How to Measure a Window Gearbox for Replacement

How to Measure a Window Gearbox for Replacement

Why getting measurements right matters

Replacing a window gearbox is a common task for both tradespeople and competent DIYers. A correctly measured gearbox ensures smooth operation, longer life for the handle and keeps, and avoids unnecessary returns or on-site delays. Window gearboxes vary by spindle type, projection, fixing positions and handedness; matching these accurately is essential whether you are ordering a direct replacement or a universal unit.

Tools and preparation

Prepare the right tools and workspace before you start:

• Cross-head and flat screwdrivers
• Digital calliper or steel rule for precise measurements
• Tape measure
• Torx set if required by the handle or locking mechanism
• Pen and labelled bags for screws and components
• Camera or smartphone to photograph positions and markings

What to record before removing the gearbox

Record these details while the window is still assembled; this saves time and reduces errors when ordering replacements:

• Type of window: casement, flush casement, tilt-and-turn, or top-hung
• Which side the handle is on and which way the window opens; this gives the gearbox handedness
• Manufacturer part number or stamp on the old gearbox, if visible
• Photograph the handle position at closed and fully open points

Step-by-step measurement process

Follow these steps for reliable measurements. Work carefully and keep small parts secure.

• Remove the handle: depending on the style, remove the rose or grub screw and gently pull the handle off the spindle. Note whether the handle is fitted with a sprung washer or cover plate.
• Measure the spindle projection: from the face of the gearbox to the end of the spindle; measure both sides if the spindle is reversible. Record in millimetres.
• Measure spindle size and shape: use callipers to measure the width across flats for a square spindle. Common sizes include 7mm and 8mm but check precisely; some older or specialised windows use different dimensions.
• Identify spindle profile: square, splined, offset or round. Note whether the spindle is solid or hollow and whether it has flats or serrations for the handle to locate onto.
• Measure the gearbox body: note the overall height and depth of the gearbox case as these affect the sash routing and screw clearance.
• Measure fixing centres: measure the distance between the centres of the mounting holes; measure both vertically and horizontally if there are multiple fixings. Accurate centre-to-centre figures are crucial when matching plates or bodies.
• Check the handle rose or backplate: if the rose attaches through the gearbox, measure the distance from spindle centre to the screw holes on the rose; many replacements are sold with different rose sizes.

Identifying handedness and operation

Handedness affects the direction the gearbox moves the locking rods or espagnolette when the handle is turned. Determine handedness by observing the window from the inside with the handle in the closed position: if the hinges are on the left, the unit is generally left-hand; if the hinges are on the right, the unit is right-hand. Note whether the handle operates clockwise to lock or to open; some units are reversible but many are not, so record the direction.

Removing the gearbox safely

When you remove the gearbox, support the sash so it cannot drop or move unexpectedly. Keep all screws and plates together and label them. If the gearbox forms part of a multi-point espagnolette assembly, do not force the rods out; instead slacken the securing screws and slide the assembly out as one unit where possible. Take photographs of how the rods engage with keeps and brackets; this will assist reassembly.

Ordering a replacement: trade tips

When ordering, provide the supplier with the following: measurements you recorded; photos from three angles; manufacture stamps or numbers; the handle and rose type; and whether the gearbox is for a PVCu, timber or aluminium window. Where possible, bring the old gearbox or its spindle to the supplier. Trade suppliers can cross-reference part numbers and recommend direct replacements or universal gearboxes that match the critical dimensions.

DIY fitting and adjustment advice

Fit the replacement gearbox dry first: insert, check spindle projection and handle engagement, then secure loosely and operate the handle through a full cycle to check for binding. Grease sliding parts sparingly with a specialist window lubricant; do not use general purpose grease that can attract dirt. Tighten fixings progressively and re-check operation. If locking rods no longer meet the keeps correctly, small adjustments at the keeps or replacing worn keeps will restore proper engagement.

Common pitfalls and how to avoid them

• Assuming spindle size without measuring; a few tenths of a millimetre difference can stop a handle fitting
• Ordering by length only; centre-to-centre fixing positions and spindle projection are equally important
• Ignoring handle return positions; the handle may have a different rest angle that affects closure
• Failing to check for worn keeps; a new gearbox will not function correctly if the keeps are out of alignment

Further reading and standards

For more about espagnolette locking systems and keeps, see our detailed resource on espagnolette window locks. If you are fitting gearboxes as part of a refurbishment, consider the age and condition of the surrounding hardware; sometimes replacing the sash keeps with matching modern keeps reduces future call-backs.

Recommended Products

Based on this article, you may find these products helpful:

• ERAB Espagnolette Window Lock Universal Lock Replacement
• Avocet Slimline Offset Window Espag
• In-Line Window Mechanism
• Offset Window Mechanism
• Multipoint Gearbox GU Old Style

Conclusion and next steps

Accurate measurement of the spindle, gearbox body and fixing centres prevents wasted time and incorrect orders. For best results: take clear photographs, measure with callipers, record handedness and handle orientation, and keep the old part until the new unit is fitted successfully. If you are unsure about a measurement or have a non-standard window, contact your local trade supplier with photos and the figures you have taken; they can usually identify the correct part or recommend a compatible alternative.