Spacers

When preparing a slot car for competition —or simply wanting it to run more accurately— the small details make a difference. And few details are as decisive as the exact position of each element on the rear axle. Axle spacers are those tiny pieces that keep crowns, wheels, bearings, and other components exactly where they should be. Without them, everything tends to shift with vibrations, impacts, or temperature changes, generating play that translates into erratic behaviour, uneven tyre wear, and, in the worst case, premature mechanical failures.

If you are looking for spacers because something does not fit well on your axle, because you want to replicate a setup that has been recommended to you, or because you are simply starting out in slot car preparation, this guide will explain everything you need to know: what they are exactly, how to choose them correctly, what types exist, and how to use them without making the most common mistakes. Let’s get to the point.

What are axle spacers and why are they important

An axle spacer is a cylindrical piece —generally made of technical plastic, brass, or aluminium— that is mounted on the rear axle of the slot car to occupy the excess space between the various mechanical components. Its main function is twofold: on one hand, to prevent lateral movement of crowns, wheels, and bearings; on the other, to eliminate axial play that occurs when the axle has more usable length than necessary for the elements that compose it.

In a well-configured axle, each piece has its defined position and does not move. The crown is centred with respect to the motor pinion, the wheels are symmetrical with respect to the chassis, and the bearings work without lateral effort. When a spacer is missing or its thickness is not appropriate, all this is distorted: the crown may rub against the chassis, the wheels may become uncentred with respect to the body, or the bearings may suffer loads for which they are not designed.

In a competition context, where adjustments are measured in tenths of a millimetre and every gram of parasitic friction counts, spacers are mandatory components that require periodic inspection. But even in casual or collectible cars that want to be kept in good mechanical condition, their use prolongs the lifespan of the assembly and ensures smooth and silent running.

How to choose spacers correctly

The choice of a spacer depends on several factors that should be analysed before purchasing:

• Inner diameter: it must match exactly with the diameter of the axle on which it will be mounted. The most common axles in 1/32 scale cars are 3 mm, although there are also 2 mm, 2.38 mm (3/32"), or 1/8" options. Check the specifications from your car's manufacturer or measure with a micrometer if in doubt.
• Thickness (axial length): this is the most critical parameter. Each manufacturer markets spacers in different thicknesses (0.1 mm, 0.2 mm, 0.5 mm, 1 mm…) so you can combine them to achieve the exact fit. Measure the free space on your axle before ordering and consider whether you need a fixed thickness spacer or a combination of several.
• Material: plastic spacers are the most economical and sufficient for casual or basic level competition cars. Brass or aluminium spacers are stiffer, do not deform with heat, and are the preferred option in high-performance setups.
• Compatibility with the car: brands like NSR, Slot.it, Avant Slot, or Scaleauto have their own catalogues of spacers adapted to their axles and chassis. Using components from the same manufacturer facilitates adjustment, although generic spacers also work if the diameter is correct.

If you are preparing a car for a regulated category, also check if the regulations specify anything about materials or the type of axle mounting, as some categories limit the use of certain components.

Main types and differences

Although at first glance all spacers seem the same, there are important differences that affect their use:

Technical plastic spacers

These are the most common and the ones that most manufacturers mount as standard. Lightweight, very low cost, and easy to cut or sand if you need to adjust the thickness. Their main limitation is that they can deform slightly with high temperatures or excessive tightening. Perfect for casual cars and basic configurations.

Brass spacers

Brass is the favourite material of the most demanding preparers. It is dimensionally stable, does not deform with heat, and works well with steel axles. Its higher specific weight compared to plastic can be considered by those looking to distribute mass on the axle, although in practice the impact is minimal given its small scale.

Aluminium spacers

Lighter than brass and with good dimensional rigidity. They are used in setups where the goal is to minimise rotating weight as much as possible. Less common than brass, but available in the catalogues of specialised brands.

Adjustable or multi-piece spacers

Some systems allow stacking several spacers of different thicknesses to achieve measurements that do not exist in a single component. This is the most flexible way to achieve very precise adjustments when there is no exact measurement spacer available on the market.

Technical aspects we need to know

To understand the role of spacers well, it is important to have a clear idea of how the rear axle of a slot car works as a whole:

• The axle rotates within the bearings, which are housed in the chassis or directly in the chassis.
• On the axle, the crown (which receives the movement from the motor pinion), the wheels with their tyres, and, in some designs, a flywheel or fixing collar are mounted.
• The motor mounting determines how the motor is oriented with respect to the axle: in a sidewinder mounting, the motor is parallel to the rear axle; in an inline mounting, it is perpendicular (longitudinal); in an anglewinder mounting, it is at an angle. Each configuration generates different forces on the axle and, therefore, different positioning needs for the crown.
• The axial play (movement of the axle from side to side within the bearings) should be minimal but not zero: a completely locked axle with no play can generate heat and premature wear. The goal is to have no perceptible play but for the axle to rotate freely.

Spacers act precisely on this last point: they fill the excess space in the axle so that the assembly is tightly controlled, without play but without locking.

Another important aspect is the centering of the crown. If the crown is not perfectly aligned with the motor pinion, the transmission is not efficient and accelerated wear occurs on both parts. Spacers allow the crown to be shifted laterally until the correct alignment is achieved.

