Automotive Powertrain Deep Hole Drilling: Crankshafts, Connecting Rods, and Transmission Components

A Tier 1 automotive supplier producing connecting rods for a high-performance diesel engine (36MnVS4 micro-alloyed forged steel, 300–340 HB) was drilling a Ø3.5 mm × 115 mm oil passage (L/D 33:1) at a 30° angle to the connecting rod axis. The existing gun drilling process (Vc = 85 m/min, f = 0.025 mm/rev, 80 bar coolant) was producing 3.5% scrap due to drill breakage at the carbide tip-to-shank brazed joint, occurring at 80–115 mm depth when the drill exited the large-end bore and re-entered the opposite wall. A redesigned gun drill with a reinforced brazed joint (1,100 MPa shear strength silver brazing alloy), a 15° primary bevel angle to reduce impact forces during the interrupted cut, and increased coolant pressure (80 to 120 bar) reduced breakage to 0.2% and increased tool life from 450 to 1,200 bores between regrinds.

Automotive Deep Hole Drilling Applications ​

Powertrain Component Drilling Parameters ​

Multi-Step Combination Tooling ​

Production Systems and Quality ​

Gun Drilling Machine Configurations for Automotive Production ​

Quality Requirements for Automotive Powertrain Bores ​

FAQ ​

What are the most common deep hole drilling applications in automotive powertrain components? ​

The most common deep hole drilling applications in automotive powertrain manufacturing are oil passage bores in crankshafts, connecting rods, camshafts, and transmission shafts. Crankshafts require 8–16 oil passages per shaft: a main oil gallery running through the crankshaft from the front to the rear main bearing journal (typically Ø5–8 mm × 200–500 mm), and angled cross-drilled passages connecting the main gallery to each crank pin journal. These bores intersect with each other at precisely controlled positions and must be free of burrs at the intersections to prevent oil flow obstruction. Connecting rods have a single oil passage (typically Ø2.5–5 mm × 80–150 mm) drilled at an angle from the big-end bearing bore to the small-end bushing. Camshafts have a through-bore oil gallery (typically Ø3–6 mm × 300–800 mm) that may extend the full length of the shaft, with radial cross-drilled holes feeding each cam lobe bearing journal. Transmission shafts have internal oil galleries (typically Ø4–10 mm × 200–500 mm) that deliver lubrication oil to gear bearings and synchronizer assemblies. Additional automotive deep hole drilling applications include: fuel injector control bores (Ø1–3 mm × 30–120 mm with H6–H8 tolerances); turbocharger shaft oil bores in superalloys; ABS sensor bores in wheel hubs; and brake caliper piston bores. The automotive industry is the largest consumer of gun drilling tools worldwide, accounting for approximately 35–40% of all gun drill production.

What are the challenges of drilling angled oil passages in crankshafts? ​

Drilling angled oil passages in crankshafts presents several challenges. Drill entry on a curved surface — the oil passage bore typically starts on the cylindrical surface of a main bearing journal or crank pin journal, meaning the drill enters on a curved surface at an angle (typically 15–75° to the radial direction). This causes the drill to skid or walk on the curved surface before penetrating, leading to positional deviation of the bore. The solution is to use a spot-facing operation before drilling to create a flat entry surface, or to use a specialized gun drill geometry with a reduced point angle (20–25°) and a smaller chisel edge to improve centering. Interrupted cutting — many crankshaft oil passages intersect with existing bores (the main gallery) or with the crank pin bore. When the drill passes through an existing cavity, it experiences an interrupted cut — the cutting forces drop to zero as the drill crosses the cavity and then spike as it re-enters the opposite wall. This repeated impact loading is the primary cause of gun drill breakage in crankshaft drilling. Deep L/D ratios — crankshaft main oil galleries typically have L/D ratios of 30:1–80:1, requiring careful control of cutting parameters and coolant delivery to maintain tool life and bore quality. Chip evacuation from angled bores — chips from angled bores must be evacuated against gravity (for downward-angled bores) or may fall back into the bore (for upward-angled bores). High-pressure coolant (80–150 bar) is essential to ensure positive chip evacuation. Burr control at intersections — every oil passage intersection must be deburred to prevent metal particles from entering the engine oil system. Burr removal from intersecting bores (typically Ø3–6 mm) is difficult to automate and often requires manual inspection and deburring.

