Duramax LLY – PISTON/CONNECTING ROD OVERHAUL – Piston Selection and Fit

Production engines are equipped with different grade size pistons. The different grades allow factory select fitting of pistons. Correctly, three grades are used with the ID letters JT, KT, and GT or J, K, G, or T. The different grades are to accommodate minor variances in bore size. The ID letters are on the cylinder block pan rails adjacent to each cylinder bore, or the piston itself. The grade sizes are described in the piston and bore size chart at the end of this section.

Some general selection recommendations are:
• If an original piston is in good condition and meets all specifications, it can be reused.
• If the cylinder bore is OK but the piston was damaged, just use a new piston of the same grade size.
• If the piston grade size is not marked on the piston or pan rail, and the original piston is not reusable, select a piston that will provide the required piston-to-wall clearance. It is only necessary to know the actual bore size.
• The largest oversize piston available is 0.020 in. (0.50 mm).
• It is not necessary to rebore all the cylinders on a low mileage engine. Only the damaged cylinder needs to be repaired. This is permissible because factory service replacement and original pistons are all the same weight.
Balance is not affected. However, it is recommended that all the pistons be replaced and the cyliners be rebored on high mileage engines.

• If a cylinder bore only requires honing to correct a problem, use the next grade size piston. Avoid going to the largest oversize piston unless absolutely necessary.
• Verify that a new piston is properly aligned on the connecting rod. The rod ID mark and the relief (combustion chamber) in the piston crown should be on the same side.


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