Balancing the rotating mass

Hot-rodding the Nash/Rambler 195.6 overhead valve six

7 June 2010

The bottom end of this motor is drop-deal reliable and grossly overbuilt. Crank and rods are forged, journals are huge; oil supply is overkill. Though it is fully counterbalanced, the stroke is very long, the rods and pistons fairly massive and there are only four main bearings so it will never be very high-revving by current standards.

But imbalance costs power, eats parts, is plain old unpleasant, and pushing another 500 - 1000 practical RPM from this underpowered engine is a very large gain.

Given the inherent inline six geometry at least partial balancing is worthwhile. I chose to match the masses of the six rod, pin and piston sets, and leave the crank and flywheel alone for cost purposes.

I used the scale at work to simply guide me in buzzing metal off the conn rods until they were within 1 gram of each other. Though I do realize that the big end makes a big orbital excursion with the crank, it was impractical for me to balance big and little ends separately; I checked a couple, couldn't detect much of a difference anyways.

DO NOT MIX AND MATCH ROD SETS WITHOUT A GRAM SCALE!

I did "discover" one of those mysteries that could really hurt if undetected -- it turns out that in 1963 or later, AMC cast connecting rods with the exact same part number and casting number, but which weighed 60 - 90 grams more than earlier rods! They are visually identical. Upon closer inspection, the little end may visually reveal: on my heavy set, the little end hole is not centered exactly in the casting, which I first took to be sloppy machine work. It is not; all of them are slightly offset to the same side (I do not recall which side). That metal alone doesn't account for the difference, but it might serve as visual indicator. The light rods had the little end hole centered in the casting.

Rod assemblies were adjusted to be 640.5 grams each. This is the AMC "light" set.

Connecting rod, cap, nuts, initial weights, rounded to nearest .5.
1648.5
2653.0
3642.0
4643.5
5646.0
6640.5

The pistons+pins were very close in mass, no surprise there. The rods varied in mass; see table below. I used the extremely high-tech process of flat-filing the piston skirts to remove mass. I didn't take any pictures of the rods, but I took off the casting part line and in general smoothed thicker portions with abrasive rolls on a big Foredom hand grinder.

All pistons were filed to be 568.5 grams, within approx. 0.2g. Piston brand unknown.

Piston plus pin, initial weights, rounded to nearest .5.
1569.5
2570.0
3570.5
4571.0
5570.0
6568.5