Window Gearbox Buying Guide: Sizes, Types and Compatibility

Window Gearbox Buying Guide: Sizes, Types and Compatibility

A reliable window gearbox keeps casement and tilt-and-turn windows operating smoothly and securely. Whether you are a trade professional replacing multiple units on site or a homeowner undertaking a single repair, choosing the correct window gearbox is crucial for function and longevity. This guide explains how gearboxes work, what measurements matter, the common types available, and practical steps for selection and installation.

What a window gearbox does

The window gearbox is the internal part of the handle mechanism that converts the rotary motion of a handle into the movement that opens or locks the window. In casement windows the gearbox drives an espagnolette or a locking mechanism; in tilt-and-turn windows it forms part of a more complex multipoint mechanism. Faulty gearboxes cause stiff or loose handles, incomplete locking and poor weather performance; replacing a worn gearbox is often a cost-effective repair compared with changing the whole window.

Key measurements and what to check

Accurate measurement is essential when ordering a replacement. Take clear photographs before you remove anything and record the following dimensions and details:

• Spindle type and size: identify the spindle shape (square, flattened or splined) and measure the square size; 7 mm square is common but confirm on site.
• Spindle projection: measure how far the spindle protrudes from the gearbox face to the end; several handle types require a minimum projection to engage properly.
• Case dimensions: width and height of the gearbox case, and the position of the spindle relative to the screw holes; these determine whether the new unit will sit in the routed frame pocket.
• Backset or depth: the distance from the gearbox faceplate to the spindle centre or to the screw fixing line; essential where faceplates differ in length.
• Orientation and handle side: note whether the gearbox is left or right handed, and whether it operates clockwise or anti-clockwise for locking.
• Manufacturer marks or part numbers: many gearboxes have codes stamped on the metal; record these and the handle brand if known.

For trade orders, provide clear photos of the gearbox mounted in the frame with the handle removed plus close-ups of any numbers. That avoids costly returns and delays.

Common types of window gearbox

There are several gearbox types in current use; choose the one that matches the original or improves performance for the application:

• Standard casement gearbox: a simple rotary gearbox that operates a single lock or latch; suitable for most domestic casements with single-point locking.
• Espagnolette gearbox: designed to operate espagnolette rods; compatible with espagnolette furniture and usually has a longer case or specific fixing centres.
• Tilt-and-turn gearbox: a more robust unit made to interface with multipoint keeps and arms; these often require exact matching due to sash geometry.
• Adjustable or reversible gearbox: offers some adjustability in spindle projection or handle direction; useful where frames are non-standard or left/right use varies.
• Heavy-duty/commercial gearboxes: built for frequent use and higher loads; used on larger or high-traffic windows and some doors.

Adapters and conversion spindles are available to convert between spline and square spindles or to change projection length; these can be useful for temporary fixes but fit like-for-like where possible for longevity.

Compatibility with handles and espagnolette systems

Compatibility depends on spindle shape, projection and the handle rose or backplate. Modern espagnolette systems are modular; however, older gearboxes may not line up with new handle screw centres. Key points:

• Match spindle shape and size: a mismatched spindle will either not engage or will strip the gearbox.
• Check handle screw positions: handles with dissimilar fixing centres may not fit without new through-set holes or replacement backplates.
• Ensure the gearbox stroke and travel match the espagnolette rod engagement: incorrect travel can leave the sash partially locked.
• When replacing an espagnolette gearbox, confirm the rod connection method: some use hooks, clips or pins; the replacement must accept the original connection.

If in doubt, remove the handle and gearbox and carry the parts to a supplier or send detailed photos; trade counters can often identify components from brand marks and fixings.

Practical trade advice and DIY guidance

For tradespeople: keep a small stock of common sizes and reversible gearboxes on the van; a selection of adjustable spindles, splined adaptors and various screw-pack sizes speeds up first-time fixes. Record recurring window types at each site so future visits need fewer measurements.

For homeowners tackling a replacement yourself: these basic steps apply to most installations:

• Tools: Phillips and flat screwdrivers, hex keys, tape measure, marker and PPE such as gloves and eye protection.
• Preparation: open the window, support the sash if needed, and remove the handle. Photograph and label any removed parts.
• Removal: unscrew the gearbox fixing screws and withdraw the gearbox; note the orientation and any shims or packing pieces.
• Fitment: test the new gearbox in place without fully tightening screws; fit the handle and test operation through the full travel before final tightening.
• Adjustment and lubrication: apply a small amount of appropriate grease to the spindle and check for smooth engagement; adjust spindle projection or use spacers if necessary.
• Safety: do not force a stiff handle; if the gearbox does not move freely after fitting, check for debris, misalignment or a damaged espagnolette rod.

If the window also forms part of a security-rated installation, or if the gearbox interfaces with an alarm sensor or multipoint locks, consider using a competent installer to ensure compliance and warranty retention.

Where to get parts and support

Window Hardware Store stocks common window gearboxes, spindles and adaptors for trade and DIY customers. For espagnolette-specific issues consult the espagnolette window locks resource at /guides/espagnolette-window-locks which covers rod types and keeps compatibility. For unusual or legacy systems send photos showing the gearbox face, spindle and any stamped codes so we can recommend an exact match.

Recommended Products

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

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

Conclusion and next steps

Choosing the correct window gearbox reduces callbacks and improves window life. Measure carefully, match spindle shape and projection, and check case and fixing positions before ordering. For trade customers, stocking a small range of reversible gearboxes and adaptor spindles speeds repairs. For DIYers follow safe removal and test-before-tightening practice; if you encounter complex multipoint systems or alarmed fittings consider a qualified installer.

Next steps: measure your existing gearbox and take clear photos; contact Window Hardware Store with measurements, photos and any part numbers; or browse our range of window gearboxes and spindles online to find the correct replacement.