A spacer has a plain bore and a fastener passes through it; a standoff carries its own thread and the fastener screws into it.

That single difference changes how the assembly goes together. A spacer needs one fastener passing through the whole stack; a standoff can be fixed at each end independently.

Standoffs therefore allow layers to be built up and taken apart one at a time, which is why multi-board electronics assemblies use them.

Spacers are simpler and cheaper, and are the right choice where a single through-bolt already exists.

A spacer contributes no strength to the joint; a standoff carries the thread and so carries load.

The two are often confused in catalogues, so checking whether the bore is threaded is worth doing before ordering.

Because tolerances add up across a stack, and an uneven stack means the two parts are no longer parallel.

Four spacers at the corners of a board each a little different in length will tilt it, and the tilt is the sum of the differences.

That matters for circuit boards with connectors that must align, for optical and sensor mountings, and anywhere a seal has to compress evenly.

Machined spacers hold much closer tolerances than cut tube, at a higher cost per piece.

Where the absolute distance matters less than the parts being parallel, buying all spacers from one batch helps considerably.

Shims are the usual correction when a stack is found to be out.

Rarely - once the fastener is tightened the spacer is captive, and until then it spinning is harmless.

The spacer takes no torque and provides no locking, so rotation has no effect on the joint.

It matters during assembly if the spacer has to be held in place while a fastener is started, which is awkward in blind or overhead positions.

It also matters where the spacer's outside surface is decorative or has a feature that must stay aligned.

Hex and square spacers exist for those cases, and self-adhesive or shouldered spacers solve the assembly problem another way.

For most panel and board work, round is the right default.

Metal where it must carry load or conduct, polymer where it must insulate or isolate.

Aluminium and brass are the common metals: aluminium for light weight, brass for machinability and corrosion resistance, stainless where the environment demands it.

Nylon and other polymers insulate electrically, weigh very little and cost less, and are the default in electronics where isolation matters.

Glass-filled and high-temperature polymers extend the range where a plain nylon spacer would soften or creep.

Ceramic spacers are used where both high temperature and electrical isolation are required.

In mixed-metal assemblies the spacer can be chosen specifically to break a galvanic path.

Yes - polymers creep under sustained compression, and the joint relaxes as they do.

The effect is small at room temperature over short periods and significant over years or at elevated temperature.

The symptom is a joint that has gone slack with nothing having turned at all.

Engineering polymers and glass-filled grades resist creep far better than commodity plastics.

Where the clamp force must stay constant, a metal spacer removes the question entirely.

Designs that tolerate some relaxation - using a spring washer or a compliant element - are the other approach.

Enough to assemble easily, and no more than needed to keep the stack concentric.

A close bore keeps the spacer centred on the fastener, which matters visually and for any assembly that must be positioned accurately.

A generous bore makes assembly easier through misaligned holes and lets the spacer sit off-centre.

Where the spacer is visible, an off-centre spacer is immediately obvious.

For high-vibration assemblies, a close bore reduces the small movements that wear the bore over time.

Standard spacer series are sized against nominal fastener sizes, and their clearance suits general work without further thought.

Metal tube spacers can, with care about squareness; moulded polymer spacers usually should not.

The ends of a spacer are its bearing faces, and a cut that is not square tilts the assembly.

Cutting in a lathe or with a proper tube cutter and then facing the end gives an acceptable result.

Burrs on the bore and the end face must be removed, or they sit between the bearing surfaces.

Moulded polymer spacers often have features at the ends - a chamfer, a shoulder, a moulded flat - that a cut removes.

For anything where the distance is critical, buying the correct length is more reliable and usually cheaper than the labour of cutting.