250 Pioneer
.00175" - .002" 
.0045" - .005"
250 Plonker
 .001" - .0015"
.0035" - .004"
250 Stiletto
 .002" - .0025"
.0045" - .005"
250 T.T. & D. M. R.
.0025" - .003" 
.005" - .0055"
175 Pioneer
.002" - .0025"
.0035" - .004"
175 Stiletto
.0025" - .003"
.0045" - .005"

FIG. 71

Section 6:

Piston Sizing Information

1. The numbers and letters stamped on the top of the piston indicate its size in millimeters. Fig. 69 illustrates this.

2. The piston sizing chart, Fig. 70, illustrates the measured size of all OSSA pistons.

3. Fig. 71 gives correct piston to cylinder clearances to be observed when reboring. This chart also shows the maximum allowable clearances.

4. Even though exacting procedures are followed during the manufacture of pistons, they are not all precisely the same size. These very slight differences are denoted by a letter stamped on the top of the piston, either an A. B., C., or D.  A "B" piston is. 0002" (2/10 of one thousandth) larger than an "A".  A "C" piston is .0002" larger than a "B".  A "D" piston is .0002" larger than a "C".

5. This small difference in piston size is particularly helpful at the factory when the cylinders and pistons are selected for proper fit. Due to the large number of cylinders that the factory must produce, the finished inside size of each cylinder can vary perhaps .0002"to .0004" (2/10 to 4/10 of a thousandth). When each engine is being assembled, the assembler selects the piston (either A, B, C, or D) that gives the correct piston to cylinder clearance, thereby insuring maximum performance and piston life from that engine.

6. Many OSSA dealers who carry a large stock of pistons also use this sizing method when they rebore a cylinder. The piston size chart (Fig. 70) gives the exact size of each individual 175 and 250 OSSA piston.

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