Fitting or removing a washer without taking the fastener out - which is often the difference between a five-minute job and stripping an assembly.

A closed washer has to be threaded onto the bolt, so the bolt must come out first.

A slot lets the washer slide in sideways under a slackened head or nut.

That is decisive where the bolt is captive, where the nut is welded, or where withdrawing the fastener would release something being supported.

It also suits adjustment work where washers are added and removed repeatedly to set a clearance.

The trade-off is that an open ring is structurally weaker than a closed one.

No - the slot interrupts the ring, so the washer spreads and distorts at much lower load.

A closed washer carries hoop stress all the way round, which is what keeps it flat under a concentrated head load.

Cutting the ring removes that path, so the two arms either side of the slot bend outwards.

Once it has spread, the bearing area is reduced and the washer may work loose from the joint altogether.

Under a high-preload fastener the effect is immediate rather than gradual.

For loaded joints a closed washer is the correct part, and dismantling to fit it is the correct method.

Wide enough to clear the shank comfortably, which is more than the nominal bore.

The washer must slide on past a thread, and a slot cut to the bore dimension will catch on the thread crests.

Manufacturers state slot width separately from bore, and the two are different numbers.

Too wide and the washer has very little material either side, so it distorts even more readily.

For repeated adjustment work the wider slot is usually worth the reduced stiffness, since the loads are low.

Cutting a slot into a standard washer on site gives a rough edge and no control over either dimension, so a made part is preferable.

Yes, and it is one of their main uses - stacked slotted washers set a clearance without disassembly.

Adding or removing thickness under a fastener is exactly the adjustment slotted washers are made for.

Purpose-made slotted shims in graded thicknesses do the job more precisely, and are the better choice where the dimension matters.

Stacking creates multiple interfaces, each of which can settle, so a stack is less stable than one washer of the right thickness.

Slots in a stack should be staggered rather than aligned, so the weakness is not concentrated in one direction.

Once the adjustment is settled, replacing the stack with a single closed washer or shim of the final thickness is the durable answer.

They can migrate out of the joint if they spread, and vibration makes that more likely.

A washer that has opened under load is no longer captive, and there is a clear path sideways out of the joint.

That is a specific hazard where the joint is above anything or anyone.

Orienting the slot away from the direction of any vibration or flow reduces the risk.

Keeping the fastener properly tightened is the main control, since a loose fastener lets the washer move at all.

Where vibration is present and a washer must be added without disassembly, treating it as a temporary measure is the honest position.

As soon as the assembly is next dismantled - and immediately if it has spread.

A slotted washer that has visibly opened has already lost bearing area and will continue to distort.

Any joint that will be taken apart for other reasons is the opportunity to fit a proper closed washer.

Where the slotted washer was used to make an adjustment, the final dimension can be achieved with a closed washer or a shim once the value is known.

In a designed connection a slotted washer should be regarded as a temporary expedient.

Recording where slotted washers have been fitted makes the eventual replacement possible rather than forgotten.

Yes - the same reasoning applies to insulating and sealing layers that must be fitted without disassembly.

Retrofitting electrical isolation to an existing joint is a common case, and a closed washer would require the fastener to come out.

Polymer slotted washers distort more readily than metal ones, so the load limitation is even tighter.

Sealing applications are generally unsuitable, because the slot itself is a leak path.

Where isolation is being retrofitted, the shank still contacts the bore, so the isolation is partial unless a sleeve is fitted too.

As with metal, the proper fix is a closed component at the next opportunity.