Square Packers vs Horseshoe Shims: Which Do You Need and Why?
04/06/2026
If you have landed here unsure which product you actually need, you are not alone. Square packers and horseshoe shims look similar, are often used in the same kinds of jobs, and are stocked by the same suppliers. The difference matters though, and ordering the wrong one wastes time.
The distinction comes down to one thing: whether there is already a bolt or fixing in the way.
What Is a Square Packer?
A square packer is a flat metal plate, typically square or rectangular, with no slot or hole in it. It sits under a load to raise, level, or align it before a fixing goes in. You place the packer, get everything to the right height or position, then fix through or around it.
The shape means it covers the full bearing area beneath whatever it is supporting. That is the point. A square packer distributes load evenly, does not concentrate stress at a slot, and gives you the maximum contact area between the structure and whatever it is sitting on.
Think of a structural steel column sitting on a concrete base. Before the holding-down bolts are tightened, packers go under the base plate to bring it to the correct level. The packers carry the load of the column while grout is poured, or they remain in place permanently as part of the bearing detail. There is no bolt through the packer itself, so a solid plate is exactly what you want.
Square packers are also used extensively in timber frame construction under sole plates, below pre-cast concrete elements, under machinery bases, and anywhere a flat bearing surface needs to be raised by a precise amount before fixing takes place.
What Is a Horseshoe Shim?
A horseshoe shim is a flat metal plate with a U-shaped slot cut into one edge. That slot is the whole point. It allows the shim to slide sideways around an existing bolt, stud, or fixing rather than requiring you to lift the component clear of the fixing to get the shim underneath.
In practice this means you can introduce a horseshoe shim into a joint or assembly that is already partially fixed, or where removing the fixing entirely is not practical. You slide the shim in from the side, around the shank of the bolt, and it sits in position just as a solid packer would, but without the need to dismantle anything first.
The most common application is window and door frame installation. The frame goes into the opening, fixings go through the frame into the surrounding structure, and horseshoe shims are slid in around those fixings to fine-tune the position of the frame before the fixings are fully tightened. You can pack one side up, check for square, add or remove shims, and adjust without pulling the fixing out each time.
The same logic applies to steel connections where a bolt is already partially in place, to machinery that needs levelling after initial positioning, and to any situation where you need to shim around an existing fixing rather than before one goes in.
The Practical Difference in Real Scenarios
Levelling a structural post on a base plate. The post arrives, the base plate sits on the concrete, and the holding-down bolts are waiting to go in. Nothing is fixed yet. Square packers go under the base plate to bring it to the correct level. You stack the right combination of thicknesses, check level, then tighten the bolts down. The packers stay in place permanently. This is a square packer job.
Aligning a window frame in an opening. The frame is in the opening, the fixings are partially driven to hold it in place, and you need to adjust the frame to get it perfectly plumb and level before finishing. You cannot easily remove the fixings to slip a solid packer in behind the frame. A horseshoe shim slides in around each fixing point, lets you build up the exact amount of packing needed, and stays in place when the fixing is tightened. This is a horseshoe shim job.
Setting machinery on a factory floor. Heavy equipment gets positioned, feet are loosely bolted down, and the machine needs to be levelled precisely before the bolts are torqued to final setting. If the bolt holes in the feet are accessible, horseshoe shims can be slid in around the bolts to level each foot independently. If the feet are unbolted and the machine is lifted, square packers go underneath first. Which you use depends on the sequence of the job and whether the fixings are in the way.
Choosing the Right Thickness
Both square packers and horseshoe shims are available in a range of thicknesses, and in most jobs you will use a combination to build up to the exact gap you need to fill.
Standard thicknesses run from 1mm up to 6mm, with the most commonly used sizes being 1mm, 1.5mm, 2mm, 3mm, and 5mm. For fine adjustment, thinner shims in the 1mm to 2mm range give you the precision. For larger gaps, start with the thickest shim that fits within your target and use thinner ones to make up the difference.
As a general rule, use the fewest shims possible to reach your target thickness. A stack of ten 1mm shims is less stable than two 5mm shims and a 1mm shim. Where the shim stack will be carrying significant load, keep the stack compact and consider whether a thicker custom part would be more appropriate.
If the gap you are filling falls between standard thicknesses, or if your application calls for a non-standard size, custom shims are available to order and are cut to your specification with the same tolerances as the standard range.
Does Material Choice Change Between the Two Types?
No. Both square packers and horseshoe shims are available in mild steel, stainless steel, and aluminium, and the same material selection logic applies to both. Mild steel for dry, indoor, and structural applications. Stainless steel for external, damp, or food-adjacent environments. Aluminium where the shim is working within an aluminium structure or where weight or on-site trimming matters.
If you are unsure which material to specify, the full material selection guide covers this in detail.
| Situation | Use |
|---|---|
| No bolt in place yet, packing under a load before fixing | Square Packer |
| Bolt or fixing already in position, shimming around it | Horseshoe Shim |
| Structural column or beam base plate levelling | Square Packer |
| Window or door frame alignment during installation | Horseshoe Shim |
| Machinery base levelling before bolts go in | Square Packer |
| Machinery levelling with bolts partially in place | Horseshoe Shim |
| Maximum bearing area and load distribution needed | Square Packer |
| Adjustability without removing fixings | Horseshoe Shim |
Shop the Range
Square Packers are available in mild steel, stainless steel, and aluminium across the full standard thickness range, with bulk pricing available for larger orders. Horseshoe Shims are stocked in the same materials and thicknesses, sized to suit standard fixing diameters. If you need a non-standard size, slot width, or thickness, custom parts are cut to order with a typical lead time of 3 to 4 working days.
All parts are manufactured at our facility in Braintree, Essex. Free delivery on orders over £100 ex VAT.
Need a little extra help?
If you are unsure about material selection, design suitability or which product is best for your application, contact us.