In technical documentation, especially when working with imported equipment and vehicles, the term is often found shim, the translation into Russian is not always clear for a novice mechanic. This word comes from the English verb "to shim", meaning "to wedge" or "to wedge". In the Russian-language technical tradition, this element is most often called gasket, shim or wedge gasket. Understanding the exact value is necessary for the correct selection of spare parts and high-quality repairs of components.
Usage shim critically important in assemblies where minimizing backlash and precise positioning of parts is required. Incorrect translation or ignoring this element can lead to accelerated wear of the bearings, engine overheating or disruption of the suspension geometry. In this article we will look at how to correctly identify these parts in catalogs and what parameters to consider when replacing them.
⚠️ Attention: Using shims of the wrong thickness may compromise motor clearance or bearing preload, causing the unit to seize.
The main function of shims is to compensate for tolerances when assembling mating parts. In the automotive industry, where quality standards require micron precision, shim plays the role of the final gap calibrator. Often in catalogs they are marked by thickness, for example, 0.15 mm or 0.20 mm.
Exact meaning of the term and classification
With literal translation shim - This is a thin strip or wedge. However, in the context of auto parts and mechanical engineering, the term becomes more specific. Depending on the design and purpose, this word is translated into Russian differently. Mechanics may use the terms "gasket", "washer", "plate" or "liner".
The key difference from a conventional gasket is the material and purpose of use. If gasket serves for sealing and is often made of rubber or cardboard, then shim - This is always a rigid metal element intended for mechanical adjustment. It does not deform under load, maintaining the specified size.
In technical documentation you can find the following translation and classification options:
- 🛠️ Adjustment shim — an adjusting washer, used to adjust the clearances in valves or bearings.
- 🛠️ Shim washer - a flat washer, often used under nuts or bolts to align the planes.
- 🛠️ Shim plate — an adjustment plate, a larger element for aligning assemblies.
It is important to distinguish shim and spacer (spacer sleeve). Although both elements take up space, the spacer is usually cylindrical in shape and tall, while the shim is a thin disk or plate. Confusion about terms when ordering parts can result in a part that does not physically fit into place.
Materials of manufacture and physical and chemical properties
Selection of material for shim depends on the environment in which it will operate. In an internal combustion engine, gaskets are exposed to high temperatures, oil and aggressive chemical compounds. Therefore, ordinary soft metals will not work here.
The most common material is hardened steel. It has high hardness and does not wrinkle when tightening fasteners. For components subject to corrosion or in the food industry (refrigerator compressors), stainless steel is used. In some applications where electrical insulation or lightness is important, polymer composites or brass are used.
When replacing valve shims, always wipe them with a degreaser before measuring with a micrometer - an oil film can distort the readings by 0.01-0.02 mm.
Particular attention should be paid to thermal expansion. Material shim must have an expansion coefficient close to the material of the mating parts. If the washer expands more than the housing, it may cause seizure when the engine warms up. If it is less, there will be a knock when cold.
In high-precision components, such as Common Rail fuel equipment, calibrated washers from special alloys. Their surface undergoes additional grinding to achieve ideal flatness. The use of analogues of dubious quality in such places is unacceptable.
Size table and tolerances (GOST and ISO)
When selecting analogues, it is necessary to focus not only on the internal and external diameters, but also on the accuracy class. Metric (ISO) dimensions shim standardized, but tolerances may vary. Below is a table of the correspondence of common standard sizes.
| Inner diameter (ID), mm | External diameter (OD), mm | Thickness (T), mm | Standard |
|---|---|---|---|
| 10.5 | 18.0 | 0.10 - 0.50 | ISO 2171 |
| 12.0 | 20.0 | 0.10 - 0.60 | DIN 988 |
| 15.0 | 25.0 | 0.10 - 1.00 | JIS B 1730 |
| 17.0 | 28.0 | 0.15 - 0.80 | GOST 15736 |
Please note that the thickness increments of standard washers are often 0.05 mm or 0.10 mm. In some engines (for example, Honda or Toyota), the pitch may be even smaller - 0.025 mm. This requires a complete set (shim kit) to carry out high-quality cylinder head repairs.
