and caps using a piece of drill rod as a planishing tool. The thin metal was easily worked and provided a bearing surface. I didn't get deep enough into his design to understand the oil system, but there doesn't appear to be any conventional oil grooves in the liners.
Had I originally planned on using silver, and knowing what I now know, I would have also planned for thinner bearings than the .053" ones I now need. During construction I had two opportunities to choose the bearing thickness, but since I was planning to use SAE 660 bearing bronze, I selected a convenient machining thickness for that alloy on my lathe.
I thought about using silver after realizing that I had not been able to turn the crankshaft's main journals perfectly round by cutting them on a lathe. A softer bearing material would allow me to plan for an initially tight fit that would quickly open up just enough to conform to the journals' imperfections. Babbitt came to mind, but it likely wouldn't stand up to the pounding it would receive in a model IC engine. Pure silver has less than half the hardness of bearing bronze, and since I already had some hidden away in a closet I decided to give it a try.
I learned a lot from my failed attempt to press out a first bearing from a chunk of silver inserted between the halves of a steel die set that I had machined. My 20 ton press deformed the dies as much as it formed the silver. I spent the next few days playing with the remains of my dies as well as my re-melted scrap silver in order to get some some hands-on experience in working with the two.
The first and most disappointing lesson learned was that in order for me to have any hope of forming the bearings using dies, the silver blanks would have to start out much closer to their final thickness than I had hoped. The shop roll that I was using to roll out my one ounce ingots exerted enough force, but its control was very coarse. The rolling had to be done in multiple passes, and so the final thickness was a crapshoot. A miniature jeweler's rolling mill is probably better suited for this type of work, but lacking one I ended up re-melting and re-rolling my ingots several times. My empirical and very lofty goal for the starting blank thickness eventually evolved to .053" +.003"/-.000".
The second lesson learned was that it was much better to finesse the silver into its final shape by beating on the dies with a wooden mallet instead of brutalizing them in a hydraulic press.
Using my new-found knowledge I re-purposed a few of the original dies and created some new ones to manually form the silver with much less trauma to the dies. The dies are very simple, but since my crankcase has two different web widths, I (needlessly) duplicated a couple of the dies to accommodate two different bearing widths. I included a feature on the Final ID Die to press-form a .055" wide x .020" deep oil groove. All the dies except for the large ones were heat treated. The large dies were machined from Stressproof since I didn't have any large diameter harden-able steel.
The forming process starts out with an annealed silver blank having the target thickness and its dimensions closely trimmed to the bearing's final length and width. The blank is inserted between the ID Starting Die and the OD Starting Die, and a mallet is used on the ID Starting Die to rough form the bearing half shell.
The rough formed shell is then inserted on the Width Die, and the two sides of the blank are sanded against 220g/400g paper on a hard flat surface to bring the shell to its final width.
The shell is then inserted on the Height Die, and the two ends are simultaneously sanded until the shell has been brought to its finished height. The Height Die has an additional thousandth crush height built into its design.
The shell is returned to the Starting OD Die, and this time the mallet is used to pound the Final ID Die into the shell blank in order to start the formation of the oil groove. The goal of this step is to get the Final ID die deep enough into the blank so the combination can be inserted into the Final OD Die which is where the groove will actually be formed. It turned out that centering the ID Starting Die during this step was the most critical part of the entire forming process. All my scrapped parts originated from poor centering of the oil groove during this step.
The bearing shell, which is now sandwiched between the Final OD Die and the Final ID Die, is set on a pair of sturdy metal support blocks so a large wood mallet can be used to complete the forming process. The combination is rotated on the support blocks while the periphery of the Final OD Die is pounded with the mallet over the top (only) 180 degree portion of the die resting on the bearing. The Centering Guides help keep the blank centered in the die, and they are designed to provide visibility to the bearing as it's being formed. As the oil groove is deepened, excess metal is displaced and the shell grows a bit in width. There's no positive stop in this die set to halt the forming process when the the final shell thickness is reached except for an increase in forming resistance. It's difficult but not impossible to go too far. The Height and Width Dies are used once more to finish the final edge dimensions.
The bearing is finally inserted between the Height Die and the Starting OD die to verify the final bearing dimensions. The mallet is used against the combination resting on a hard flat surface to set the final bearing dimensions without damaging the oil groove.
The process sounds more complicated than it really is. The most difficult step is creating the starting blanks; and, frankly, I don't recommend my method for creating them. I've done some research and silver sheet is available in a wide range of gauges from online jewelry supply houses. I plan to design the rod bearings around commercially available silver sheet instead of rolling any more ingots. After making the first two bearing halves, the fabrication time dropped to less than five minutes per shell.
For my particular bearing design I was able to get six half shells out of a single one-ounce ingot. At the current price of silver the cost of each bearing pair worked out to about $5.00. My bearings were thicker than they needed to be, and this ran up their cost as well as the effort to make them. I hope to do better planning for the rod bearings.
I sanity checked my first pair of bearings using Plastigauge on the best-behaved number seven journal. Due to the max .0018" TIR that I previously measured on the three center journals, my planned target clearance for all the bearings was .003"; and this is very close to what I measured on the number seven journal. The next step is to drill the holes in the top shells for the oil injectors which will also serve as anti-rotation stops. The rest of the bearings will then be installed and measured. - Terry
