Tower Bolt Sizes: 4-inch vs 6-inch, Light vs Heavy

What the size actually changes

The nominal size of a tower bolt is the length of its barrel, and what that governs is how far the pin travels into the receiving socket.

A 4-inch bolt shoots a shorter pin. That is fine where the door is light, well fitted and not carrying a security burden — internal doors, bathroom doors, cupboard doors, light shutters.

A 6-inch bolt puts more pin into the socket. Two things follow. It resists forcing better, because more steel sits across the gap between door and frame. And it copes better with doors that have moved, because a deeper socket still engages when the alignment has drifted a few millimetres — which every door does with seasonal humidity.

For a main entrance, an external door, or any tall or heavy leaf, use the 6-inch.

Light versus heavy

Size is length. Light and heavy describe the gauge of the material and the quality of the mechanism, and the two are independent — you can have a heavy 4-inch or a light 6-inch.

A light bolt uses thinner extruded section. The pin is slimmer, the barrel walls are thinner and the mounting plate flexes more under load. It works, and for a bathroom door it works indefinitely.

A heavy bolt uses a thicker barrel and a more substantial pin, and the slide action is usually better damped. On a door that is bolted and unbolted several times a day, that difference shows up as a mechanism that still slides cleanly in five years rather than one that has started to bind.

The gauge matters more than the length for durability. A heavy 4-inch will generally outlast a light 6-inch on the same door.

Our range: SA-35 is a two-piece set in 4-inch and 6-inch, for fitting top and bottom in one purchase. SA-36 is available in light and heavy in both lengths.

Two per door, top and bottom

Standard practice on a main door is one bolt near the top and one near the bottom, which is why SA-35 is sold as a pair.

The reason is torsion, not just strength in numbers. A single bolt in the middle leaves the top and bottom corners free to flex away from the frame. On a tall leaf that flex is substantial, and it is exactly where a door is levered. Bolting both ends holds the whole edge in plane.

Fitting

Get the alignment right before you drive a single screw. Close the door, hold the bolt in position, and slide the pin out to mark precisely where the socket must sit. A socket that is a few millimetres out will force the pin at an angle, and a pin driven at an angle wears the barrel until it jams.

Drill the socket hole slightly deeper than the pin travel so grit and paint cannot stop it seating fully. On a floor socket this matters more than on a frame, because floors collect dust.

If the bolt is going into an aluminium section rather than timber, use self-tapping screws intended for aluminium, not wood screws. A wood screw in thin aluminium strips its thread and the mounting plate works loose.

Finish

Tower bolts are made in aluminium and stainless steel, in the usual plated and coated finishes. In a bathroom, or anywhere with standing humidity, stainless is worth the difference for the same reason it is on a window lock: once the plating on a mild steel bolt is scratched, corrosion runs underneath it and the mechanism stiffens.