Where's the best place to remove weight from a rod to even out a 16g. difference in weight.?

I'll assume that you have a very accurate gram scale and the ability to use it. I've made a practice of weighing each end of the conrod separately, and matching the heavier to the lighter end-by-end. To do this, you will need to make a fixture that will suspend the rod horizontally at the centerline of each end (either big-end bearing bore or wristpin bushing bore) so that you can rest the other end on the scale platform. You will immediately note that the weight of the two ends added together does not equal the total weight f the rod. This isn't really a problem.

Don't forget to weigh and balance the pistons, wristpins, circlips and piston rings (after setting end-gaps.) You can mix and match new components to achieve the closest balance, but I hesitate to mix used parts. Also, if there is a discrepancy on the total weight of the piston/pin/ring/circlip package, use the heavier piston assembly with the rod that has the lighter small end, saving some grinding.

On the big end, I remove material around the bolt bosses, using a belt sander. On the small end, I go right around the outside diameter of the rod, keeping the material removal even, and not making any gouges or scratches. To match the two rods' total weight, I remove material from the shaft, on the top and bottom as viewed from the small end when mounted on the bike. It's a laborious process, and one that you won't want to repeat.

When you order BMW rods, they ate weight-marked on the side, with an engraving marker. The mark will usually be between -10 and +10, meaning grams deviation from a standard weight (which I can't remember). No mark means no deviation or less than one gram.

BMW considers deviation of less than ten grams rto be insignificant, and i tend to agree. In 1977 and 1978, we conducted a lengthy series of experiments to reduce vibraton on the R100 models. In the end, we discovered that the single greatest factor that led to excessive vibration was too much bearing clearance in the front main bearing. Connecting rod and piston imbalance was a distant second place.

On final assembly, make sure to use assembly lube (moly is my choice) on the rod bearings, wristpin bushings and inside the pistons at the pin bosses. Add a few drops of oil to the moly assembly lube to make a viscous slurry, smeared evenly on the bearing. Right before you bolt up the rod (the
pins on the mating surface go toward the front of the motor) liberally flood the crankshaft journal with fresh motor oil fron a clean squirt can. This will rinse away any dust particles, and create an immediate oil wedge on startup.

A couple warnings come to mind: 1) Some vendors are selling BMW car rod bearings instead of the ones made for the motorcycles. They are the same dimension, but they are radiused differently at the edges, and can cause some oiling and wear problems. Insist on motorcycle bearings, part number 11 24 1 258 460.

2.) Don't over-oil the pistons and rings. After final honing the cylinders, spray a little WD-40 or equivalent into the cylinder and wipe thoroughly with a lint-free cloth. Then squirt a little oil on the piston skirts and rub it around evenly. Don't oil the rings at all. On startup, raise the RPM up to around 3000 RPM for 60 seconds or so, and you will greatly increase the likelihood of good ring seating. For the first 500 miles, keep engine RPM up, but engine LOAD down, by downshifting early and using a lower gear than normal.

There's much more, but you can get most of it from the FAQ's on the Airheads and IBMWR lists. Or you can get Oak's book on top-end assembly. He and I disagree on a few specifics, but following his recommendations or mine will get you where you're going.


-Tom Cutter

(Originally posted to the Airheads list)