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T-Head Bolts
A comprehensive engineering reference for EPC contractors, machine tool engineers, structural specialists and procurement heads — covering T-head bolt types, T-slot engagement mechanics, dimensional standards, property classes, material grades, surface treatments and full project documentation requirements.
T-Head Bolt Types, Engineering Function
& T-Slot Engagement Mechanics
Machine Table · Extrusion Profile · Formwork · Clamp Bolt
Definition and Engineering Function
A T-head bolt (also called a T-bolt, T-slot bolt or sliding bolt) is a fastener with a flat, rectangular or square head that is dimensioned to slide freely along a T-shaped slot machined or extruded into a structural member, machine table or extrusion profile — and then lock in place transversely when a nut is tightened on the protruding threaded shank. The T-shaped head provides two fundamental engineering functions simultaneously: anti-rotation — the rectangular head bears against the slot side walls and is prevented from rotating as the nut is tightened; and sliding adjustment — the head can be positioned anywhere along the slot length before clamping, providing infinitely variable fastening locations without drilling additional holes.
The critical geometry of the T-head bolt is the relationship between head width, head height and the T-slot dimensions. The head must be wide enough to bear on the slot lips under clamping load (preventing pull-through), narrow enough to insert through the slot opening before rotating 90° or sliding from the slot end, and tall enough to fill the slot depth and engage the full height of both slot lips. An incorrectly dimensioned T-head bolt — particularly one with insufficient head height — will pull through the slot lips under clamping load, destroying the slot geometry and releasing the fixture without warning.
When a T-head bolt is tightened, the nut develops bolt preload which is reacted entirely through the T-head bearing on the underside of the slot lips. The bearing stress on the slot lip underside is: σ_bearing = F_preload / (2 × A_lip), where A_lip is the bearing area of each slot lip. For cast iron machine tables (typical DIN 650 T-slot material), the allowable bearing stress is approximately 60–120 MPa depending on grade. Excessive bolt preload or incorrect T-head geometry producing a reduced bearing area will cause slot lip yielding or fracture — a costly and irreversible failure mode that destroys the machine table surface.
T-Head Bolt Types — Complete Engineering Descriptions
T-Slot Engagement Mechanics — Critical Geometry
| Type | Standard | Head Profile | T-Slot System | Material | Primary Application |
|---|---|---|---|---|---|
| Standard T-Head | DIN 186 / DIN 261 | Square / rectangular | DIN 650 machine T-slots | CS / SS | Machine tables, fixture plates, tooling |
| Wide T-Head | DIN 261 / custom | Wide flat rectangular | DIN 650 (high load) | CS | Heavy machine clamping, large fixtures |
| Hammer Head T-Bolt | Extrusion profile std. | Rectangular (profile-matched) | Al extrusion profiles | CS / SS | Modular frames, machine guarding, fixtures |
| Clamp / Step Bolt | DIN 787 / clamping std. | Step or offset T | DIN 650 machine T-slots | CS | Milling fixtures, workholding step clamps |
| Flanged T-Head | Custom / structural | T + flange | Structural steel / Al | CS / SS | Structural connections, soft substrate |
| T-Head Stud | Custom / formwork | T-head + plain stud | Formwork channels | HDG CS | Concrete formwork tie-rod systems |
Dimensional Data, Governing Standards
& T-Slot Sizing Reference
Head Width · Head Height · Shank Diameter · Thread Length
T-Head Bolt Dimensions — DIN 186 / DIN 261
| Thread | Pitch (mm) | Head Width A (mm) | Head Length B (mm) | Head Height H (mm) | Shank Dia. d1 (mm) | DIN 650 T-Slot Width (mm) | Tolerance |
|---|---|---|---|---|---|---|---|
| M6 | 1.00 | 10.0 | 14.0 | 6.5 | 6.0 | 12 | h9 |
| M8 | 1.25 | 12.0 | 16.0 | 8.0 | 8.0 | 14 | h9 |
| M10 | 1.50 | 14.0 | 18.0 | 9.5 | 10.0 | 16 | h9 |
| M12 | 1.75 | 16.0 | 22.0 | 11.0 | 12.0 | 18 | h9 |
| M16 | 2.00 | 22.0 | 28.0 | 14.0 | 16.0 | 22 | h9 |
| M20 | 2.50 | 28.0 | 36.0 | 17.5 | 20.0 | 28 | h9 |
| M24 | 3.00 | 34.0 | 44.0 | 21.0 | 24.0 | 34 | h9 |
| M30 | 3.50 | 42.0 | 54.0 | 26.0 | 30.0 | 42 | h9 |
| M36 | 4.00 | 52.0 | 64.0 | 31.0 | 36.0 | 52 | h9 |
DIN 186:1990 / DIN 261:1991. Head width A is the critical fit dimension — h9 tolerance against the T-slot width per DIN 650. Head height H must match the slot depth. Always verify actual machine table T-slot width and depth before ordering — there is no universal standard for all tables and the slot dimensions must be measured on the specific machine. All dimensions in mm.
