Every crankshaft carries a network of small deep oil passages drilled at awkward angles through the main and pin journals. They are gundrilled in volume, and getting them right is what keeps a crankshaft line running — and a bearing alive.
A crankshaft main bearing feeds oil into a main journal gallery, which crosses to the pin journal through angled passages drilled at shallow angles to the axis. These are the holes that keep a rod bearing alive at high RPM — they must be positioned, sized and chip-free. Because the passages meet other drilled holes and break out of curved bearing surfaces, they are among the hardest small deep holes in production machining.
| Passage | Diameter | Depth | Material | Notes |
|---|---|---|---|---|
| Main-to-pin gallery | Ø5–8 mm | 50–150 mm | Nodular iron / forged steel | Angled 20–35°, intersects other holes |
| Pin bleed hole | Ø3–5 mm | 30–80 mm | Nodular iron | Short, high count per shaft |
| Main bearing oil feed | Ø6–10 mm | 40–120 mm | Steel / iron | Radial entry off the bearing face |
Starting parameters for gundrilling nodular iron: Vc 60–100 m/min, feed 0.02–0.05 mm/rev at Ø6 mm, coolant 50–120 bar. For forged steel drop Vc to 50–80 m/min.
Machine a flat landing at the gallery entry so the gundrill starts square to the surface.
Drill a short pilot or use a starting bushing at the entry.
Feed continuously with high-pressure coolant. Watch for pressure dips as the drill crosses intersecting galleries.
Break the exit burr and check position relative to the bearing target.
Carbide-tipped gundrills with refined point geometry and chip-breaker contours; TiN/TiAlN coated for iron. Use short, stiff drills with guide-pad widths sized to the intersecting-bore duty. On CNC machining centers, coolant-through spindles and a bushing plate hold entry alignment. For highest volume, dedicated gundrilling machines with two spindles and automatic loading run both sides of the shaft simultaneously.