Home » Duramax LLY » Duramax LLY – CYLINDER BLOCK SERVICE – Cylinder Boring and Honing Information

Duramax LLY – CYLINDER BLOCK SERVICE – Cylinder Boring and Honing Information

Check the ID letters stamped on the cylinder block pan rails. These letters identify the production standard piston grade size and cylinder bore. The letters used are J, K, G, and T. Grade and sizes are listed in the service piston and bore chart at the end of this section. Use this information as a check – before ordering parts or performing any machining operations.

Production and oversize pistons from the factory are all the same weight, therefore engine balance is not affected. This means that a low mileage block does not have to have all the cylinders rebored if only one or two require this. However, on high mileage blocks, it is better to rebore all the cylinders to maintain equal cylinder pressures.

The main bearing caps should be installed and tightened to specified torque before commencing boring operations. The block decks should also be clean, smooth, and free of nicks and burrs. The boring bar must rest on a flat, even surface to avoid an off center bore. Boring and honing plates are also recommended. Keep boring bar cutting tool speed and feed rates within the tool manufacturers recommendations. Excessive feed or speed rates can produce unwanted tool marks. Extra honing operations will be required to correct this.

Boring should be a two step operation consisting of: a rough bore to remove about 90% of the required stock, and a finish bore to within 0.003-0.005 in. (0.0762-0.1270 mm) of the desired final size. The final bore size, including required piston-to-bore clearance, will be achieved during honing operations. Cylinder honing is required to break the glaze, or to achieve final surface finish and piston clearance after boring operations. Glaze breaking hones are designed for hand operation in a drill motor. This type of honing only requires breaking through the varnish buildup on the walls. Machine operated cylinder hones, such as those built by the Sunnen Corp., are required for achieving final finish and size.

Honing operations on rebored cylinders should also be a two step procedure. A rough hone to within 0.0005 in. (0.13 mm) followed by a finish hone to size. The rough honing establishes size while finish honing generates the required 45° cross-hatch pattern. The crosshatch pattern provides a surface that will retain oil for proper wall lubrication. Perfectly smooth finishes are to be avoided.

NOTE: Service oversize pistons are available in one size only which is 0.020 in. (0.50 mm) over. This is the maximum allowable cylinder oversize. Note this before initiating boring operations.


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