| T-Slot Width A1 (mm) | Slot Depth t (mm) | Slot Opening A2 (mm) | Slot Base Width A3 (mm) | Corresponding Bolt Thread | Max Clamping Force (kN) — CI Table |
|---|---|---|---|---|---|
| 12 | 13 | 9.5 | 14 | M6 – M8 | 5–10 |
| 14 | 16 | 11.0 | 16 | M8 – M10 | 8–16 |
| 16 | 19 | 13.0 | 18 | M10 | 12–22 |
| 18 | 22 | 14.5 | 20 | M12 | 18–32 |
| 22 | 28 | 18.0 | 25 | M16 | 32–60 |
| 28 | 36 | 23.0 | 32 | M20 | 55–100 |
| 34 | 44 | 28.0 | 38 | M24 | 85–145 |
| 42 | 55 | 34.0 | 48 | M30 | 140–240 |
| 52 | 67 | 42.0 | 58 | M36 | 220–370 |
DIN 650:1989. Max clamping force range accounts for cast iron grade variation (GG-20 to GG-30). Clamping force is limited by slot lip bearing stress, not bolt tensile capacity. Always use the table clamping force — not the bolt proof load — as the design limit for machine table T-bolt assemblies.
| Profile Series | Slot Width (mm) | Head Width (mm) | Head Height (mm) | Thread Sizes | Bolt Length Range |
|---|---|---|---|---|---|
| 20×20 series | 6.0 | 5.8 | 2.8 | M4 – M5 | 8–50 mm |
| 30×30 series | 8.0 | 7.8 | 3.8 | M5 – M6 | 10–80 mm |
| 40×40 series | 10.0 | 9.8 | 4.8 | M6 – M8 | 10–100 mm |
| 45×45 series | 10.0 | 9.8 | 4.8 | M6 – M8 | 10–100 mm |
| 40×80 / 80×80 | 10.0 | 9.8 | 4.8 | M6 – M10 | 12–120 mm |
| 80×80 heavy | 14.0 | 13.8 | 6.8 | M8 – M12 | 16–150 mm |
Head dimensions vary between extrusion profile manufacturers (Bosch Rexroth, Item, Misumi, Aluprofil). Always verify head dimensions against the specific profile series drawing before ordering — a head that is 0.5 mm too wide will not slide and will jam. Head tolerance is typically −0.2/−0.0 mm on width (always on the minus side to ensure free sliding).
Governing Standards
DIN 186 covers standard T-head bolts with square or rectangular heads for use in T-slots per DIN 650, for metric thread sizes M6 to M36 with short and long shank variants. DIN 261 covers T-head bolts with wider (table bolt) head profiles for larger bearing area on slot lips. DIN 650 defines the T-slot geometry itself — slot width, depth, opening and lip dimensions — to which the T-head bolt head must be matched. DIN 650 T-slots are the standard for machine tool tables (milling machines, boring mills, grinding machines, CMM bases and precision fixture plates). ISO 888 provides an international reference for bolt dimensions applicable to T-head bolts. For aluminium extrusion profile T-slots, dimensions are defined by the individual extrusion system manufacturer (Bosch Rexroth, Item, Misumi) and there is no single overarching standard — the T-bolt must be matched to the specific profile series.
