Hi David, do you have a source for your information, I have always understood that the crankshaft is balanced to a percentage of the weight of the connecting rod, piston, rings etc.matchless wrote:Hi, Just some additional info on the twin con rods (Yes they were polished and weight matched). When rebuilding an engine it is imperative to match the conrods with the correct crank.
Early rods (1949 - 1951) had a small cross section rod and were very light. These must be matched to the early (Light) cranks with the large centre main bearing.
Later model cranks had three different weights (Different balance factors ) and must be matched with a choice from three different (weight) types of conrods.
It should be noted that as the factory increased the capacity from 500 to 550 to 600cc, the cranks were increasingly made heavier, consequently, you will find the con rods were made heavier to equalise the balance factors.
Even though all the twin conrods have the same part number, Do not mix any of the earlier rods with later rods, otherwise, the engine will be out of balance.
Regards............David
Looking at the 1960 parts list both the 500 and 650 have common connecting rods.