Types of shims
Types of shims, and how to choose one.
A shim takes up a gap, sets a height or brings two parts into line. This guide explains the types of shims by material and shape, when to use shimstock, cut shims or shim packs, and how to specify one so that the right shim is ordered first time.
- Metal shim thicknesses we cut
- 0.005 to 3 mm
- Colour-coded Polymex® thicknesses
- 12 to 3,000 µm
What a shim does
What a shim does, and why the type matters.
A shim is a thin piece of material fitted between two parts to fill a gap, set a height, correct an alignment or adjust end float. Machine feet are shimmed to bring a motor into line with a pump; bearings and gears are shimmed to set preload and backlash; brackets and panels are shimmed so that they sit flat and true. A shim does not have to seal: that is a gasket's job.
The material decides whether a shim holds the gap over time. It must not compress, creep, corrode or swell where it is fitted. The shape decides how easily it goes in and comes out, and the form it arrives in, from a sheet of shimstock to a finished cut part, decides how quickly a fitter can use it.
We cut most of the types below to your drawing, with laminated shims on request, and our colour-coded Polymex® shims show their thickness by colour. Each section explains what a type is good at and where to order it.
Custom shims cut to your drawingBy material
Shims by material: Polymex® plastic, metal, laminated and specialist.
Most shims are plastic or metal. Plastic shims are light, insulating and corrosion proof, and colour-coded Polymex® shows its thickness at a glance. Metal shims are stiffer, take more heat and carry heavier loads. Laminated and specialist shims solve particular problems.
This is general guidance; the data sheet for the material we supply applies.
Polymex® is a trademark of PSG. Kapton® is a registered trademark of its owner. Nomex® is a trademark of Arclin or its affiliates. Other names are the trademarks of their owners.
Types of shims by material
General guidance, not a specification
| Shim type | Best for | Watch for |
|---|---|---|
| Plastic shims | ||
| Polymex® A polyester, colour-coded | Thin, precise shims and shim washers; insulating; resists oils and most solvents | Service range -70°C to +130°C (PSG data) |
| Polymex® B polypropylene, colour-coded | Thicker packers and spacers; light; resists water and many chemicals | Softening point 144°C (PSG data); petrol and paraffin attack it |
| Nylon and rigid PVC | Low-cost insulating spacers and packers | Creep under a heavy steady load |
| Metal shims | ||
| Steel and stainless steel | Heavy loads, machine feet and heat; stainless resists corrosion | Carbon steel rusts unless protected |
| Brass and phosphor bronze | Bearing shims; corrosion resistant and non-sparking | Softer than steel |
| Aluminium and copper | Light shims (aluminium); soft, conductive shims (copper) | Galvanic corrosion against other metals |
| Laminated (peelable) shims | Foil layers bonded into one shim and peeled on site to the gap | Thickness changes in whole layers only |
| Specialist shims | ||
| Kapton® polyimide film | Very thin shims that also insulate, to about 400°C short term | Thin gauges only |
| PTFE | Corrosive assemblies and chemical plant, -200°C to +260°C | Creeps under a sustained load |
| Nomex® and compressed fibre | Insulating shims in motors and transformers; hot, oily joints | Compress more than plastic or metal |
Plastic shims
- Polymex® A polyester, colour-coded
- Thin, precise shims and shim washers; insulating; resists oils and most solvents
- Watch for: Service range -70°C to +130°C (PSG data)
- Polymex® B polypropylene, colour-coded
- Thicker packers and spacers; light; resists water and many chemicals
- Watch for: Softening point 144°C (PSG data); petrol and paraffin attack it
- Nylon and rigid PVC
- Low-cost insulating spacers and packers
- Watch for: Creep under a heavy steady load
Metal shims
- Steel and stainless steel
- Heavy loads, machine feet and heat; stainless resists corrosion
- Watch for: Carbon steel rusts unless protected
- Brass and phosphor bronze
- Bearing shims; corrosion resistant and non-sparking
- Watch for: Softer than steel
- Aluminium and copper
- Light shims (aluminium); soft, conductive shims (copper)
- Watch for: Galvanic corrosion against other metals
- Laminated (peelable) shims
- Foil layers bonded into one shim and peeled on site to the gap
- Watch for: Thickness changes in whole layers only
Specialist shims
- Kapton® polyimide film
- Very thin shims that also insulate, to about 400°C short term
- Watch for: Thin gauges only
- PTFE
- Corrosive assemblies and chemical plant, -200°C to +260°C
- Watch for: Creeps under a sustained load
- Nomex® and compressed fibre
- Insulating shims in motors and transformers; hot, oily joints
- Watch for: Compress more than plastic or metal
Polymex® figures are PSG data (Polymex® data sheets); other entries are general guidance.