σ_lip = F_clamp / (2 × A_lip) // Bearing stress on slot lips [MPa]; must not exceed material allowable
σ_lip ≤ f_c // Allowable: ~80 MPa for GG-20 cast iron, ~120 MPa for GG-30
// WORKED EXAMPLE: M16 DIN 186, DIN 650 22mm slot, F_clamp = 40,000 N, head length B = 28 mm
A_lip = (25 − 18) / 2 × 28 = 3.5 × 28 = 98 mm² per lip
σ_lip = 40,000 / (2 × 98) = 204 MPa // EXCEEDS allowable → reduce clamping force or use wider head bolt
Material Grades, Mechanical Properties
& Surface Treatments
Zinc Plate · Black Oxide · HDG · Dacromet · Passivation
| Grade | Standard | Tensile Str (MPa) | Yield Str (MPa) | Hardness | Temp (°C) | Corrosion | Application |
|---|---|---|---|---|---|---|---|
| CS Grade 5.8 | ISO 898-1 | ≥500 | ≥400 | ≤220 HV | −50 to +300 | Low | Standard machine table, formwork, light structural |
| CS Grade 8.8 | ISO 898-1 | ≥800 | ≥640 | 225–300 HV | −50 to +300 | Low | Heavy machine clamping, structural T-bolts, fixtures |
| CS Grade 10.9 | ISO 898-1 | ≥1040 | ≥940 | 320–380 HV | −50 to +250 | Low | High-load fixture bolts, precision clamping |
| A193 B7 Alloy | ASTM A193 | ≥1000 | ≥862 | 235–321 HB | −101 to +399 | Low | High-temp pressure equipment, anchor bolt T-heads |
| SS 304 (A2-70) | ISO 3506-1 | ≥700 | ≥450 | ≤220 HV | −196 to +650 | High | Food, marine-adjacent, extrusion frame, outdoor |
| SS 316L (A4-70) | ISO 3506-1 | ≥700 | ≥450 | ≤220 HV | −196 to +650 | Very High | Offshore, chloride, chemical plant, cleanroom |
| Duplex 2205 | ASTM A182 F51 | ≥620 | ≥450 | ≤310 HB | −50 to +300 | Very High | Offshore structural, sour service environments |
| Brass (CuZn37) | BS 2872 | ≥370 | — | ~120 HV | −50 to +180 | Good | Electrical enclosures, marine, decorative |
| Inconel 625 | ASTM B564 | ≥827 | ≥414 | ≤241 HB | −200 to +980 | Extreme | High-temp, chemical process, turbine clamping |
Material Selection for T-Head Bolt Applications
The most common error in machine tool T-bolt specification is selecting too high a property class. A Grade 10.9 T-head bolt in a DIN 650 machine T-slot can develop a bolt preload (at rated torque) that exceeds the slot lip bearing capacity of a standard GG-20 cast iron table by 200–300%, causing slot lip yielding or fracture. Property class 5.8 is the correct default for machine table T-bolts — it limits the achievable preload within the slot lip bearing capacity of standard cast iron. Grade 8.8 is only appropriate when the machine table is hardened steel or grey iron GG-30 and the specific table rating confirms the higher clamping force. Never use Grade 10.9 in standard cast iron machine table T-slots without a specific calculation confirming the slot lip bearing capacity.
Stainless steel T-head bolts sliding in stainless T-slots (common in food-grade and pharmaceutical equipment) carry a high galling (cold welding) risk due to the combination of the long sliding contact length of the head in the slot and the identical alloy families of bolt and slot. Apply MoS₂ paste or PTFE lubricant to the T-head bearing faces before installation, and use SS 316L T-bolts in SS 304 slots (or vice versa) — dissimilar SS grades have reduced galling tendency compared to same-grade SS-to-SS contact.
T-head bolts deployed in H≶S service zones must comply with NACE MR0175 / ISO 15156 hardness limits — maximum 22 HRC (237 HB) throughout the full body including the T-head. The T-head forming process (hot or cold forging) can locally work-harden the head section. NACE compliance verification requires cross-section hardness mapping of the T-head body — not just the shank. Duplex 2205 or super duplex T-head bolts are specified for offshore structural T-slot connections in sour atmosphere zones where both corrosion resistance and NACE compliance are required.
| Finish | Standard | Thickness (µm) | Salt Spray (hrs) | Max Temp (°C) | Notes & Application |
|---|---|---|---|---|---|
| Plain / Bare | — | — | <24 | — | Machine table use (indoor); VCI for export |
| Black oxide + oil | MIL-DTL-13924 | 0.5–2 | 24–48 | 150 | Standard machine table T-bolts — dry film lubricant retained in slot |
| Phosphate + oil | MIL-DTL-16232 | 5–15 | 48–96 | 120 | Machine tool T-bolts; consistent K-factor for torque calculation |
| Zinc electroplate SC3 | ASTM B633 SC3 | 13 min | 200 | 150 | Outdoor structural T-bolts; verify head dimension post-coating |
| Hot-dip galvanise | ASTM A153 Class C | 45–86 | 500+ | 200 | Formwork T-bolt studs, outdoor structural; head width increases — verify slot fit post-HDG |
| Dacromet / Geomet | ISO 10683 | 8–12 | 720+ | 300 | High-strength T-bolts; no H₂ risk; minimal dimensional addition to head |
| Zinc-nickel alloy | ASTM B841 | 8–15 | 500–1000 | 200 | Offshore-adjacent structural T-bolt connections |
| SS 316L passivation | ASTM A380 | N/A | 1000+ | 870 | SS T-bolts for offshore, food, pharma, cleanroom extrusion systems |
| PTFE / Xylan | Whitford spec. | 15–30 | 400+ | 230 | Anti-galling SS T-bolts in SS slots; low K-factor for torque control |
CRITICAL: For T-head bolts in T-slots, any coating that adds thickness to the head width (zinc electroplate, HDG) must be accounted for in the head width tolerance. A head with a nominal width of 22.0 mm and a zinc electroplate of 13 µm per face becomes 22.026 mm after plating — marginal but acceptable. HDG at 45–86 µm adds 0.09–0.17 mm per face, making the 22.0 mm head 22.18–22.34 mm after galvanising. This typically renders the T-head unable to slide in a 22.0 mm h9 T-slot. Specify "post-HDG head width not to exceed slot h9 upper limit" explicitly on purchase orders for hot-dip galvanised T-head bolts.
