Fitting Partial-Extension Drawer Runners — Step by Step
A partial-extension runner allows a drawer to open to approximately 75%, whilst the rear section remains inside the cabinet carcass, making it ideal for light to medium-duty loads.
Fitting drawer runners requires precision and exact adherence to installation dimensions. A deviation of just a few millimetres inevitably leads to sticking drawers or uneven gap margins across the furniture front. In this technical guide, we walk through the correct installation of a partial-extension runner, address dimensional tolerances, and explain the structural differences between runner types.
Technical Fundamentals: Runner Types and Extension Lengths
Before taking out your tools, the type of runner must be defined. A partial-extension runner is often used in furniture construction where full access depth is not strictly necessary or where reducing the tipping moment of the cabinet when the drawer is fully extended is desirable.
In practice, fitters encounter two primary designs of partial-extension runners: The ball-bearing runner is side-mounted to the drawer and consists of interlocking metal profiles supported by steel ball bearings. The roller runner is usually fitted as an undermount or bottom-edge installation. Here, a plastic roller glides inside an epoxy-coated steel profile. Suitable models for simple designs are frequently found amongst roller drawer runners.
Difference: Partial Extension vs. Full Extension
The structural difference lies in the number of telescopic segments. A full-extension runner features at least three interlocking profiles, allowing the drawer to be pulled out 100% clear of the front edge of the carcass. A partial-extension runner generally consists of only two profiles. The mechanical stop engages earlier. If you are planning a drawer where unrestricted access to the rearmost corner is essential, switching to full-extension drawer runners is advisable, provided the installation width (often 12.7 mm for ball-bearing runners) is identical.
The lengths of the runners depend on the carcass depth. Common nominal lengths start with very short 250 mm drawer runners for bathroom furniture or narrow recesses, extending in 50 mm increments up to 800 mm.
Required Tools and Installation Fixings
For a professional, tension-free installation, prepare the following tools:
- Cordless screwdriver with torque adjustment
- Bit sets: PZ (Pozidriv) size 2 or TX (Torx) size 15/20, matching your chosen screws
- Tape measure or folding ruler
- Accurate try square
- Pencil or scribing awl
- Spirit level (when installing inside large, already erected carcasses)
- Chipboard screws (countersunk, typically 3.5 x 15 mm or 4 x 16 mm) or Euro screws (for pre-drilled 5 mm hole patterns)
Preparation and Calculating Clearances
The most crucial measurement when fitting a side-mounted ball-bearing runner is the installation width (the clearance required between the drawer side and the carcass internal wall). On standard versions, this is exactly 12.7 mm per side. If the drawer box is too wide, the mechanism will bind. If it is too narrow, the runners will disengage or the ball bearings may fall out of the cage.
With a classic roller runner, the rail is screwed beneath the drawer base at the side. Here, a lateral clearance of 1 mm to 1.5 mm per side is generally required, depending on the material thickness of the runner.
Step-by-Step Installation Guide
This guide refers to the installation of a side-mounted ball-bearing runner (partial extension), as this requires the most precise alignment.
1. Separate the Carcass Rail from the Drawer Member
Almost all ball-bearing runners feature a disconnect mechanism. Extend the runner fully. Inside the inner profile, you will see a small black plastic release lever. Push this lever up (or down) and slide the inner profile all the way out. You now have the carcass rail (wider outer member) and the drawer member (narrower inner profile) separated.
2. Mark the Fixing Positions on the Carcass
Determine the height at which the drawer should sit. Draw a horizontal reference line on the inside face of the carcass wall. Use your try square against the front edge of the carcass to ensure accuracy. If the carcass features a System 32 hole pattern, you can secure the runner directly with Euro screws into the 5 mm drill holes. The standard distance from the carcass front edge to the first fixing hole is 37 mm.
3. Mount the Carcass Rail
Position the carcass rail against the marked line. The runner should sit flush with the front edge of the carcass or set back slightly (approx. 2 mm) so that the drawer front rests completely flat against the carcass when closed. Fasten the runner initially only through the vertical slotted holes. This allows for roughly 2 mm of vertical height adjustment afterwards. Tighten the screws hand-tight. Repeat the procedure as an exact mirror image on the opposite side of the carcass.
