The top ring (See Figure 2-18) is a compression ring made of keystone high strength iron with a molybdenum face.
It is a keystone design, which is a tapered ring fitting into the tapered land of the Ni-resist insert-molded cast iron full groove insert.
The second ring is also a compression ring; cast iron construction and chrome faced. Two rings are used to reduce the pressure drop across each ring. The third ring is an oil control ring and two types are utilized depending on emissions application: Lt. duty (under 8.500 lbs. GVWR) uses a 3-piece. Heavy duty (over 8.500 lbs. GVWR) uses a 2-piece. The 3-piece design is made up of 2 segment rails which wipe the cylinder wall and one expander which controls the 2 segment rails.
In 1983 and later both Lt. and Heavy duty will use the two 2-piece. This improves high mileage durability.
This piston has a half-clover shaped identation on the piston face, which at T.D.C. outlets to the pre-chamber, Figure 2-19. As the piston rises during.the compression stroke, the air swirl begins in these two indentations. When the piston reaches the pre-chamber opening, the air swirl is increased in the spherical pre-chamber. This is a Ricardo Turoidal Piston design and is used with the Ricardo COMET V Pre-Chamber for more complete mixing of the air and fuel.
The 6.2L piston is match fitted to each cylinder bore of the engine. This is accomplished by measuring the internal diameter of the cylinder bore and stamping the corresponding size code on the pan rail of the case. The piston outside diameter is measured and the size code stamped on the piston face. When the piston is assembled to the cylinder case, these size codes are matched to insure that proper fit and clearance between the cylinder and piston is maintained. There are six matching cylinder bore sizes. Size codes A, B, C, D, E, G are used to match the piston and cylinder bore. “A” size pistons are assembled to “A” size cylinder bores, “ B” size pistons to “ B” size cylinder bores and so forth.
1. Check USED piston to cylinder bore clearance as follows:
a. Measure the “Cylinder Bore Diameter” with a telescope gage 2 V2 ” (64mm) from the top of cylinder bore.
b. Measure the “ Piston Diameter” (at skirt across center line of piston pin). (Fig. 2-10).
c. Subtract piston diameter from cylinder bore diameter to determine “ Piston to Bore Clearance” .
d. For Bohn Pistons #1 thru 6 .089-.115mm (.0035-.0045 in.)
For Zollner Pistons #1 thru 6 .112-.138mm (.004-.005 in.)
For Bohn Pistons #7 and 8 .102-.128mm (.004-.005 in.)
For Zollner Pistons #7 and 8 .125-.151mm (.0049-.0059 in.)
2. If used piston is not acceptable, determine if a new piston can fit cylinder bore.
3. If cylinder bore must be reconditioned, measure new piston diameter (across center line of piston pin) then hone
cylinder bore to correct clearance.
4. Mark the piston to identify the cylinder for which it was fitted.
There will be two different suppliers of pistons used in the 6.2L; and you may encounter either one, they differ in finish on the exterior of the piston, so there are two different piston to bore clearance values. Bohn Pistons identified by the word “ Bohnna Lite” near the pin boss, will have a clearance of .089-.115mm (.0035-.0045 in.).
Zollner Pistons identified by the letter Z with a circle around it, also near the pin boss. It carries a clearance of .112-. 138mm (.004-.005 in.)
