# Fastener Selection: Cap Screws, Inserts, Dowel Pins > Fastener selection for CNC assemblies: socket head cap screws, set screws, dowel pins, helicoils, grades, torque values and locking methods. https://www.sinbo-machining.com/wiki/threads/fasteners/ --- ## On This Page - Which Fastener Do You Need? - Fastener Types at a Glance - Socket Head Cap Screws (SHCS) - Grades — Which Class? - Head Clearance Requirements - Set Screws (Grub Screws) - Point Types — Choose the Right One - Dowel Pins - Types - Press Fit — H7/m6 - Helicoil / Thread Inserts - When to Use Inserts - Installation - Fastener Grades - Torque Values - Metric Fasteners (Nm) — Dry, Zinc-Plated - Inch Fasteners (ft-lbs) — Dry, Zinc-Plated - Tightening Sequence - Locking Methods - Common Mistakes # Fastener Selection Most assembly failures trace back to the wrong fastener, wrong grade, or wrong torque — not a bad design. This guide covers what you actually use on CNC machined parts: socket head cap screws, set screws, dowel pins, thread inserts, and the grades and torque values that matter. ## Which Fastener Do You Need? Stop here. Match your application to the right fastener type before worrying about grades or torque. | Application | Use This | Why | General clamping, two parts bolted together | Socket Head Cap Screw (SHCS) | Strongest head geometry per size. Fits in counterbored holes. Most common CNC fastener. | Low head clearance, cosmetic surface | Button Head Cap Screw | Low-profile dome head, no sharp edges. 20–30% weaker than SHCS of same size. | Flush surface required (no protrusion) | Countersunk Screw | 90-degree flat head sits flush. Requires countersunk hole. Weakest head type — avoid for high load. | Secure a component to a shaft | Set Screw (Grub Screw) | Headless, drives into a shaft via threads. Point type matters — see below. | Large structural joint, wrench access | Hex Bolt | External hex head, wrench-driven. Cheapest for M10+. Standard for structural steel. | Pivot point, hinge, sliding joint | Shoulder Bolt | Precision ground shoulder acts as a bearing surface. Expensive but eliminates a separate pin. | Precision location between two parts | Dowel Pin | No threads. Press-fit locates parts to ±0.01mm. See dowel pin section below. | Stripped threads, soft material, frequent disassembly | Helicoil / Thread Insert | Creates a strong steel thread in aluminum, plastic, or damaged holes. See insert section below. | Axial retention on a shaft | Cotter Pin | Cheap, one-time use. Works with castle nut or through-hole in shaft. | Retain a bearing or collar on a shaft | Retaining Ring (Snap Ring) | Sits in a groove. Axial retention only. Cheap and fast to install. | Load-bearing stud (nut on both sides) | Stud | Threaded both ends or full length. Used when one side has no wrench access (e.g., engine block). ## Fastener Types at a Glance | Type | Drive | Head Style | Load Type | Relative Cost | Common Sizes | SHCS | Hex socket (Allen) | Flat-top cylinder | Tension + shear | 1.0x | M2–M30 | Button Head | Hex socket | Low dome | Tension + shear | 1.1x | M3–M16 | Countersunk | Hex socket / Phillips | 90° flat | Tension only | 0.8x | M3–M12 | Hex Bolt | External hex (wrench) | Hex head | Tension + shear | 0.6x | M5–M42 | Set Screw | Hex socket | Headless | Axial retention | 0.7x | M2–M12 | Shoulder Bolt | Hex socket | Hex head + shoulder | Shear + pivot | 3.0–5.0x | M5–M16 | Dowel Pin | N/A (press fit) | None | Shear (location) | 0.5x | 3–20mm dia. | Cotter Pin | N/A (split leg) | None | Axial retention | 0.2x | 1–12mm dia. | Retaining Ring | Pliers (snap ring) | None | Axial retention | 0.3x | 3–200mm dia. ## Socket Head Cap Screws (SHCS) The workhorse of CNC assemblies. ISO 4762 (metric) / ASME B18.3 (inch). Strong head geometry means you can use a smaller SHCS than a hex bolt for the same clamp load. The hex socket drive keeps the head compact — fits in counterbores where a wrench can't reach. ### Grades — Which Class? | Class | Tensile (MPa) | Yield (MPa) | Hardness | When to Use | 8.8 | 800 | 640 | 22–32 HRC | Non-critical, cost-sensitive. Low-stress assemblies. | 10.9 | 1000 | 900 | 32–39 HRC | Default for CNC assemblies. Good strength, not brittle. Covers 90% of cases. | 12.9 | 1200 | 1080 | 39–44 HRC | High-stress, space-constrained. Engine mounts, mold clamps. More brittle — avoid in impact/vibration. Default recommendation Use Class 10.9 SHCS as the standard for all