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Coach bolts are a standard fixing across construction, joinery and site work. Also known as carriage bolts, they are commonly used for timber connections where a secure fixing and clean finish are required.
This guide covers coach bolts, carriage bolts, lag bolts and cup square hex bolts, including DIN 603 dimensions, materials and how to choose the right product.
A coach bolt is a round head bolt with a square neck underneath. When installed into timber, the square section grips into the material to prevent rotation while tightening.
This allows installation using a single tool on the nut side, making them practical for site use and repeat fixing.
In UK trade use, coach bolts and carriage bolts are the same product.
Cup square hex bolts is another naming variation used in some specifications.
Lag bolts can refer to coach bolts in some searches, but may also mean heavy-duty timber screws. Always check the product form before ordering.
Coach bolts are manufactured to DIN 603, which defines head and square neck dimensions.
The diagram below shows the main DIN 603 coach bolt dimensions referenced in the table, including head diameter, square neck and thread length notation.
DIN 603 coach bolt dimensions diagram.
| D | M5 | M6 | M8 | M10 | M12 | M16 | M20 |
|---|---|---|---|---|---|---|---|
| Pitch | 0.80 | 1.00 | 1.25 | 1.50 | 1.75 | 2.00 | 2.50 |
| dk max | 13.55 | 16.55 | 20.55 | 24.65 | 30.65 | 38.80 | 46.80 |
| dk min | 12.45 | 15.45 | 19.35 | 23.35 | 29.35 | 37.20 | 45.20 |
| v max | 5.48 | 6.48 | 8.58 | 10.58 | 12.70 | 16.70 | 20.84 |
| v min | 4.52 | 5.52 | 7.42 | 9.42 | 11.30 | 15.30 | 19.16 |
| f max | 4.10 | 4.60 | 5.60 | 6.60 | 8.75 | 12.90 | 15.90 |
| f min | 2.90 | 3.40 | 4.40 | 5.40 | 7.25 | 11.10 | 14.10 |
| k max | 3.30 | 3.88 | 4.88 | 5.38 | 6.95 | 8.95 | 11.05 |
| k min | 2.70 | 3.12 | 4.12 | 4.62 | 6.05 | 8.05 | 9.95 |
| b | Not specified | ||||||
Scroll sideways on smaller screens to view the full DIN 603 table.
General purpose use for internal applications.
For higher load and structural applications.
Used where corrosion resistance is required.
Yes. They are different names for the same fixing.
The standard covering coach bolt dimensions.
Timber connections, fencing, decking and structural timber work.
Not always. Some refer to heavy-duty timber screws.
For comparison with alternative fixings, see our coach bolts vs coach screws guide.
No. Thread length is not defined under DIN 603.
Contact TC Fixings for product advice and availability.
Posted: 10th January 2020
Author: TC Fixings
