[Sketch notes: The crosshead is a large block at the top of the connecting rod, which slides up and down between the guide faces. The slippers (guide shoes) are attached to the crosshead on each side. A common method: the slipper is bolted to the crosshead via a dovetail or a bolted joint, with a shim between the slipper and the crosshead to allow adjustment of the guide clearance. The slipper has a white-metal (or other) bearing surface that bears on the guide.]

The crosshead carries the guide shoes (slippers) on each side. The slipper is attached to the crosshead by bolts, with a shim (adjusting plate) between the slipper and the crosshead so that the guide clearance can be adjusted by adding/removing shims. The slipper has a bearing surface (white metal or a special material) that slides on the guide face. The attachment must be rigid to transmit the side thrust from the connecting rod to the guide, and the shims allow the clearance to be set.

The guide clearance (the clearance between the slipper and the guide face) is strictly limited because:

1. If the clearance is too large, the crosshead and piston can move laterally (sideways) excessively, causing the piston to strike the liner (piston slap), damaging the piston, rings, and liner, and causing noise and vibration.
2. A large clearance allows the connecting rod to tilt excessively, increasing the side thrust and the load on the slippers and guide, and causing uneven wear.
3. If the clearance is too small, the slipper can bind on the guide, causing overheating, wiping, and seizure.

The clearance is therefore set to a small, controlled value (within maker's limits) to keep the piston and crosshead correctly aligned while allowing the necessary oil film.

Lubricant is fed to the slippers (not the guides) because:

1. The slipper is the moving part and the oil can be fed through the crosshead to the slipper bearing surface, where it is needed to form the oil film between the slipper and the guide.
2. Feeding oil to the slipper ensures the oil is delivered directly to the loaded bearing surface (the slipper face) as it slides on the guide, giving a reliable oil film.
3. The guide is a large, fixed surface; feeding oil to the guide would not deliver it reliably to the loaded area and would waste oil.
4. The oil fed to the slipper also lubricates the crosshead bearing and the connecting rod, and the oil drains back to the crankcase.

So the oil is fed through the crosshead to the slipper faces, ensuring the sliding surfaces are lubricated.