CROSSHEAD AND GUIDE SHOE CLEARANCES AND ALIGNMENT IN LARGE TWO-STROKE ENGINES

Crosshead bearing clearance:

• The crosshead bearing (top-end bearing) clearance is measured with the piston/ crosshead accessible. Using a feeler gauge at the parting faces, or the maker's clearance gauge/ plastigage/ lead-wire method, the diametral clearance between the bearing shell and the crosshead pin is measured and compared with the maker's specified range (typically 0.05-0.15 mm per 100 mm of pin diameter, but per maker).
• The clearance is checked with the bearing cap torqued to specification; the reading is recorded for trend comparison.

Guide shoe (slipper) clearance:

• The guide shoe (crosshead guide/ slipper) runs on the guide (the vertical guide faces in the engine frame). The clearance between the guide shoe and the guide face is measured using a feeler gauge inserted between the shoe and the guide, at the top and bottom of the shoe, on both sides (port and starboard).
• The clearance is checked with the crosshead in the correct position (e.g. at mid-stroke) and compared with the maker's specified range (typically 0.1-0.3 mm per side, but per maker). The guide shoe clearance is important to allow for thermal expansion and to prevent binding while maintaining guidance.
• The guide shoe is also checked for wear of its white-metal/ bearing surface and for correct contact (bluing).

Checking alignment:

• Crosshead alignment is checked by measuring the crosshead pin relative to the engine centreline and the guide. Using a dial test indicator (DTI) mounted on the crosshead or on the guide, the crosshead is moved through its stroke and the lateral (side-to-side) and fore-and-aft movement is measured to detect misalignment.
• The alignment is also checked by measuring the piston rod/ crosshead relative to the cylinder bore (the piston should be central in the liner) and by checking the guide shoe clearances on both sides are equal.
• Crankshaft deflection readings and the piston/liner clearance (top and bottom) also indicate crosshead/ guide alignment.

Adjusting alignment:

• If the crosshead is misaligned (e.g. the guide shoe clearances are unequal, or the piston is off-centre), the guide shoes are adjusted by adding/ removing shims behind the guide shoe (between the shoe and the crosshead) to bring the crosshead central and parallel to the guide.
• The guide shoe clearances are set to the maker's specification on both sides, and the piston is re-checked for centrality in the liner.
• If the misalignment is due to a bent piston rod, worn guide, or a distorted frame, the cause must be rectified (renew the rod, machine/ renew the guide, or correct the frame) before re-setting the clearances.
• After adjustment, the engine is barred over and the clearances re-checked at several positions to confirm correct alignment throughout the stroke.