Fit Bolts (Reamed Hole) & Speed Bolts
In structural steel erection, the critical path is often defined by how quickly joints can be aligned and secured. Standard bolts allow for “play” within the clearance hole, leading to potential joint slip under transverse loads. Fit Bolts (Reamed Hole) and Speed Bolts solve this by combining the function of a drift pin with a high-strength fastener.
Designed with a precision-ground shank (tolerances k6 or h6) that exceeds the thread diameter, these bolts are engineered to fit snugly into reamed holes (H7 tolerance). This creates a “body-bound” connection that eliminates slip and transmits shear forces directly through the solid shank rather than the threaded area. For erection crews, the tapered lead-in (dog point) acts as an alignment tool, speeding up the “fitting up” process of heavy girders and reducing the need for separate drift pins.
- Transmits 100% shear load via shank.
- Precision k6/h6 shank tolerance.
- Tapered tip aligns mismatched holes.
- Eliminates joint slip/movement.
- Replaces temporary drift pins.
- Available in 8.8 / 10.9 Grades.
Technical Specifications
Product Name
Hexagon Fit Bolts (Reamed Hole) / Speed Bolts / Fitting Up Bolts
Common Standards
DIN 609 (Long Thread), DIN 610 (Short Thread), ISO 4014 (Modified Shank)
Material
Medium Carbon Steel, Alloy Steel (42CrMo4 / SCM435)
Shank Tolerance
k6 (Interference/Transition fit) or h6 (Precision clearance)
Strength Grades
Class 5.6, 8.8, 10.9 (Quenched & Tempered)
Diameter Range
M10 – M52 (Custom Inch sizes available for US Speed Bolt market)
Surface Finish
Plain (Oiled), Black Oxide, Zinc Phosphated (for lubricity during insertion)
Certifications
EN 10204 3.1, Ultrasonic Testing (UT) for internal integrity
The “Body-Bound” Advantage (Shear Transmission)
The Pain Point: In standard bolted connections using clearance holes (e.g., an 18mm hole for an M16 bolt), the joint can slip until the bolt hits the side of the hole. This “bearing stress” is often concentrated on the threads, which is the weakest part of the bolt.
The Engineering Solution: Our Fit Bolts feature a shank diameter larger than the thread diameter. When installed into a reamed hole, the shank creates a tight fit. This ensures that transverse shear loads are carried entirely by the solid, unthreaded shank, significantly increasing the shear capacity and rigidity of the joint.
2. Accelerated “Fitting Up” Process
The Pain Point: Erecting steel at height is dangerous and time-consuming. Aligning multiple layers of splice plates usually requires hammering in drift pins, removing them, and then inserting bolts.
The Engineering Solution: The “Speed Bolt” variant features a specialized tapered nose (podger point). As the bolt is driven in, the taper physically forces the steel plates into alignment. One fastener does the job of both the drift pin and the connecting bolt, reducing cycle time by up to 30%.
3. Vibration & Fatigue Resistance
The Pain Point: In dynamic machinery (turbines, couplings) or bridges, repetitive loading causes micro-movements in standard clearance holes, leading to fretting corrosion and loosening.
The Engineering Solution: The interference fit of a DIN 609/610 bolt eliminates all radial clearance. With zero play, the joint acts as a monolithic structure, drastically improving fatigue life and preventing the onset of fretting.
Standard dimensions for Hexagon Fit Bolts with Long Thread. Note the tolerance difference between Shank (ds) and Thread (d).
| Thread Size (d) | Shank Dia (ds) (k6) | Head Width (s) | Head Height (k) | Thread Length (b) (L ≤ 50) |
| M10 | 11 mm (+0.012/+0.001) | 17 mm | 6.4 mm | 17.5 mm |
| M12 | 13 mm (+0.012/+0.001) | 19 mm | 7.5 mm | 20.5 mm |
| M16 | 17 mm (+0.012/+0.001) | 24 mm | 10.0 mm | 25.0 mm |
| M20 | 21 mm (+0.015/+0.002) | 30 mm | 12.5 mm | 28.5 mm |
| M24 | 25 mm (+0.015/+0.002) | 36 mm | 15.0 mm | 33.5 mm |
| M30 | 32 mm (+0.018/+0.002) | 46 mm | 18.7 mm | 40.5 mm |
> Note: The hole must be reamed to H7 tolerance to match the k6 shank for a proper transition fit.
1. Hole Preparation (Reaming)
Do not attempt to drive a Fit Bolt into a standard drilled clearance hole. The mating plates must be clamped and then reamed together to an H7 tolerance.
Example: For an M20 Fit Bolt (Shank ϕ 21mm k6), the hole should be reamed to exactly 21.00mm H7.
2. Installation Force
While “Speed Bolts” are designed for alignment, avoid excessive sledgehammer force on the head, which can deform the material structure. Use a soft-face mallet or a hydraulic puller if the interference is high. Ensure the tapered point has fully passed through all plies before tightening the nut.
3. Lubrication
Due to the tight metal-to-metal contact of the shank, applying a Molybdenum Disulfide (MoS2) paste to the shank (not just the threads) is recommended to prevent galling (cold welding) during insertion, especially for Stainless Steel or high-grade alloy variants.
Related Products
FAQ
What is the main purpose of a Fit Bolt (Reamed Hole)?
A Fit Bolt is designed to transmit transverse shear loads and prevent alignment shifts in critical machinery or structural joints. Unlike standard bolts, it features a precision-ground shank (typically k6 tolerance) that fits tightly into a reamed hole (H7 tolerance), eliminating radial clearance and acting as a dowel pin.
How does a "Speed Bolt" differ from a standard structural bolt?
A Speed Bolt (often used in fitting up) combines a tapered “podger” tip with a specific thread design (sometimes coarse or quick-lead). The tapered tip helps align the steel plates as the bolt is driven in, and the specialized thread allows for rapid nut installation, significantly accelerating the steel erection process compared to using separate drift pins and standard bolts.
What is the difference between DIN 609 and DIN 610?
Both are hexagon fit bolts with reamed hole shanks. The difference lies in the thread length: DIN 609 has a longer thread, allowing for more adjustability in grip length, while DIN 610 has a very short thread, maximizing the shear-bearing shank length for compact heavy-duty joints.
Why do Fit Bolts require a reamed hole?
Reaming creates a perfectly round, smooth hole with a precise H7 tolerance. Since the Fit Bolt shank is manufactured to a k6 (slightly larger/interference) tolerance, this precision is required to ensure a tight fit that transmits shear load without movement. Using a standard drilled hole would result in an loose fit, negating the bolt’s engineering purpose.
Can you act as a fitting up bolts distributor for custom lengths?
Yes, as a fitting up bolts supplier and manufacturer, we can produce custom shank lengths and diameters. We frequently supply modified Speed Bolts with specific “dog points” or tapered leaders tailored to the thickness of the splice plates being assembled on site.