What type of gun drilling machines are used for high-volume automotive production? ​

High-volume automotive production uses three main types of gun drilling machines. Multi-spindle gun drilling machines — these machines have 2–8 independent gun drill spindles that can drill multiple bores simultaneously on the same or different workpieces. For connecting rod production, a 4-spindle machine can drill four connecting rod oil passages simultaneously, achieving a cycle time of 10–15 seconds per bore (versus 30–40 seconds for a single-spindle machine). The spindles are independently programmable, allowing different drilling depths and parameters for different bore positions. Dual-ended gun drilling machines — for through-bores (crankshaft main galleries, camshaft oil galleries, transmission shafts), dual-ended machines drill from both ends simultaneously, reducing the cycle time by approximately 50% compared to single-ended drilling. The two gun drill heads are precisely aligned on a common axis, meeting at the center of the workpiece with an alignment tolerance of < 0.1 mm. Rotary transfer machines — for very high volumes (> 1,000,000 components per year), rotary transfer machines integrate gun drilling stations with other operations (spot-facing, drilling, tapping, deburring, gauging) in a single automated cycle. The workpiece is clamped once and rotated through 6–20 stations, each performing a specific operation. Rotary transfer machines are most commonly used for connecting rods and small transmission components. In all high-volume configurations, automated workpiece handling (gantry loaders, robots, or transfer systems) is standard, and the gun drilling spindles are equipped with tool breakage detection systems that stop the machine within 0.1 seconds of a drill fracture.

How are burrs removed from intersecting oil passage bores in crankshafts? ​

Burr removal from intersecting oil passage bores in crankshafts is one of the most critical and difficult operations in powertrain manufacturing. Burrs form at the intersection of the cross-drilled oil passage with the main oil gallery, and any loose burr particles can cause catastrophic engine failure if they enter the oil system. The standard deburring methods for crankshaft oil passage intersections are: abrasive flow machining (AFM) — a semi-solid abrasive media is forced through the oil passages under high pressure (50–150 bar), flowing through the intersections and removing burrs by abrasion. AFM is the most effective method for complex intersecting bore geometries, achieving consistent burr removal at all intersections in a single cycle of 30–120 seconds. Thermal energy deburring (TED) — the crankshaft is placed in a sealed chamber filled with a combustible gas mixture (typically natural gas and oxygen). The gas is ignited, creating a brief (2–5 millisecond) high-temperature (2,500–3,000 °C) flame that burns off all burrs without affecting the bulk material. TED is fast (30–60 seconds per cycle) and can deburr all passages simultaneously, but it requires careful process control to avoid oxidation. Manual deburring with specialized tools — small flexible abrasive tools (flap wheels, abrasive brushes, or ceramic stones) are inserted into the oil passages by hand or by robot to mechanically remove burrs at the intersections. This method is labor-intensive but provides the most reliable burr removal for critical applications. Electrochemical deburring (ECD) — the crankshaft is immersed in an electrolytic solution with a shaped electrode positioned at each intersection. An electric current dissolves the burr material preferentially at the sharp edges. ECD is effective for small, thin burrs but less effective for large or thick burrs. The choice of method depends on production volume, burr size, and quality requirements — high-volume automotive production typically uses AFM or TED for their consistency and automation capability.

What is the trend toward electric vehicle powertrain components and deep hole drilling? ​

The transition to electric vehicles (EVs) is changing the deep hole drilling requirements in automotive powertrain manufacturing. In internal combustion engine vehicles, the primary deep hole drilling applications are in engine and transmission components — crankshafts, connecting rods, camshafts, and transmission shafts. As EV adoption increases, these components are being phased out and replaced by electric drive unit components with different deep hole drilling requirements. EV-specific deep hole drilling applications include: e-drive rotor shafts — hollow shafts in electric motor rotors that may require gun-drilled bores for cooling oil passages or for weight reduction. These shafts are typically smaller and have lower L/D ratios than engine crankshafts. Stator cooling channels — some e-drive motors have cooling channels that require deep hole drilling in the stator housing or in the motor housing. Gearbox shafts — EV gearboxes (typically single-speed or two-speed) require transmission shafts with oil galleries for gear and bearing lubrication, similar to conventional transmission shafts but with different geometry requirements. However, the total volume of deep hole drilling per vehicle is significantly lower in EVs — an EV powertrain may have 2–4 deep-drilled bores compared to 15–25 in an internal combustion engine vehicle. The net effect is a gradual reduction in the growth rate of automotive deep hole drilling, but not an immediate decline — the transition to EVs is occurring over 15–25 years, and the installed base of ICE vehicles will continue to require replacement components. Additionally, EV powertrain components tend to have tighter tolerance requirements (H7–H8 versus H9–H10 for ICE components) and use higher-strength materials, which increases the value per bore even as the volume decreases.

Disclaimer: The automotive powertrain drilling parameters, tooling designs, and process recommendations presented in this article are based on published industry data and manufacturing experience with deep hole drilling in automotive production. Actual parameters depend on the specific workpiece material (including heat treatment condition and batch variation), machine condition, tool quality, and coolant system performance. The tool life, cycle time, and quality data are typical values and may vary significantly in production. All powertrain drilling applications should be validated through production process qualification (PPAP, run-at-rate) before series production. Burr control and cleanliness requirements must comply with the specific engine or transmission cleanliness specification. No guarantee of specific tool life, bore quality, or production performance is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.