Duramax LLY – CYLINDER HEAD OVERHAUL – Cleaning and Inspection

Clean the cylinder heads and valve components in parts cleaning solvent. Use carb cleaner and a wire brush to remove stubborn carbon deposits on the heads valves and prechambers.
Check flatness of intake/exhaust manifold seating surfaces with straightedge. Surfaces must be flat to within 0.006 in. (0.15 mm).

Check cylinder head flatness with straightedge and feeler gauge (Figure 2-137). Overall (end-to-end) flatness should not vary by more than 0.006 in. (0.152 mm). Replace the head if warped or distorted. Do not machine the head.

Inspect the combustion prechambers (Figure 2-138). Minor cracks under 3/16 in. (0.48 mm) in length are not cause for replacement. Install new chambers only when cracks extend beyond the indicated length (Figure 2-138).

Check fit of each pre-chamber in the cylinder head. Replace loose (or cracked) chambers. Oversize chambers are available when needed. The oversize parts are marked with ID code “08.” The code numerals appear on the chamber crown, side, or interior (Figure 2-139)

Measure valve stem clearance with a dial indicator. Required clearance is 0.001-0.003 in. (0.026-0.069 mm) for intake and exhaust valves. If clearance is beyond limits, ream the valve guide with reamer (Figure 2-140) and install new valve with 0.0035 in. (0.089 mm) larger stem.

Check the valve stems for wear, scoring, or distortion. Inspect the faces for pitting, cracks, or burnt segments. Also inspect the lock grooves at the stem top. Be sure the grooves are free of cracks, worn or broken lands, or chipped spots. Replace any valve exhibiting the foregoing conditions. Grind the valve faces only lightly to obtain the required 45° face angle (Figure 2-141). Take care to avoid cutting into the valve margin. Quik Way, Van Norman, Sioux or similar quality equipment is recommended.

Grind the valve seats to the required 46° angle with the previ ously recommended equipment. Or, use a 4-6 blade carbide cutter to dress the seat.
Lightly lap the valves and seats with a fine grit compound. Clean the valves and seats and inspect the lap marks. The lap marks, which represent the actual contact areas on the valves and seats, should be concentric, and equal in width throughout. Ideal seat width is 0.063 in. (1.6 mm).
If the lap marks are thick-thin or eccentric, the valves or guides are worn beyond tolerance. Verify by rechecking valve stem clearance and valve seat runout with a dial indicator.

 

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