4. Mount the Drawer Member to the Drawer Box
Take the narrow inner profile. Mark a central, horizontal line along the side of the drawer box. Align the profile so that it sits flush with the front edge of the drawer box. Fasten the profile initially through the vertical slotted holes using countersunk screws. It is imperative that the screw heads sit completely flush in the metal recess. A protruding screw head will later catch against the outer member and destroy smooth running performance.
5. Insert the Drawer and Test the Action
Guide the drawer box with the fitted inner members horizontally into the carcass rails. The runner must glide smoothly like a piston inside a cylinder – without noticeable resistance. On the first insertion, you will feel a slight resistance as the ball cage engages. Push the drawer fully closed once to set the mechanism.
6. Fine Adjustment and Final Fixing
Check the gap margins of the drawer front. If the drawer sits crooked, loosen the screws in the slotted holes slightly and adjust the height. Once everything is perfectly aligned, insert the final securing screws into the round fixing holes of the runners. This permanently locks the runners in position.
Key Mechanisms: Soft-Close and Opening Aids
In modern furniture assembly, standard runners are increasingly being upgraded with mechanical features. It is essential not to confuse the terminology:
- Soft-Close: This is a damping system. A hydraulic cylinder absorbs the momentum of the drawer over the final few centimetres and pulls it shut gently and silently. Such systems can be found under soft-close drawer runners.
- Self-Closing: A purely mechanical spring mechanism that pulls the drawer shut over the final few centimetres, but without hydraulic damping. Corresponding components are designated as self-closing drawer runners.
- Push-to-Open: This mechanism is the exact opposite of soft-close damping. It is used for handleless furniture designs. Pressing the front releases a spring that pushes the drawer outward. Those planning handleless designs require push-to-open drawer runners. Mechanically, push-to-open and soft-close cannot be combined in standard slide designs.
Common Installation Mistakes
Even experienced tradespeople can make errors if preparation is rushed. Watch out for the following issues:
The drawer jams in the rear third: Often, the carcass has not been assembled or glued square. If the carcass tapers and becomes narrower at the back than at the front (trapezoidal shape), the runners are compressed against the drawer box. Measure the internal width of the carcass precisely at both the front and back.
Grating noises when opening: This almost always indicates incorrect screws. If roundhead screws are used instead of countersunk screws, or if screws have been driven in at an angle, the inner profile will rub against the screw head. Replace the screws immediately, as this will otherwise damage the ball cage.
Sagging under load: A partial-extension runner naturally exerts less leverage on the carcass than a full-extension runner. Nevertheless, every runner has a defined load capacity (e.g. 15 kg or 25 kg). Exceeding this rating causes the profile to distort. For workshop units or heavy pan drawers, switch to heavy-duty drawer runners rated for higher loads in Newtons or kilograms.
FAQ – Frequently Asked Questions about Partial-Extension Runners
Can I easily replace a partial-extension runner with a full-extension runner? Yes, this is generally possible, provided the installation width (often 12.7 mm) and the nominal length match precisely. However, the fixing hole patterns on the carcass rail usually differ, meaning new pilot holes will need to be drilled.
What does "17 mm partial extension" mean? The 17 mm measurement refers to the width of the groove that must be routed into the side of the drawer box. These specialised ball-bearing grooved runners are recessed into the drawer side, creating a slimmer profile than surface-mounted standard ball-bearing runners.
How do I clean and maintain ball-bearing runners? Ball-bearing runners come pre-greased with a special lubricant from the factory. Never use aggressive solvents or penetrating oils (such as WD-40), as they wash away the heavy lubricant grease. If dirty, wipe the tracks clean with a dry cloth and apply resin-free roller bearing grease if required.
Why can I not unhook my runner? Not every runner features a disconnect release lever. Basic roller runners are removed by pulling the drawer out to the stop, lifting the front slightly, and easing it over the front carcass roller.
Relevant Categories for Your Project
If you require further components for cabinetry or internal storage alongside your runner hardware, explore the following ranges:
- Drawer systems – Complete metal box drawer sides and systems including integrated runners.
- Heavy-duty drawer runners – For load capacities exceeding 45 kg.
- Full-extension drawer runners – When 100% access to the drawer contents is required.