Connecting Bolts Compared: Connecting Cabinets Securely with the Right Hole Pattern

March 30, 2026

A connecting bolt clamps two furniture carcasses together flush via a metric thread and a sleeve, whereas a classic wood screw cuts directly into the chipboard material.

When several cabinets are joined into a continuous run – whether in the kitchen, a living room wall unit, or a wardrobe – precise and stable element coupling is essential. Only when the carcasses are firmly bolted together will the front gaps align uniformly. In addition, the load is distributed evenly. For this task, fitters essentially have two options: using two-part connecting bolts or using direct wood screws.

Both systems have their place in furniture assembly. The choice depends on material thickness, required demountability, and visual requirements.

What are connecting bolts?

Connecting bolts are specialised furniture connectors consisting of two parts: a threaded screw and a matching sleeve with an internal thread. They are inserted through a clearance hole that passes completely through both cabinet walls being joined. As the screw is tightened, the two broad heads pull together and clamp the side walls of the cabinets with high force.

Typical designs in furniture making are based on thread sizes M4 or M6. The length of connecting bolts is variable, as the screw can be wound into the sleeve to different depths. To join two standard carcass sides made of chipboard at 16 mm thickness each (32 mm in total), one generally selects a bolt covering a clamping range of 28 mm - 36 mm.

The major advantage of this system lies in load introduction. Tensile force acts on the flat, wide heads of the metal fittings rather than on soft wood or timber-based panels. As a result, the joint is extremely sturdy and can be loosened and retightened as often as required without tearing out the material.

What are classic wood screws for carcass connection?

As an alternative to the connecting bolt, many fitters use standard wood screws and furniture screws, typically in the form of classic chipboard screws. Here, no through-sleeve is used. Instead, the first cabinet wall is drilled through and the screw is driven directly into the solid core of the second cabinet wall.

The thread cuts deeply into the wood fibres or chips of the MDF board, creating the necessary hold. For such connections, wood screws 4mm with a length of 30 mm are frequently used (for 2 x 16 mm panel thickness) to prevent breaking through the second wall.

Differences in direct comparison

To make the right choice for a given assembly project, consider the technical differences between both fixing screws:

Drilling and preparation: For a connecting bolt, an exact clearance hole must be drilled through both cabinet walls. For an M4 connecting bolt, this is usually a 5 mm diameter hole. With a wood screw, only the first panel is pre-drilled at 4 mm to 5 mm, while the second panel remains untouched so the thread can bite.
Demountability: A cabinet joined with connecting bolts can be dismantled without damage. The metric thread does not wear out. When standard furniture screws are driven directly into wood and later removed, the drill hole loses material integrity. Repeated dismantling eventually causes the thread to strip.
Tensile strength and stability: Connecting bolts offer significantly higher tolerance against lateral shear forces thanks to the metal heads bearing on both sides. Wood screws can pull out of chipboard under heavy loads.
Interior appearance: Connecting bolts present a visible, usually nickel-plated pan head on both sides of the cabinet. When using wood screws, the screw head is visible inside only one of the two cabinets.

Which screw for which scenario?

In practice, the choice between the two systems depends on the furniture requirements.

Scenario A: Modular furniture construction and kitchen installation (connecting bolts)

When installing kitchen base units or modular wardrobes, connecting bolts are the technical standard. Here, carcasses must be joined perfectly flush and permanently tight. Existing system holes (the System 32 with 5 mm diameter) are frequently used to pass the sleeves through, eliminating extra drilling entirely. If heavy loads are anticipated, professionals opt for heavier M6 or M8 connecting bolts, which require correspondingly larger drill holes (usually 8 mm or 10 mm).

Scenario B: Quick, concealed fixing (wood screws)

If furniture units will remain permanently in one location and later dismantling is ruled out, fixing with wood screws is sufficient. This is often faster because exact alignment of a clearance hole is not needed. For dark carcasses, fitters often use black wood screws, as they visually blend into the shadows of the cabinet interior.

The correct hole pattern and assembly

Installing carcass connectors requires precision. If cabinets are not aligned accurately before screwing them together, uneven gaps will result.

Step 1: Aligning the carcasses

Before driving any screws, the cabinets must be perfectly level. To achieve this, adjust the furniture legs so both carcasses sit at exactly the same height and their front edges are flush.

Step 2: Clamping

Screw clamps act like an unrelenting third hand during carcass alignment, keeping components positioned to the millimetre. Apply one clamp at the front edge and one at the rear edge. Use scrap wood pads underneath to avoid damaging the decor finish.

Step 3: Drilling

If matching System 32 holes are not present, new ones must be drilled.

For M4 connecting bolts: Drill through both side walls using a 5 mm wood drill bit. Ensure the drill is held horizontally. To avoid breakout on the exit side, press a scrap block of wood firmly against the interior of the second cabinet for the bit to exit into.
For wood screws: Drill through the first cabinet wall only (e.g. with 4 mm or 5 mm).

Step 4: Screwing together

Insert the sleeve from one side and the threaded screw from the other side through the hole. Most connecting bolts feature a combi-slot or a PZ drive. Frequent installers should ensure they use the correct bit size; a size 2 cross-head is standard. Matching wood screws PZ2 or wood screws PH2 require the same bit, avoiding tool changes during assembly. Tighten the screws hand-tight until the heads press slightly into the chipboard surface.

Aesthetics: Concealing screw heads

Visible screw heads inside cabinets often disrupt the overall look, especially in high-end furniture. Various solutions exist to conceal fixing screws.

A classic screw cover for wood is the plastic cover cap. These small caps feature a small pin on the rear that presses directly into the cross-head drive (PZ or PH) of the screw. Screw cover caps are available in numerous colours to match various decors (e.g. white, grey, Sonoma oak).

An even flatter alternative is a self-adhesive screw cover. These circular stickers made of melamine-coated paper or durable plastic are simply stuck over the screw head. Adding virtually no bulk at a thickness of often just 0.3 mm, they conceal both connecting bolts and countersunk wood screws almost invisibly.

FAQ – Frequently asked questions about carcass connection

What length of connecting bolt is needed for two 18 mm thick panels? Two panels at 18 mm thickness give an overall thickness of 36 mm. You need a connecting bolt whose clamping range accommodates this dimension. A typical M4 bolt with a clamping range of 31 mm - 40 mm is ideal here.

Which drill bit is used for M4 connecting bolts? The sleeve of an M4 bolt generally has an outer diameter of 5 mm. Therefore, pre-drilling with a 5 mm wood drill bit is necessary. A 4 mm bit is too small to pass the sleeve through.

Can I use furniture connectors instead of connecting bolts? Yes. For more complex constructions where panels meet at a 90-degree angle, furniture connectors such as cam connectors (Minifix) are used. However, for straightforward side-by-side joining of two upright cabinets, the connecting bolt remains the sturdiest and simplest solution.

Which screw drive is better: PZ or PH? Both drives are widely used. Pozidriv (PZ) provides superior torque transmission thanks to additional radial ribs, reducing the risk of the bit slipping out (cam-out effect). The essential factor is that the bit fits the screw head precisely to prevent damage during tightening.