Connecting rods should not be lightened, or even polished, unless you intend going all the way in this direction and will finish the job by having the part shot-peened. Forgings acquire a tough skin in the process of being pounded into shape, and I know of instances where connecting rods that were entirely satisfactory in standard condition promptly broke after having been polished. I do think, on the other hand, that there is a margin of safety to be gained by smoothing off the rough edges where the flash has been sheared away from the forgings. Notches are, in the engineer's language, “stress raisers” and you can do the connecting rod no harm in removing them. Lightening the connecting rod is, however, a poor choice of ways to use one's time, because a rod intended for the loads at, say, 8000 rpm is going to be overstressed at 10,000 rpm and if anything, material should be added to the rod, not removed. On the other hand, one sometimes can improve bearing reliability by opening slightly the oil channels at the ends of the connecting rod. I do not recommend that you actually cut into the bearing surface, but oil delivery to the bearing will be improved by tapering the entry. Do not extend the taper all the way to the bearing surface, as the sharp edges thus formed will flake away as the engine runs and cause a bearing failure.Crankshaft main bearings seldom are troublesome, except in engines that have
been in storage for a long time and have had corrosion at work in these bearings -or unless the bearings have been mishandled. Bearing steels are very tough, but you definitely can pound small pits in the races by injudicious use of a hammer, and pits also can be formed by rusting. Bearings damaged in either fashion should be replaced, as the pits will soon spread and become minor trenches, as a result of an activity called “Brinelling”, which actually is a form of work-hardening. The bearing's rollers and races have casehardened surfaces, but the metal under this thin case is relatively soft, and it is compressed and released (at any given point) as the bearing turns under a load. If the load is high enough, or the bearing in service long enough, the repeated compressions will literally fatigue the metal, and tiny particles of the surface will start flaking away - which becomes visible as the “tracking” seen in the races of a worn-out bearing. Any bearing will start flaking at some point in its life; bearings with races damages by rust, etc. will begin such flaking almost immediately. Incidentally, in very highly loaded bearings the flaking may be started by the sharp edges around any interruption in the bearing's surface, if the rollers pass over those edges. Oiling slots in the rod's big-end are prone to develop this kind of failure, and the same sort of flaking is sometimes observed around the oil feed holes in the crankpins of engines equipped with “direct-injection” oiling systems, like the Suzuki’s and Kawasaki’s. Remove the sharp edges, and you remove the problem - if any. There is sufficient margin of strength in stock production engines so that the problem does not occur; you may find it in the course of reaching for crank speeds substantially above the stock specification.
Somebody is always telling me about having an engine “balanced”, and I always smile nastily when the engine in question has fewer than four cylinders. In point of fact, the single-cylinder motorcycle engine cannot be brought into dynamic balance, for if you counterweight the crankshaft to compensate for the full weight of the piston and rod, you will simply have moved the shaking force from being in-plane with the cylinder axis 90-degrees. "Balancing" one of these engines consists of finding a balance factor, in percentage of reciprocating mass, which is kind to the engine's main bearings and does not excite resonance in the motorcycle's frame. In-line twin- and three-cylinder engines always have a rocking couple. By and large, the stock crankshaft counter-weighting will be correct for most applications, and unless you want to get into a really lengthy experimental program there is nothing to be gained in making changes.
