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.

Above: Figure 2 showing the older set with castelated nuts used only in the rods of the early engines;

Below: Figure 3 showing the newer system with 44mm compression heigth pistons to accomodate four rings, retaining pin clips and rod nut security lockers.