Home » Duramax LLY » Duramax LLY – CYLINDER BLOCK SERVICE – Cylinder Bore and Piston Measurement

Duramax LLY – CYLINDER BLOCK SERVICE – Cylinder Bore and Piston Measurement

Check surface flatness of the cylinder block deck with a straightedge and feeler gauge (Figure 2-104). Deck surface flatness must not vary by more than 0.002 in. (0.05 mm) in any six inch (15 cm) length, or 0.006 in. (0.15 mm) overall. Replace the cylinder block if deck flatness is not within stated limits. Do not attempt to salvage a block by machining the deck surface. This practice will result in valve-to-piston contact.

Check condition of the cylinder head and front cover dowel pins (Figure 2-104). Remove and replace any pins that are loose.

Inspect the main bearing saddles, bearing caps, front cover surface, block webs, and pan rails (Figure 2-105). Replace the block if cracks, severe scoring, or distortion is evident at any point.

Measure degree of wear and taper in each cylinder bore with bore gauge J–8087 (Figure 2-106).
a. Refer to the Service Piston and Cylinder Bore Specifications data at the end of this section for bore dimensions.
b. Center the bore gauge in the cylinder and zero the dial indicator (Figure 2-106).
c. Work the gauge slowly up and down the cylinder from top to bottom. Take measurements at cylinder axis points “A” and “B” (Figure 2-107). Note that most wear occurs in the top 2 1/2 in. (63.5 mm) of the cylinder.

NOTE: Cylinder number 7 and 8 require an additional 0.0005 in. (0.013 mm) for piston clearance. Take this into account when checking bore wear.

d. Maximum allowable service wear and taper is 0.005 in. (0.013 mm).
e. Rebore the cylinder and replace the pistons if wear and taper exceed allowable service limits.

NOTE: Replace the cylinder block if bore diameter exceeds 4.075 in. (103.521 mm) on cylinder 1-6 and 4.076 in. (103.534 mm) on cylinders 7 and 8.

Measure piston skirt diameter at the centerline of the piston pin but at a right angle (opposite) to the pin. Refer to the “Service Piston and Cylinder Bore Specifications” data at the end of this section for dimensions.

Remove the piston rings, insert the piston in the bore, and check piston-to-bore clearance with a feeler gauge. Clearance should not exceed 0.004 in. (0.120 mm) on cylinders 1 through 6, and 0.005 in. (0.133 mm) on cylinders 7 and 8.
• Replace the piston if the skirt is severely scored, collapsed, or worn below specified limits.
• Reuse the piston if in good condition and wear is minimal. However, if the cylinder bore is worn and requires reboring, the piston must be replaced, even if it’s in good condition.
• Replace the piston if the bore is OK but piston-to-bore clearance is greater than specified.

Remove all the expansion/freeze plugs with a punch and hammer. Coat the edges of replacement plugs with Permatex #2, or a high temp RTV sealer. Seat the new plugs in the block with a suitable size socket and hammer. Be very sure each plug is firmly seated. Replace any plug that becomes distorted during installation; do not salvage it.


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