If the original part is not available, use shim the nearest size, but only if the difference does not exceed the tolerances specified by the engine manufacturer. Exceeding the gap leads to noise, decreasing it leads to burnout of the valves.
Is it possible to use several thin washers instead of one thick one?
Technically this is possible, but it is highly not recommended. The multi-layer design creates an unstable package that can move under vibration. It is better to use one washer of the calculated thickness.
Application in engine and transmission
In a car engine shim occurs at several critical nodes. The most famous example is adjusting the thermal clearances of valves. Here the washer is installed between the camshaft cam and the valve tappet. Its thickness determines how tightly the valve will close and whether it will be “pressed” when heated.
The second important place is the crankshaft. Adjusting washers (thrust half rings) limit the axial play of the shaft. Wear of these parts leads to a characteristic knocking sound and rapid failure of the crankshaft thrust collars. Packages in automatic transmissions shim used to adjust pressure in the valve body and friction disc clearances.
- 🚗 cylinder head: valve adjustment (overhead cam).
- 🚗 Crank mechanism: crankshaft thrust washers.
- 🚗 Transmission: Adjustment of differential and shaft bearings.
When assembling an engine after a major overhaul, mechanics often use the selection method. The actual gap is measured and the required thickness is calculated mathematically shim. The formula is simple: New Thickness = Old Thickness + (Measured Gap - Nominal Gap).
Instructions for replacement and wear diagnostics
Replacing shims requires care and cleanliness. Dirt getting between the washer and the pusher will result in an incorrect reading of the gap. Before starting work, it is necessary to dismantle the necessary components, providing access to the gas distribution mechanism or bearings.
The replacement process is as follows: first, the old washer is removed, the seat is cleaned, and the thickness of the removed part is measured with a micrometer. Based on this measurement and the current state of the gap, a new part is selected.
☑️ Algorithm for replacing washers
⚠️ Attention: Never reuse rusted or warped washers. Even microscopic corrosion changes the geometry and disrupts the operation of the unit.
After installing the new part, you need to turn the engine shaft several times (manually, with a key on the pulley) and re-measure the gap. This is necessary for the washer to “sit” in place. If the readings float, it means the surface is dirty or the washer is installed crookedly.
Frequent mistakes when selecting analogues
One of the most common mistakes is trying to replace shim an ordinary washer from a hardware store. Such washers (DIN 125) have low hardness and high surface roughness. In the engine they will quickly crumble, turning into mush, which will lead to an emergency.
Another mistake is ignoring the temperature regime. Aluminum washers cannot be installed in units operating at temperatures above 200°C, as aluminum “floats”. Also, do not try to grind the washer down on sandpaper to adjust the size - the parallelism of the planes is disrupted.
Always check the labeling. Sometimes manufacturers apply a thickness code with a laser, which can be erased. In such cases The only reliable way to determine the size is to measure with a micrometer with an accuracy of 0.01 mm.
Saving on high-quality shims when repairing an engine can lead to the need for repeated disassembly and expensive repair of the cylinder head after several thousand kilometers.
FAQ: Frequently asked questions
Is it possible to make a shim yourself from sheet metal?
Theoretically, it is possible if you use a high-precision press and hardened steel, but in garage conditions it is almost impossible to achieve the necessary parallelism of planes and hardness. The risk of damaging an expensive component is too great.
What is the difference between shim and gasket?
Shim (adjusting shim) is used to change the geometry and clearance and is made of hard metal. Gasket (sealing gasket) is used to seal joints and is made of soft materials (rubber, paronite, graphite).
How can you tell if the valve shims are worn out?
The main symptom is the appearance of a metallic clatter on a cold or warm engine (depending on the type of adjustment), which does not disappear after warming up. Compression may also drop or fuel consumption may increase.
Does the shim need to be lubricated before installation?
Yes, usually the washers are installed with a thin layer of engine oil to prevent dry friction in the first seconds of starting the engine. However, the oil layer should not be excessive so as not to affect the accuracy of the gap.