What Are Cap Washers and Why Do Sim Rig Builders Use Them?

Quick answer

Cap washers are aluminium covers that sit over bolt heads on your rig's aluminium profile, giving a flat, finished surface instead of an exposed fastener. Builders use them for three reasons: they protect hands, arms, and cables from sharp or protruding bolt ends, they make a cockpit look intentional rather than unfinished, and they let you colour-match hardware to the frame. If you only care about clamping force, a standard washer does that job; cap washers add the finish and safety layer on top.

Once your rig is assembled, the last thing you notice is usually the hardware: rows of bolt heads sticking out of slot 8 profile. Cap washers are the small, cheap part that turns that into a clean line. They are not structural, and they do not replace a proper washer under load, but they solve a real problem on rigs that get touched, leaned on, and crawled around.

This guide answers the questions builders actually ask before ordering: what cap washers do, where they belong, how to pick the right size, and when a standard washer is the smarter choice.

What does a cap washer actually do?

A cap washer is a shallow aluminium disc that sits over the head of a bolt, hiding it and giving the joint a flat, rounded edge. On a sim rig it is purely a finishing and safety part: the bolt underneath still does the clamping, and the washer does not add meaningful rigidity to the frame. What it does add is a surface you can run your hand along without catching a thread or a hex head.

That matters more than it sounds. Rig owners routinely reach past uprights to adjust a wheelbase, route a cable, or climb in and out of a bucket seat. Exposed bolt heads are where knuckles get scraped and where cable insulation gets rubbed through over months of use.

• Use cap washers on bolts you can see or touch, not on hidden joints inside the frame.
• They are cosmetic and protective, not a substitute for a correctly torqued bolt.

Cap washers vs standard washers: what is the difference?

A standard washer is a flat disc that spreads clamping load under a bolt head or nut. Its job is mechanical: it protects the profile from being dented and keeps the joint tight. A cap washer is a cover. It sits on top of the finished joint and hides the fastener, which is why it is usually made from aluminium and offered in colours.

In practice, most rigs use both. The standard washer goes on first, under the bolt head, and the cap washer goes over the top as the visible layer. If you remove the standard washer to fit a cap washer, you lose the load-spreading benefit and risk marking the profile. Treat them as two different jobs rather than two options for the same job.

• Standard washer: load spreading, always under the fastener.
• Cap washer: appearance and edge protection, always on the outside.

Which size cap washer do I need?

Match the cap washer to the bolt size, not to the profile. An M8 bolt needs an M8 cap washer, an M6 bolt needs M6, and so on. If you mix sizes, the cap will either sit loose and rattle or refuse to seat over the bolt head. Before ordering, walk your rig and note which bolt sizes appear most often, because most frames use a mix.

The Sim-Lab Cap Washers come in M8, M6, or M5, in sets of six, with black, blue, or red finishes. Each set also includes extra bolts and washers so the caps fit the common brackets in the range. Counting first avoids a second order for one size you forgot.

• Count bolt sizes per area: wheel deck, pedal plate, seat frame, and uprights often differ.
• Order by the dominant size first, then fill in the smaller counts.

Where should you place them on a rig?

Start where skin and cables meet metal. The inside faces of the uprights beside your seat, the front edge of the pedal plate, and any bolt head near where a cable runs are the highest-value spots. Those are the places you brush against when getting in, adjusting pedals, or tidying wiring.

After that, finish the visible exterior: the outer faces of the main frame, the wheel deck, and any bracket you see from your seating position. There is no need to cap bolts buried inside the frame where nothing touches them. Spending caps on visible and reachable hardware gives the biggest change for the smallest outlay.

• Priority one: anywhere hands, arms, or cables pass.
• Priority two: any bolt head visible from the driver's seat.

Do cap washers affect rigidity or performance?

No. Rigidity comes from the profile, the brackets, and how tight the bolts are. A cap washer adds a few grams and a finished look, nothing more. If a joint feels flexy, the fix is a better bracket, a gusset, or correct torque, not a different washer.

That is also why cap washers are a safe upgrade at any point in a build. You can add them to a finished rig without disassembling anything structural, and you can remove them later without affecting how the frame performs.

• Do not use cap washers to fix a loose or flexing joint.
• Check bolt torque first if something moves that should not.

Cap washer quick reference

Do I need cap washers on every bolt?

No. Cap the bolts you can see or touch, and leave hidden internal joints bare. Focus on uprights, wheel deck, pedal plate edges, and cable routes.

Should I remove the standard washer to fit a cap?

Never. Keep the standard washer under the bolt head for load spreading and add the cap on top. Two washers, two jobs.

How do I choose between M8, M6, and M5?

Match the cap to the bolt thread size, and count how many of each size your rig uses before ordering. Most rigs need more than one size.

Parts that pair with cap washers

• Sim-Lab Cap Washers (Set of 6): Aluminium cap washers with the Sim-Lab logo engraved, available in black, blue, or red and in M8, M6, or M5, six pieces per set, with extra bolts and washers included to fit common brackets. The colour and size options let you match existing hardware and cover the bolt sizes your rig actually uses, while the included extras reduce the chance of a second order.
• Cross Cable Binder Block (Set of 10): Cable management clips designed for slot 8 profiles, supplied in packs of 10 and used with tie wraps to organise and secure cables. If you are already tidying exposed hardware, routing cables along the profile keeps them away from bolt heads and makes the finished rig look deliberate.
• P1X Pro Simracing cockpit: An aluminium profile cockpit built for rigidity and adjustability, supporting direct drive wheels and loadcell pedals, with a modular design and wheelmount and monitor mount options. A profile rig is where cap washers make the most sense, because the frame exposes many bolt heads along the uprights and wheel deck.

Cap washers are a small, low-cost way to make a profile rig safer to live with and cleaner to look at. Count your bolt sizes, cap the spots you actually touch, and leave the hidden joints alone.