Practical tips for use, maintenance, and preparation

• Measure before buying: use a micrometer or, at a minimum, a digital caliper to measure the free space on the axle before acquiring spacers. Estimating by eye often leads to tenths of a millimetre errors that are noticeable in the car's behaviour.
• Combine thicknesses: if you need to fill 0.7 mm, use a 0.5 mm spacer and a 0.2 mm one. Do not try to adapt a spacer by filing or cutting if you do not have experience: it is easy to lose perpendicularity and worsen the result.
• Check spacers periodically: especially plastic ones. With use and heat, they can deform slightly or compress, generating play where there was none before. In each car inspection, check that the axle does not have excessive lateral play.
• Do not exceed tightening: spacers should hold components in position, not lock them. If the axle does not rotate freely after mounting the spacers, there is too much material: reduce the total thickness.
• Clean before mounting: remove any lubricant or dirt from the axle before placing the spacers. Any trapped particle can uncentre the assembly and add unwanted friction.
• Note your setups: when you find the combination of spacers that makes your car run perfectly, write down the measurements and positions. This way, you can replicate the configuration in future inspections or in another car of the same preparation.

Common mistakes to avoid

Using spacers of incorrect size

This is the most common mistake. A spacer that is too thick locks the axle or generates tension in the bearings; one that is too thin leaves play. Prior measurement is essential.

Mixing materials indiscriminately

Combining spacers of different materials on the same axle is not necessarily bad, but you must ensure that all have the exact same inner diameter. A small difference in the central hole can cause the spacer not to be centred on the axle and generate vibration.

Forgetting to check the centering of the crown

Changing spacers without checking afterwards that the crown is still well aligned with the pinion is a costly mistake in wear and noise. Always, after mounting or changing spacers, visually and tactilely verify the alignment of the transmission.

Reusing deformed spacers

Plastic spacers that have suffered excessive heat or impacts may have lost their shape. If a spacer does not remain perfectly perpendicular to the axle, replace it: its cost is minimal and the impact on performance can be significant.

Not considering the type of motor mounting

The optimal position of the crown on the axle varies depending on whether the car has the motor in sidewinder, inline, or anglewinder configuration. Before determining where to place the spacers, be clear about the correct position of the crown for your type of motorisation.

Recommendations for beginners and advanced users

If you are starting out

The most important thing is to learn to identify when an axle has excessive lateral play. Hold the axle between two fingers and move it from side to side: there should be no perceptible movement. If there is, you need spacers. Start with the spacers recommended by your car's manufacturer —NSR, Slot.it, Avant Slot, and Scaleauto have very well-documented kits— and do not modify more than one variable at a time so you can identify what change produces what effect.

A good starting point is to check specialised forums or ask in stores like Ministry of Hobby: the accumulated experience of other enthusiasts saves a lot of trial and error time.

If you already have experience

Advanced users work with brass spacers of very tight tolerances, combining different thicknesses to achieve adjustments of a tenth of a millimetre. It is common to carry a small assortment of spacers in the competition bag to fine-tune the setup in the pit between sessions.

It is also worth exploring the effect of spacers on mass distribution on the axle: although their individual weight is small, moving mass from one side to the other of the axle can have a measurable impact on the dynamic balance of the car, especially in very high precision setups.

Frequently asked questions

What is the most common axle diameter in 1/32 slot cars?

The most widespread is 3 mm, but you will also find axles of 2.38 mm (equivalent to 3/32 inch) in many cars from Anglo-Saxon brands. Always check your car's specifications before buying spacers.

Can I use spacers from a different manufacturer than my car?

Yes, as long as the inner diameter is correct. Generic spacers or those from other brands work perfectly if they fit well on the axle. The key is the measurement, not the logo.

How much axial play is acceptable in a slot axle?

In competition, the goal is for the play to be practically imperceptible to the touch, but for the axle to rotate without resistance. A common starting point is between 0.05 and 0.1 mm of total play, although each preparer has their own criteria.

Do spacers affect the speed of the car?

Directly, no. But a poorly adjusted axle (with too much play or excessive friction) can reduce transmission efficiency and increase motor consumption. A correct adjustment with the right spacers helps the car perform at its best.

How often should I check the spacers?

It depends on usage. In competition cars, a complete axle inspection before each important session is recommended. In casual cars, a check every 10-15 hours of use is sufficient.

Can I sand or cut a spacer to adjust its thickness?

Technically yes, but it requires practice and the right tools to avoid losing perpendicularity. It is safer and quicker to combine spacers of different thicknesses until reaching the desired measurement.

Are brass spacers always better than plastic ones?

They are more dimensionally stable and more durable, but they are not necessarily "better" for all uses. For a casual or beginner car, plastic spacers perform their function perfectly and their cost is much lower.

What happens if I mount too many spacers and the axle is tight?

The axle will have difficulty rotating freely, the bearings will suffer excessive axial load, and the motor will work harder than necessary. This can generate heat, premature wear, and loss of performance. Reduce the total thickness until the axle rotates without resistance but without lateral play.

Are spacers necessary in all slot cars or only in competition ones?

In all. Even casual cars come with spacers as standard. The difference lies in the precision of the adjustment: in competition, the exact tenth is sought; in casual use, it is enough that there is no excessive play.

How do I know if the crown is well centred after changing the spacers?

Mount the body, turn the axle by hand, and observe (or listen) for any rubbing or noise in the transmission. With the car on the track, visually check that the pinion and crown are well aligned. If in doubt, use a magnifying glass or a phone camera to zoom in on the image.

Is there any difference between spacers for sidewinder, inline, or anglewinder mounting?

The spacer itself is the same component, but the position where it is placed on the axle varies according to the type of motorisation, as the crown must be at a different point on the axle to align with the pinion in each configuration. Before mounting, clearly identify what your motor's mounting is.