If you are buying scaffolding for a construction project, the U-head jack is one of those small components that quietly carries a lot of responsibility. It sits at the top of a vertical standard, holding timber or steel beams, formwork, and slabs in place while crews fine-tune the height. But a U-head jack is not one single product. It comes in different rod diameters, lengths, thread types, and cradle sizes, and ordering the wrong one can mean rework, delays, and even safety issues on site. This guide walks through the key specifications of a scaffolding u head jack so you know exactly what to check before you place an order.
What a U-head jack does
A U-head jack, also called a forkhead or U-head screw jack, is an adjustable support installed at the top of a scaffold standard or shoring prop. Its U-shaped cradle holds the beam or formwork and stops it from sliding sideways, while the threaded rod and nut let the crew raise or lower the support in small, controlled steps. It is widely used in formwork shoring for slabs and beams, in ringlock and cuplock systems, and in tube and clamp scaffolding. It is the counterpart of the base jack, which does the same job at the bottom of the scaffold but uses a flat plate for ground contact. Getting the two mixed up is a common mistake, and it is one of the easiest ways to end up with an unstable setup.
Core specifications explained
When you compare u head jack specifications from different suppliers, you will see the same handful of parameters again and again. Here is what each one actually means.
Rod diameter and type. The threaded rod comes in two basic forms: solid and hollow. Common solid diameters are 30 mm, 32 mm, and 34 mm, while hollow rods are typically 38 mm or 48 mm. Solid rods are lighter and well suited to general-duty work, while hollow rods carry heavier loads and are the usual choice for shoring and formwork applications. The rod diameter also needs to match the system you are using, so check it against the vertical standard or prop you plan to pair it with.
Thread type and pitch. Most U-head jacks use an ACME or rolled thread, with a pitch of roughly 6.35 mm or 10 mm. ACME threads are stronger and wear more evenly under repeated adjustment, which matters when the same jack is used across many cycles on site. A clean, consistent thread also means the nut turns smoothly by hand, without needing a hammer or a pipe to force it.
Length and adjustment range. Total length typically runs from about 400 mm to 700 mm, with an adjustable height of up to 450 mm on standard units and up to 600 mm on heavy-duty units. The adjustment range tells you how much vertical play you have once the jack is in place, so it is worth matching it to the level differences you expect on your slab or beam lines.
U-head size and depth. The cradle itself is usually square, in sizes such as 100 x 100 mm, 120 x 120 mm, or 150 x 150 mm, with a depth of roughly 50 to 75 mm. The cradle needs to be wide enough to seat the beam securely and deep enough to keep it from tipping out under load.
Base plate, material, and finish. The plate under the cradle is typically around 150 x 120 x 50 x 5 mm. Most jacks are made from Q235 or Q355 carbon steel, and the finish is where suppliers differ the most: hot-dip galvanized, electro-galvanized, powder coated, or painted. For outdoor or coastal sites, hot-dip galvanizing gives the best corrosion protection and the longest service life.
Load capacity and weight. Depending on the rod size, material, and cradle dimensions, a U-head jack typically carries between 3 and 6 tons. Unit weight usually falls in the 3.5 to 5.5 kg range. A typical hollow 38 x 4 x 600 mm jack with a 150 x 120 x 50 x 5 mm plate weighs around 3.6 kg, which gives you a useful benchmark when comparing quotes.
Common standard sizes
Most manufacturers work from a fairly short list of standard sizes. The table below shows the combinations you are most likely to come across.
| Rod diameter | Type | Length | Cradle / plate size | Finish |
|---|---|---|---|---|
| 30 mm | Solid | 400 / 600 mm | 120 x 150 x 50 x 4 mm | Galvanized / painted |
| 32 mm | Solid | 400 / 600 mm | 120 x 150 x 50 x 4 mm | Galvanized / painted |
| 34 mm | Solid | 400 / 600 mm | 120 x 150 x 50 x 4 mm | Galvanized / painted |
| 38 mm | Hollow | 600 mm | 150 x 150 x 50 x 5 mm | Galvanized / painted |
| 48 mm | Hollow | 600 mm | 150 x 150 x 50 x 6 mm | Galvanized / painted |
Solid or hollow: which one do you need?
The choice between a solid and a hollow rod usually comes down to the load and the application. Solid rods are lighter, cheaper, and perfectly adequate for light to medium duty, such as access scaffolding and simple leveling. Hollow rods are stiffer for their weight and are the standard choice for formwork shoring, bridge works, and other heavy-duty jobs where the jack is supporting a concrete slab or a heavy beam. If you are not sure, ask the supplier for the load rating of each option rather than guessing.
How to choose the right U-head jack
Start with the system you are using. A jack for a tube and clamp setup, for example, needs a rod that fits the 48.3 mm standard tube, while ringlock and frame systems have their own matching sizes. Then work through the u head jack dimensions one by one: rod diameter and type, total length and adjustment range, cradle size and depth, and plate size. Confirm the finish against your site conditions, and always state the required load capacity in your inquiry. Listing these details in your RFQ is the fastest way to avoid receiving a jack that does not fit.
What to check when the jacks arrive
A quick receiving inspection catches most problems before they reach the scaffold. Turn the nut along the full thread by hand, with gloves on, and it should move smoothly and consistently. Check the welds where the cradle meets the rod and where the plate is attached, and look for even galvanizing coverage with no bare spots or flaking. If the thread binds, the weld looks rough, or the coating is patchy, that is a red flag worth raising with the supplier before you accept the batch.
Keeping U-head jacks in good condition
U-head jacks are simple parts, but they last much longer with a little care. Keep the threads clean and free of dirt and debris, and check regularly for rust or damage, since even light corrosion can weaken the load path. Never extend a jack beyond its rated adjustment range, and always make sure the cradle is seated squarely under the beam before loading. With these habits, a good-quality jack will serve many projects without trouble.
Final thoughts
The specifications of a scaffolding u head jack are not complicated, but they are easy to overlook until a mismatch shows up on site. Rod diameter and type, thread, length and adjustment range, cradle size, plate size, material, finish, and load capacity together define whether a jack will do the job. Bythai Scaffolding supplies adjustable U-head jacks and a full range of scaffolding accessories, with OEM options and flexible MOQ to match project needs. If you are planning a purchase, send your required sizes and load figures to get a quote that fits your system.