CNC assemblies. The cost difference vs 8.8 is negligible (5–10%), and you avoid the brittleness risk of 12.9. Only go to 12.9 when you've done the math and confirmed 10.9 isn't enough. ### Head Clearance Requirements The counterbore for a SHCS must be deep enough for the head to sit fully below the surface, plus a small clearance. For the hex socket drive, you need enough depth for the Allen key to engage — typically the key inserts at least 70% of the socket depth. | Size | Head Dia. (mm) | Head Height (mm) | Socket Size (mm) | Min. Counterbore Depth (mm) | M3 | 5.5 | 3.0 | 2.5 | 3.5 | M4 | 7.0 | 4.0 | 3.0 | 4.5 | M5 | 8.5 | 5.0 | 4.0 | 5.5 | M6 | 10.0 | 6.0 | 5.0 | 6.5 | M8 | 13.0 | 8.0 | 6.0 | 8.5 | M10 | 16.0 | 10.0 | 8.0 | 10.5 | M12 | 18.0 | 12.0 | 10.0 | 12.5 Common mistake Undersizing the counterbore diameter. The head diameter must have clearance around it (H7/g6 fit is typical) or the screw head binds in the bore before seating. Add 0.2–0.5mm to head diameter for clearance depending on size. ## Set Screws (Grub Screws) Headless screws threaded through a hub to bear against a shaft. They transmit torque by friction or by the point digging into the shaft. ISO 4026 (flat point) / ISO 4027 (cone point) / ISO 4028 (dog point) / ISO 4029 (cup point). ### Point Types — Choose the Right One | Point Type | Standard | How It Works | When to Use | Downside | Flat Point | ISO 4026 | Flat end bears against shaft surface | Frequent disassembly. Doesn't damage shaft. Use with a flat on the shaft for positive location. | Lowest holding power. Relies on friction only. | Cone Point | ISO 4027 | Pointed end embeds into shaft | Permanent or semi-permanent assembly. Highest axial holding power. | Damages the shaft. Not for parts that need removal. | Dog Point | ISO 4028 | Extended flat tip fits into a drilled hole in the shaft | Positive engagement without shaft damage. Best for precise axial location. | Requires a matching hole in the shaft. Adds a drilling operation. | Cup Point | ISO 4029 | Dished end digs into shaft slightly | Most common point type. Good balance of grip and reusability. | Leaves a small mark on shaft. Can work loose under vibration. Most common set screw mistake Using a cup point on a hardened shaft. The cup can't dig in, so it slips. Use a flat point with a flat machined on the shaft, or a dog point with a drilled hole. On hardened shafts (HRC 45+), set screws have very limited holding power regardless of point type — use a keyway or clamp instead. ## Dowel Pins Precision ground cylinders pressed into holes to locate two parts relative to each other. They don't clamp — they position. Use in conjunction with fasteners (which provide the clamping force). ### Types | Type | Standard | Tolerance | Material | Use Case | Straight Dowel | ISO 2338 / DIN 6325 | m6 | Hardened steel (60 HRC min) | Most common. Press-fit for precision location. | Pull-Out Dowel | DIN 7979 | m6 | Hardened steel | Threaded end for easy removal with a screw. Essential for blind holes. | Taper Pin | ISO 2339 / DIN 1 | Self-locking taper | Steel | Shear loads. Taper fit eliminates need for press. Reusable. ### Press Fit — H7/m6 The standard fit for dowel pins is H7 hole / m6 pin. This is a light press fit (transition fit technically, but behaves as press fit at small diameters). The hole should be reamed, not drilled, to achieve H7 tolerance. | Dowel Dia. (mm) | Hole (H7) Range (mm) | Pin (m6) Range (mm) | Press Fit (approx.) | 3 | 3.000–3.010 | 3.004–3.012 | 0.004–0.012mm interference | 5 | 5.000–5.012 | 5.008–5.018 | 0.008–0.018mm interference | 6 | 6.000–6.012 | 6.008–6.019 | 0.008–0.019mm interference | 8 | 8.000–8.015 | 8.010–8.023 | 0.010–0.023mm interference | 10 | 10.000–10.015 | 10.012–10.027 | 0.012–0.027mm interference Layout rule Use two dowel pins at maximum spacing for best positional accuracy. Three pins is over-constrained and causes fitting problems. Two pins + fasteners for clamping is the standard pattern. Pin diameter is typically 6mm or 8mm for most CNC assemblies — larger only for heavy machinery. Material note Dowel pins must be hardened steel (60 HRC minimum). Unhardened pins deform during installation and don't hold position. Stainless dowel pins exist (A2/A4) but are softer — use only when corrosion resistance is required and accept reduced positional accuracy. ## Helicoil / Thread Inserts A helical coil of diamond-cross-section stainless