Main babbitt bearings may be poured in much the same manner as connecting rod bearings. However, there is a slight difference. Main bearings are a little more complicated. I have found it easier to pour main bearings in halves rather than one piece. The lower halves should be poured first.
The set-up requires four sheet metal washers approximately 4 by 4 inches, with a center hole drilled to exact size of the crankshaft journal. These pieces will close off both ends of the bearings. An accompanying photo in the Image Gallery will illustrate the use of the washers. As the bearings will require a flange on one side of the bearing, it will be necessary to fabricate a 'U' shaped piece of scrap metal approximately 1 by 5 inches that can easily be bent to fit around the casting. The metal strip can be held in place by pounding the ends over the engine casting.
The crankshaft must be positioned in the center of the bearing casting. By using the connecting rod attached to the piston, in place, it will support the crankshaft when the connecting rod cap is tightened. The cap is tightened when the crankshaft rod journal bearing is at its lowest arc. This also assures clearance between the rod bearing cap and the base of the engine block. The crankshaft gear is in place against the cam shaft gear, with a shim between the gear teeth to provide clearance. This assists in supporting the crankshaft in place. To check horizontal alignment of the shaft, I use the main bearing casting surface as a reference point.
With the bearing cap studs in place, I use them for a steady rest, and with a straight edge over the top of the shaft, measure the distance from the casting surface to the straight edge. Now make four spacers of this dimension, cut a piece of pipe, or use washers as spacers over the studs. It is easy to check the horizontal position of the shaft with these spacers as a guide. The rod bearing cap may be loosened to effect a change in alignment. The inside washers are held in place by use of a wood wedge between the washer and the crankshaft web. The outer left washer is held in place by the crankshaft gear. The outer right end washer is held in place with a hose clamp around the shaft. Any gaps may be sealed with furnace cement or asbestos packing. Slight leakage, however, is not objectionable because it seals very quickly. Check the alignment again before heating the casting and the shaft with a torch. If the casting is cold it will not permit the metal to flow freely and will leave gaps in the bearing. With the molten metal hot enough to cause a pine stick to smoke when submerged and then removed from the metal, it is hot enough to pour. It only takes a moment to pour the bearing.
After it cools twenty minutes, the crankshaft may be jarred loose from its seat. The flanges may be filed to fit with a body file. There is no precise thickness of the flange required as the flywheels may be moved to control end thrust of the shaft. The corners of the bearings must be rounded to fit on the inner side of the bearing with a scraper. This lower half of the bearings should now provide perfect alignment of the crankshaft. It is advisable to check these three factors: (1) the gear teeth clearance, (2) the connecting rod and cap should have sufficient clearance at the base of the housing, depending of course on the size of the engine, (3) the horizontal alignment must permit the crankshaft to be turned freely, 360 degrees, without binding of the connecting rod bearing.
The next step is to pour the cap bearings. They may be poured one at a time. With one of the crankshaft washers in place to close the opening on the inside of the cap, secure the crankshaft in its new lower bearings with one of the old bearing caps. Cut a shim from asbestos for each side of the bearing. (Note the white appearance of the shim in the photo). This will keep the molten metal from fusing with the already finished lower bearing. Be sure it covers all the new babbitt, extending tightly to the crankshaft. Bolt the cap in place, centering it on the studs. Wedge the washer against the casting with a wood block.
Tip the engine over on its side, supported in a level position by wood blocks. To complete the cap flange, the metal clip used for the lower bearing may be wrapped around the casting and held in place with a 'C' clamp. (Note the clamp in the photo.) The metal form should be level with the flange for the lower half. Heat the bearing cap and the shaft with a torch, plug any pipe thread openings with a block of wood and proceed to pour the bearing. When cooled the cap may be jarred loose with a hammer. Repeat the procedure for the opposite bearing cap and complete the job of shimming and scraping to restore the bearings to last another fifty years.