The T-head bolt head width must maintain a sliding clearance fit (h9 tolerance) in the T-slot throughout its service life. Any coating that adds material to the head width surfaces will reduce the clearance and may cause the bolt to jam in the slot. Machine table T-bolts for indoor use should be specified in plain, black oxide or phosphate + oil finish to preserve the h9 clearance. For outdoor corrosion-protected T-bolts, use Dacromet (8–12 µm, minimal dimensional impact) rather than electroplate or HDG. If the T-slot is oversized by the user to accommodate a coated T-bolt, re-measure the slot width precisely and specify the coating thickness to ensure the post-coat head width remains within the required fit class.
Inspection, QC Protocols, Applications
& Export Documentation
Machine Tool · Extrusion Frame · Structural · Formwork · Offshore
Inspection and Quality Control
The T-head bolt head width is the critical dimension that determines fit quality in the T-slot. It must be within the h9 tolerance (negative deviation from nominal) to ensure free sliding without binding. Head width is measured with calibrated digital calipers or precision slip gauges at three points along the head length. For coated T-head bolts, head width must be re-measured post-coating — coatings that exceed the h9 tolerance range render the bolt unusable in the T-slot. Head height must be verified to confirm the head fills the slot depth and bears fully on both slot lips. Both dimensions must be stated as mandatory measured characteristics in the dimensional inspection report.
Thread gauging per Go/No-Go (ISO 1502 or ASME B1.2) is performed on each batch. For T-head bolts with long shanks (used in deep machine table setups), shank straightness is also verified — a bent shank causes the bolt to pull the T-head at an angle, reducing the contact width of the head on the slot lips and producing localised stress concentrations that can damage the slot. Shank straightness must be within 0.5 mm per 100 mm length for precision machine table applications.
Proof load testing per ISO 898-1 (for bolts) is performed on each heat/lot for structural and high-load applications. For machine table T-bolts in Grade 5.8 and 8.8, proof load verification confirms the bolt will not permanently elongate before the slot lip bearing limit is reached. Hardness testing verifies the property class and is mandatory for NACE service T-head bolts. The T-head section must be included in the hardness survey — not only the shank — as the forming process can produce different hardness in the head vs shank.
Type 3.1: Minimum for all EPC, OEM and structural project supply. Must state material standard, grade, heat/lot number, chemical analysis, mechanical test results and heat treatment. Head width and head height must be explicitly stated as measured characteristics in the dimensional inspection record. Type 3.2: Co-validated by purchaser-nominated TPI body (Bureau Veritas, DNV, Lloyds, SGS, TÜV). Mandatory for offshore, NACE and structural safety-critical applications involving T-head bolts in permanent structural T-slot connections.
Applications by Industry
T-head bolts per DIN 186 / DIN 261 in Grade 5.8 with black oxide or phosphate + oil finish are the standard clamping fastener on milling machine tables, boring mill tables, precision grinding machine tables and CMM (coordinate measuring machine) base plates. They provide infinitely adjustable clamping position along the T-slot length, accommodate workpieces and fixtures of varying sizes without drilling new holes, and allow rapid repositioning between operations. The black oxide finish provides minimal dimensional addition to the head, preserving the h9 sliding fit in the DIN 650 slot.
Hammer head T-bolts in carbon steel (zinc-plated) or stainless steel 316L are the universal fastener in modular aluminium extrusion frame systems (Bosch Rexroth, Item, Misumi, Aluprofil and equivalents). Applications include machine guarding frames, conveyor systems, EPC instrument stands, cable tray support frameworks, cleanroom wall panel frames and modular safety fencing. The bolt is inserted at the slot end or dropped in through the open slot, rotated 90° to engage, and the frame connection is made by tightening the bolt through the frame component into a hammer head T-nut in the adjacent profile. For offshore and marine modular framing, 316L stainless T-bolts with passivation prevent corrosion at the steel-aluminium contact interface.