steel wire installed into a tapped hole. The insert itself becomes the threaded surface. The parent material only sees the OD threads of the insert, which distribute load over a much larger area. ### When to Use Inserts | Situation | Reason | Without Insert | Aluminum / magnesium parts | Soft material strips under repeated load | Thread pulls out after a few assembly cycles | Frequent assembly / disassembly | Steel insert wears better than aluminum threads | Threads gall and loosen after 5–10 cycles | Stripped thread repair | Restores a damaged hole to original thread size | Part is scrap or requires oversize fastener | Plastic parts | Plastic has no meaningful thread strength | Fastener pulls out on first torque | High vibration | Insert + Loctite is more reliable than threads in soft material | Fasteners work loose over time ### Installation 1. Drill the hole to the Helicoil tap drill size (larger than standard tap drill for that thread). 2. Tap with the special STI (Screw Thread Insert) tap — NOT a standard tap. The STI tap is oversized because the insert will occupy space inside the hole. 3. Install the insert with the Helicoil installation tool. The tool drives the coil into the tapped hole. The insert has a tang on the end — break it off after installation with a punch or tang break tool. Cost impact Helicoil inserts add roughly $0.30–1.50 per hole depending on size and quantity. The tap, installation tool, and inserts are cheap in volume. For a run of 100+ aluminum parts with frequent assembly, inserts are almost always cheaper than warranty returns from stripped threads. Common mistake Tapping the hole with a standard tap instead of the STI tap. The thread will be too small, the insert won't fit, and you'll either force it in (ruining the part) or have to drill and start over. Always use the STI tap specified by the insert manufacturer. ## Fastener Grades The grade (ISO class or SAE grade) tells you the fastener's strength. Higher grade = stronger but more brittle. The marking on the fastener head identifies the grade. | ISO Class | SAE Grade | Tensile (MPa) | Yield (MPa) | Material | Head Marking | When to Use | 4.8 | Grade 2 | 400 | 320 | Low carbon steel | None | Non-critical, decorative, low load | 8.8 | Grade 5 | 800 | 640 | Medium carbon, quenched & tempered | 8.8 | General purpose, cost-sensitive | 10.9 | Grade 8 | 1000 | 900 | Alloy steel, quenched & tempered | 10.9 | Default for CNC. Structural joints. | 12.9 | — | 1200 | 1080 | Alloy steel, Q&T (higher carbon) | 12.9 | High stress, space-limited. Avoid under impact. | A2-70 | — | 700 | 450 | 304 stainless | A2-70 | Corrosion resistance needed. 70% strength of 8.8. | A4-80 | — | 800 | 600 | 316 stainless | A4-80 | Marine / chemical. 80% strength of 8.8. Hydrogen embrittlement warning Grade 10.9 and 12.9 fasteners with zinc plating are susceptible to hydrogen embrittlement if the plating process isn't properly controlled. This causes sudden, unexpected failure under load. For safety-critical applications, use plain (black oxide) or phosphate-coated fasteners, or specify baking after plating. ## Torque Values Correct torque is critical. Under-torque means the joint loosens. Over-torque strips threads or breaks the fastener. These values are for dry, zinc-plated or plain steel fasteners. Reduce by approximately 25% for lubricated fasteners (with oil, anti-seize, or Loctite). ### Metric Fasteners (Nm) — Dry, Zinc-Plated | Size | Thread Pitch | Class 8.8 | Class 10.9 | Class 12.9 | A2-70 SS | M3 | 0.5 | 1.1 | 1.5 | 1.8 | 0.9 | M4 | 0.7 | 2.5 | 3.5 | 4.1 | 2.1 | M5 | 0.8 | 5.0 | 7.0 | 8.3 | 4.1 | M6 | 1.0 | 8.5 | 12.0 | 14.0 | 7.0 | M8 | 1.25 | 21.0 | 29.0 | 34.0 | 17.0 | M10 | 1.5 | 41.0 | 58.0 | 68.0 | 33.0 | M12 | 1.75 | 71.0 | 100.0 | 118.0 | 58.0 | M14 | 2.0 | 110.0 | 155.0 | 182.0 | 90.0 | M16 | 2.0 | 175.0 | 245.0 | 288.0 | 143.0 ### Inch Fasteners (ft-lbs) — Dry, Zinc-Plated | Size | Threads/Inch | Grade 5 | Grade 8 | #10 | 24/32 | 2.5–3.0 | 4.0–4.5 | 1/4" | 20 | 6–7 | 9–10 | 5/16" | 18 | 12–13 | 18–19 | 3/8" | 16 | 20–22 | 30–33 | 1/2" | 13 | 45–50 | 65–75 | 5/8" | 11 | 85–95 | 125–140 | 3/4" | 10 | 140–155 | 200–225 ### Tightening Sequence For joints with multiple fasteners (flanges, covers, plates), the tightening sequence matters. Always tighten in a star or crisscross pattern, working from the center outward. This ensures even clamping pressure and prevents warping. | Pattern | Use When | Example | Star pattern | Circular flange (4+ bolts) | Pipe flanges, motor end bells | Crisscross (X) | Rectangular cover (4 bolts) | Gearbox covers, manifold caps | Spiral outward | Large plate (8+ bolts) | Machine bases, large flanges | Inside-out | Concentric bolt patterns | Cylinder heads, multi-bolt flanges Two-pass method For critical joints, tighten all fasteners to 30–50% torque in sequence, then do a second pass to full torque in the same sequence. This settles the joint evenly and prevents uneven loading that can distort the parts. ## Locking Methods Fasteners loosen under vibration, thermal cycling, and dynamic loads. The right locking method depends on the environment, reusability requirement, and cost. | Method | How It Works | Best For | Reusable? | Relative Cost | Limitations | Loctite (threadlocker) | Anaerobic adhesive fills thread gaps, cures to solid | General vibration. Most common method. | Medium (242/262) or permanent (271) | $0.05–0.15/bolt | Surface must be clean. Temperature limits (150°C for blue, 200°C for red). | Split Lock Washer | Sprung washer bites into surfaces under tension | Low-to-medium vibration. Quick, cheap. | Yes (replace washer) | $0.02–0.08 | Least effective method under severe vibration. Can damage soft surfaces. | Nyloc Nut | Nylon insert grips bolt threads | Medium vibration. Reusable 5–10x. | Yes (up to temp limit) | $0.10–0.50 | Nylon degrades above 120°C. Not for high-temperature use. | Castle Nut + Cotter Pin | Cotter pin through nut and bolt prevents rotation | Critical safety joints. Wheel bearings, steering. | Yes (replace cotter pin) | $0.15–0.40 | Requires drilled hole in bolt. Not for confined spaces. | Nord-Lock Wedge | Wedge-locking washer pair — tension, not friction | Severe vibration. Heavy machinery. | Yes | $0.30–1.00 | More expensive. Requires access to both sides. | Double Nut | Two nuts locked against each other (jam nut) | Adjustable tension, no special parts needed. | Yes | 2x nut cost | Bulky. Requires wrench access for both nuts. Practical advice For most CNC assemblies, Loctite 243 (medium strength, oil-tolerant) is the go-to. It works on slightly oily threads (unlike 242), holds against moderate vibration, and you can remove the fastener with hand tools. Use Loctite 271 (high strength, red) only for permanent assemblies. Never use Loctite on nylon-insert nuts — the chemical degrades the nylon. ## Common Mistakes | Mistake | What Happens | Correct Approach | Using 12.9 grade everywhere "for safety" | More brittle — snaps under impact or fatigue. Hydrogen embrittlement risk with plating. | Use 10.9 as default. 12.9 only when stress calculation confirms it's needed. | Over-torquing with an impact driver | Stripped threads, broken fasteners, distorted parts. Especially common on M5 and smaller. | Use a torque wrench. Impact drivers are for construction, not precision assemblies. | No thread engagement in tapped hole | Fastener pulls out under load. Minimum is 1x diameter in steel, 1.5x in aluminum, 2x in plastic. | Design the hole depth for adequate engagement. Use inserts in soft materials. | Using cup-point set screw on hardened shaft | Point can't dig in. Set screw slips under load. | Use flat point with shaft flat, or dog point with drilled hole. Consider a keyway. | Tapping Helicoil hole with standard tap | Insert won't fit. Hole is too small. | Always use the STI tap specified by the insert manufacturer. | Single dowel pin for location | Parts rotate around the pin. No angular constraint. | Always use two dowel pins at maximum spacing. Never three (over-constrained). | Using stainless fasteners with carbon steel plates | Galvanic corrosion at the interface. Stainless is cathodic — the steel plate corrodes faster. | Use same material family throughout, or add an insulating washer. | Not accounting for Loctite on torque | Over-torqueing. Loctite acts as a lubricant during installation, then locks when cured. | Reduce torque by 25% when using Loctite. Follow Loctite's torque chart. | Ignoring thread pitch | Coarse thread (default) vs fine thread specified on drawing but coarse supplied. | Always specify pitch: M8x1.25 (coarse) vs M8x1.0 (fine). Don't rely on "standard." | Reusing prevailing-torque nuts | Locking capability degrades with each use. Nut eventually spins freely. | Replace nyloc nuts after 5–10 cycles. Inspect prevailing-torque nuts each use.