- Published 22 May 2023
- Last Modified 29 Aug 2023
- 6 min
Coach Screw Information and Support Guide
Coach screws are your ultimate fastening ally. Unlock the power of precision and stability with our comprehensive guide.
Reviewed by Jay Proctor, Technical Support Team Leader (February 2021)
Coach Screws vs Coach Bolts
Coach screws are heavy-duty screws with a square or hexagonal head and an externally threaded cylindrical shaft that tapers to a point at the tip. They are primarily used for holding together heavy timber and fixing metal to timber and in some cases masonry or concrete. They are also known as lag screws or lag bolts but should not be confused with coach bolts or carriage bolts.
The handy chart below highlights a few differences between the two fasteners:
Characteristic | Coach Screws | Coach Bolts |
|---|---|---|
Also known as | Lag screws | Carriage bolts |
Available thread sizes | 6mm, 8mm, 10mm, 12mm | M6, M8, M10 |
Available thread lengths | 30mm-120mm | 20mm-110mm |
Shaft | Tapered tip | Dowel shaped |
Standards | DIN 571 | DIN 603, 555 |
Fastening | These are tightened or loosened using a wrench spanner or impact socket on the coach screw head | A spanner, wrench, or impact socket is used to tighten or loosen the nut that is fed onto the end of the shaft |
Application time | Time required to drill pilot hole and screw in coach screw | Time required to drill hole, hammer through bolt, and apply washer and nut |
Coach Screws
Coach screws are threaded fasteners with a pointed tip and hexagonal or square-shaped head. They are typically used for wood-to-wood or wood-to-metal connections, providing strong grip and pulling power.
Coach Bolts
Coach bolts feature a smooth, dome-shaped head and a threaded shank. They are commonly used in applications where a more streamlined appearance is desired, such as in construction or furniture assembly.
How Do I Use a Coach Screw?
Timber
All coach screws require a pilot hole, using a general-purpose drill bit such as a twist drill bit, before they are screwed into place, preventing wood splitting. You will need a wood drill bit to drill the pilot hole. They are slightly different from metal drill bits as they have a point at the end to stop them from slipping out of position.
As a rule of thumb, the pilot hole diameter needs to be half the diameter of the coach screw when using softwood and three-quarters when using hardwood. So for example, when inserting an 8mm coach screw, the pilot hole needs to be 4mm for softwood and 6mm for hardwood. Once the pilot hole has been drilled, the coach screw can be driven directly into the wood.
When using coach screws on timber, it is advisable to add a washer to avoid embedding the screw head into the wood on impact. This will also make it easier to remove the screw at any point. Another option is a flanged coach screw. This is a coach screw with an integrated washer built into the head.
Masonry
If you are using a coach screw to fix heavy objects to masonry or brickwork you will need to drill a hole with an SDS drill bit. To ensure that any heavy object fixed to the wall is secure, it is essential to use a wall plug with a coach screw. The wall plug will need to be the same diameter and depth as the hole you drilled.
Important Information
Coach screws must be tightened or loosened using the correct size spanner, wrench, or impact socket to avoid damaging the head. Measure size across the flat edges of the head, not from point to point.
What are Coach Screws Made of?
Most coach screws are made from steel and offered in several finishes:
- Stainless steel – offers high corrosion resistance making them ideal for outdoor use. They are also highly recommended for use with timber such as oak and cedar as these particular types of wood can contain acid and chemicals which can corrode metal. There are two main stainless steel grades:
- A2(304) - can handle high and low temperatures
- A4(316) - can handle high and low temperatures but has a higher corrosion resistance making them ideal for coastal areas or areas with high pollution
- Bright zinc plated – provides moderate corrosion resistance and is suitable for indoor use in a dry environment
- Green organic coating – provides a weather-resistant coating and is ideal for external use, making them ideal for projects such as decking
- Passivated – this is a two-part deep cleansing process to remove free iron or other contaminants from the surface. This process maximises stainless steel corrosion resistance
- Hot dipped galvanised – has a thicker zinc coating than zinc plated, giving them better corrosion resistance, and making them more suitable for outdoor use. The thicker coating produces a dull metal finish
Choosing the Correct Size of Coach Screw
Coach screws come in an array of different sizes and are manufactured to meet DIN 571 German national standard dimensions.
Diameter (d) | Min to Max Diameter of Shank (d1) | Min to Max Thickness of Head (k) | Min to Max Width Across Flats (s) |
|---|---|---|---|
M4 | 3.52-4 | 2.50-3.10 | 6.64-7 |
M5 | 4.52-5 | 3.13-3.88 | 7.64-8 |
M6 | 5.52-6 | 3.63-4.38 | 9.64-10 |
M7 | 6.42-7 | 4.63-5.38 | 11.57-12 |
M8 | 7.42-8 | 5.12-5.88 | 12.57-13 |
M10 | 9.42-10 | 6.55-7.45 | 16.57-17 |
M12 | 11.30-12 | 7.55-8.45 | 18.46-19 |
M16 | 15/30-16 | 9.55-10.45 | 23.16-24 |
Diameter: This refers to the screw's outer diameter of the threaded section of the shaft and is always listed first. So, an M4 screw fits a 4mm diameter hole.
Length: This refers to the shaft length from the tip to underneath the head.
Quick Tips for Coach Screw Applications
- To stop the coach screw from becoming stuck in wood, rub the threaded shaft with hand soap
- Similarly, when using coach screws with masonry, dip the screw in oil before use
- If you are unsure of the coach screw diameter, hold a drill bit against the shaft. If the drill bit matches the shaft diameter, excluding the thread, this will provide you with the screw diameter