By shape
Shims by shape: flat, slotted, ring and custom profile.
The shape decides how a shim is fitted, and whether it can be changed without taking the assembly apart.
Flat shims
Plain rectangles or blanks cut to a footprint, for packing under brackets, panels, rails and machine bases.
Slotted shims
A U-shaped slot lets the shim slide in around a bolt without removing it, so a machine can be aligned with its bolts loosened instead of taken out. The standard choice for motor, pump and gearbox feet.
Ring shims and shim washers
Thin washers that set bearing preload, gear backlash or end float on a shaft. Our guide to custom washers and spacers covers their sizes and materials.
Custom profile shims
Shims with holes, cut-outs and tabs that follow a component exactly, such as the shims that set the air gap in electromagnetic brakes and clutches. They are cut to your drawing.
Stock or cut
Shimstock, cut shims or shim packs?
Choose the situation and the table shows which form of shim suits it.
What is the job?
Choose properties to find the right material. 4 materials compared.
| Situation | Shimstock sheets | Shim packs | Cut shims | Laminated shims |
|---|---|---|---|---|
| Gap measured on site, maintenance or repair | Yes | Yes | No | Yes |
| Production assembly, the same shim every time | No | No | Yes | No |
| Unknown gaps, many thicknesses to handA pack holds a sheet of each of several colour-coded thicknesses. | No | Yes | No | No |
| Holes, cut-outs or a complex outline | No | No | Yes | No |
| Fitted around a bolt without dismantlingSlotted shims, cut to the bolt and foot size. | No | No | Yes | No |
| Fine adjustment during assembly | Yes | Yes | No | Yes |
| Least stock to holdOne laminated shim covers a range of thicknesses. | No | Yes | No | Yes |
| Thickness checked at a glanceWhen made from colour-coded Polymex®. | Yes | Yes | Yes | No |
General guidance. Polymex® comes as shimstock and shim packs and is cut into shims to your drawing; laminated shims are available on request.
Shim stacks
Stacking shims to make up a gap.
When a measured gap falls between standard thicknesses, several shims are stacked to make it up. Use as few as you can: every extra layer adds an interface that can settle, trap dirt or shift, and the thickness tolerances of the layers add together. A stack of two or three shims is steadier than a stack of eight.
With colour-coded Polymex®, each layer's colour gives its thickness, so a stack is easy to check. Our shim stack calculator takes a measured gap in millimetres or inches and finds the fewest Polymex® shims that make it up, then the fewest colours. For a toolbox, an assorted Polymex® shim pack of 22 colours, from 0.025 to 1.500 mm, is sold in our online shop.
Measure the gap at its tightest point and check the finished stack with a feeler gauge or micrometer. Where the same gap occurs on every unit, a single cut shim is quicker to fit than a stack.
Open the shim stack calculatorThickness and tolerance
Shim thickness, tolerance and the colour code.
A shim is only as accurate as its thickness. Polymex® is colour-coded so that each nominal thickness has its own colour, from the 0.012 mm Clear film to the 3.000 mm Black sheet. The colour identifies the thickness; the tolerance is set by the material, as the table shows.
We cut metal shims in seven metals, and their tolerance depends on the metal and the gauge, so we state it with the quotation. For the thinnest insulating shims, see our guide to Kapton® film.