T-head stud bolts in hot-dip galvanised carbon steel are used in modular concrete formwork systems (Doka, PERI, MEVA and equivalent) where a T-head engages a channel in the formwork waler and the stud passes through the formwork facing to be locked with a wing nut or plate on the reverse face. The T-head engagement in the waler channel provides the anchor point for the tie-rod system that holds the formwork faces apart under concrete pressure. Formwork T-bolt studs are cycled 50–100+ times and require regular inspection for thread wear, head damage and HDG coating integrity.
T-head bolts in Grade 8.8 with HDG or Dacromet coating are used in structural steel connections incorporating T-slots, channel connections and adjustable bracket systems. Examples include curtain wall attachment systems (adjustable mullion anchor brackets), modular building systems, structural steel to concrete interface connections and heavy industrial racking systems. The T-slot provides the adjustability required to accommodate construction tolerance, while the T-head bolt provides the structural clamping load. For coastal and offshore structural applications, Grade 8.8 T-head bolts with Dacromet 720+ hr salt spray protection are specified.
T-head bolts in offshore applications appear primarily in structural extrusion framing (instrument stands, cable trays, equipment skids), adjustable pipe support systems and equipment base plate anchor connections using T-slot base rails. For sour service zones (H≶S present), duplex 2205 or super duplex T-head bolts with EN 10204 3.2 MTC, PMI and TPI witness are project-standard. NACE MR0175 hardness compliance including the T-head section is mandatory, with full cross-section hardness mapping as part of the 3.2 certificate.
Export Packaging and Preservation
- T-head bolts packed in heat-sealed polypropylene bags, batch-labelled with PO number, type (DIN 186 / DIN 261 / profile series), grade, thread size, shank length, head dimensions, heat/lot number and quantity
- VCI (Volatile Corrosion Inhibitor) poly liner for plain, black oxide and phosphate-finish T-head bolts for sea freight or long-term storage
- T-head bolts must not be packed loose in bulk without separation — head edges and thread ends cause mutual abrasion; pack in foam trays or with individual tissue wrapping for precision machine table bolts
- Bags packed in double-wall corrugated cartons with foam or kraft paper void fill
- Cartons secured on ISPM-15 heat-treated timber pallets with stretch wrap and minimum two steel strapping bands per pallet layer
- MTC, head width and height inspection certificate, thread gauge certificate and all project documents in waterproof sealed envelope attached to crate exterior
- Outer crate labels: PO number, item tag, material grade, type, standard, thread size, head dimensions, shank length, heat number, quantity, gross weight, country of origin
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 for offshore / NACE / structural safety-critical applications |
| 02 | Dimensional Inspection Report | DIN 186 / DIN 261 / profile spec | AQL 1.0 per ISO 2859; head width and head height as mandatory measured characteristics |
| 03 | Head Width Gauge Certificate | DIN 650 h9 tolerance / project spec | Post-coating re-check mandatory for all coated T-head bolts |
| 04 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go per heat lot for project supply |
| 05 | Proof Load / Tensile Test Report | ISO 898-1 / ASTM A307 | Per heat/lot for structural and high-load machine clamping grades |
| 06 | Hardness Test Report (incl. T-head) | ISO 6507 / ISO 6506 | T-head section hardness mapped separately for NACE sour service bolts |
| 07 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex, alloy and exotic grade T-head bolts |
| 08 | Chemical Analysis Report | ISO 898 / ASTM A193/A307 | Included in MTC — verify against grade limits |
| 09 | Heat Treatment Certificate | ISO 898-1 / ASTM spec | Required for Q&T grades (8.8, 10.9, A193 B7) |
| 10 | Surface Coating Certificate | ASTM A153 / B633 / ISO 10683 | Required for all coated finishes; post-coat head width verification |
| 11 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include T-head bolt manufacture |
| 12 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports T-head bolts in all types — standard square T-head (DIN 186), wide T-head (DIN 261), hammer head for extrusion profiles, clamp/step bolt, flanged T-head and T-head stud bolt — in carbon steel grades 5.8, 8.8 and 10.9, stainless steel 304/316L, duplex & super duplex, alloy steel A193 B7 and brass. Full EN 10204 3.1/3.2 MTC documentation, ISO 9001:2015 QMS, TPI witness by BV/DNV/Lloyds/SGS/TÜV accommodated. M6–M36 metric and 1/4"–1-1/2" inch. Custom head dimensions to drawing. NACE MR0175-compliant grades with T-head section hardness mapping. 48-hour express dispatch on in-stock sizes.