The early 170V pistons had a diameter of 73.5mm
and a compression heigth of 38mm with three rings - two compression
rings and one oil scraper. Piston rods had a diameter of 18mm and
were of the free movement type (see figure 1).
The piston rods had a length of 200mm measured
from the center of the small end to the center of the crank bore,
and the bearing surfaces were ready finished to the proper diameter
in white metal (figure 2).
Subsquently the number of rings was increased
to four above the pin and thus the compression heigth had to be
increased to 44mm to accomodate the extra compression ring. The
pin diameter was increased to 20mm and pistons were fitted with
pin retaining clips. So as to use the same engine blocks, the length
of the piston rods had to be decreased by the same 6mm to 194mm
(figure 3) and they were designed to accept bearing halves.
Old connecting rods may be modified to accept
20mm pins and bearing halves. 75mm pistons may be used in the old
engines because they fall within the repairing range of cylinder
bores of the older models. But 200mm rods can only be used with
pistons having a 38mm compression heigth and 194mm rods with 44mm
compression heigth pistons.