Colour-coded plastic shimsShim materials from PSG
Thicknesses and tolerances
| Shim material | Available thicknesses | Thickness tolerance |
|---|---|---|
| Polymex® A polyester | 0.012 to 0.350 mm | ±5% of nominal |
| Polymex® B polypropylene | 0.375 to 3.000 mm | ±10% of nominal |
| Brass, copper, aluminium, phosphor bronze, nickel, stainless steel and steel | 0.005 to 3.000 mm | By metal and gauge, stated with the quotation |
| Kapton® polyimide film | 0.023 to 0.125 mm | To the film maker's specification |
| Nomex® aramid paper | 0.050 to 0.760 mm | To the paper maker's specification |
| PTFE | 0.500 to 50.000 mm | By grade and thickness |
| Nylon, rigid PVC and HIPS | 0.250 to 6.000 mm, by material | By grade and thickness |
| Laminated (peelable) shims | On request | By layer thickness |
- Polymex® A polyester
- 0.012 to 0.350 mm
- Thickness tolerance: ±5% of nominal
- Polymex® B polypropylene
- 0.375 to 3.000 mm
- Thickness tolerance: ±10% of nominal
- Brass, copper, aluminium, phosphor bronze, nickel, stainless steel and steel
- 0.005 to 3.000 mm
- Thickness tolerance: By metal and gauge, stated with the quotation
- Kapton® polyimide film
- 0.023 to 0.125 mm
- Thickness tolerance: To the film maker's specification
- Nomex® aramid paper
- 0.050 to 0.760 mm
- Thickness tolerance: To the paper maker's specification
- PTFE
- 0.500 to 50.000 mm
- Thickness tolerance: By grade and thickness
- Nylon, rigid PVC and HIPS
- 0.250 to 6.000 mm, by material
- Thickness tolerance: By grade and thickness
- Laminated (peelable) shims
- On request
- Thickness tolerance: By layer thickness
Polymex® thicknesses are nominal. Other materials and thicknesses on request.
Specifying
How to specify a shim.
Five details turn a gap into a shim we can quote.
- 01
The gap and its tolerance
The thickness the shim must make up, and how closely. If you have measured several gaps, say whether you need one shim or a set of thicknesses.
- 02
The shape
Flat, slotted, ring or a custom profile, with a dimensioned sketch or a drawing. For slotted shims, give the bolt size and the dimensions of the foot.
- 03
The load and the environment
What the shim carries and what it meets: heat, oil, chemicals, moisture, or a need to insulate. This decides between plastic, metal and specialist materials.
- 04
How it will be fitted
Single shims, stacks, laminated shims, or Polymex® shimstock to cut on site, and whether colour coding would help the fitter.
- 05
The quantity
A one-off, a maintenance stock or a production volume, which decides between shimstock, cut shims and the cutting method.
Shim FAQs
Shim questions, answered.
Can't find what you need? Our technical team is happy to help.
What is a shim?
A shim is a thin piece of material placed between two parts to fill a gap, set a height, align them or adjust a clearance. Shims are made in plastic, metal and specialist materials, in flat, slotted, ring or custom shapes, and unlike a gasket they do not have to seal.
Plastic or metal shims?
Choose plastic where the shim must insulate, resist corrosion, protect a soft surface or be identified by colour, as Polymex® is. Choose metal where it carries heavy or shock loads, sits in high heat or must not compress at all. Our colour-coded plastic shims page explains the Polymex® range.
What is a laminated (peelable) shim?
A laminated shim is made of many thin metal foil layers, typically about 0.05 to 0.08 mm each, bonded into a solid shim. The fitter peels layers off with a knife until the shim matches the gap, so one part covers a range of thicknesses and is adjusted on assembly. They are used where gaps vary from unit to unit, notably in aircraft and heavy engineering.
What are slotted shims used for?
Slotted shims have a U-shaped slot, so they slide in around a bolt without the bolt being removed. They are used to align motors, pumps, gearboxes and other machines on their bases: loosen the bolts, lift the foot slightly, slide the shim in and retighten. We cut slotted shims to your bolt and foot sizes.
How do I make up a gap from several shims?
Measure the gap at its tightest point, then combine as few shims as possible to reach it without exceeding it. Polymex® colours let the stack be checked by eye, and the shim stack calculator finds the fewest shims for any gap, in millimetres or inches.
What thickness tolerance do shims have?
It depends on the material and the thickness. Polymex® polyester shimstock is made to within 5% of its nominal thickness and Polymex® polypropylene to within 10%. Metal shim tolerances depend on the metal and the gauge, and we state them with the quotation. If your joint needs a closer tolerance, tell us when you ask for a quote.
Send us your drawing.
Tell us the gap, the shape, the material if you know it and the quantity, and our technical team will recommend the right type of shim and quote for it.
Gasket and shim materials
More materials and methods
- Read more
Reference
Gasket material chart
Temperature, hardness and chemical resistance compared.
- Read more
Gasket and sealing materials
Rubber gaskets
Every common elastomer compared, plus insertion, natural and butyl rubber.
- Read more
Electrical insulation and thermal
Electrical insulation materials
Papers, films and sheet cut into insulators.
- Read more
Plastics and components
Engineering plastics
Nylon, acetal, polycarbonate and PVC parts to drawing.
- Read more
Cutting methods
Die cutting
Flatbed and rotary die cutting, prototype to volume.
- Read more
Overview
Gasket and shim materials
Every material we cut, compared in one place.
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