GiroDisc Two-Piece Rotors: Why Rings Replace Separately

By Core Brake Parts Tech Team — Circleville, OH warehouse

A GiroDisc two-piece rotor is a cast iron friction ring bolted to a separate aluminum hat. When the iron wears out, cracks or runs past its wear limit, you replace only the rings and a fresh hardware kit and keep the hats. That is the whole point of the design: the part that wears is the part you buy again, and the precision-machined center stays on the car.

Below is how the ring and hat work together, how to tell when the rings are done, what comes in a replacement ring set, and what the reassembly job actually involves.

How a Two-Piece Floating Rotor Is Built

A one-piece rotor is a single iron casting: the braking surface and the center section that bolts to the hub are the same piece of metal. A two-piece rotor splits those jobs. GiroDisc machines its hats from 6061-T6 aluminum and pairs them with rings of its own cast iron, which the company says is poured in the USA and machined in house. The ring does the braking. The hat locates the ring on the hub and carries the braking torque through it.

The ring and hat are not rigidly clamped together. GiroDisc describes a floating mount that keeps axial and radial float between the hat and the disc, and says the float also lets the rotor self-center between the pads. Brembo explains the same principle on its floating discs: the braking surface and the hat can expand elastically under thermal stress, and the fixing bushes that join them allow radial expansion that helps prevent disc deformation and cracks.

That matters because iron and aluminum grow at different rates when hot, and a braking surface can run far hotter than the center of the rotor. A rigid joint would fight that growth. A floating joint lets the ring expand outward on its own.

Why the Rings Replace Separately

Friction surfaces are consumables. Every stop removes a little iron, and hard driving adds heat cycles that eventually show up as heat checking or cracks. The aluminum hat never touches a pad, so under normal use it has no wear surface to lose. Throwing it away every time the iron is done would mean paying again for the most machined part of the assembly.

GiroDisc lists the primary purpose of its system as reducing unsprung and overall vehicle weight, and the replaceable ring is what makes that lighter design economical to live with. Its replacement ring pages state it plainly: rings are sold in pairs with replacement hardware, and the only parts you reuse are your existing GiroDisc hats.

Two compatibility points to get right before ordering:

• Match the ring to the hat. GiroDisc states its replacement rings are designed to work with GiroDisc hats, not other brands and not the hats from cars that came with two-piece rotors from the factory.
• Some ring sizes are made for specific big brake kits. GiroDisc sells certain 380mm rings described as a direct fit for existing hats on Brembo or StopTech big brake kits, and separate wide-annulus versions. Confirm the diameter, thickness and annulus against what is on your car, not just the vehicle.

Signs Your Rings Are Done

A two-piece ring wears out for the same reasons any rotor does. Brembo's replacement guidance lists these conditions:

• Thickness at or below the minimum thickness (MIN TH) value marked for the disc.
• Deep circular scoring or radial cracks in the braking surface.
• Dark spots or blue spots on the disc surface, a sign of overheating.
• Noticeable deformation of the braking surface, which you feel as vibration or pulsation.

To measure, Brembo recommends a gauge reading taken in at least four different spots around the disc, using the lowest reading as the value for the whole rotor. It also warns that running below the wear limit reduces the disc's heat capacity and strength, which can lead to thermal cracks, vibration and noise, longer pedal travel and fade.

Replace rings as a pair on the same axle. Brembo states discs should be replaced on both sides even if only one is below the wear limit, and GiroDisc sells its rings in pairs for that reason.

What Comes in a Replacement Ring Set

GiroDisc's assembly instructions are direct about the hardware: the hardware kit must be replaced any time a rotor ring is replaced. Replacement ring sets include it, and kits are also sold on their own. The parts in each rotor assembly are:

1. Rotor ring
2. Brake hat (reused)
3. Drive bushings
4. Square washers
5. Spring clips
6. Flat washers (silver)
7. M5 cap screws, long
8. Belleville spring washers (dark gray)
9. M5 cap screws, short (typically 2 to 3mm shorter)

Why new hardware every time? These small fasteners live next to a component that cycles from ambient to very hot and back, over and over. Thread locker, spring washers and clips all lose their holding ability with heat and reuse. The screws are cheap; a loose ring is not.

Reassembly: The Job, Step by Step

This is bench work you can do with basic hand tools and an inch-pound torque wrench. GiroDisc's instructions cover it in five steps:

1. Clean everything. The back of the hat where it contacts the hub, the flange where the ring bolts on, and the hub faces on the car. GiroDisc stresses that all of these surfaces must be spotless. Rust or debris on a hub face causes runout that feels like a warped rotor.
2. Alternate the two screw assemblies. The long-screw assembly (square washer, spring clip, flat washer, cap screw) alternates around the hat with the short-screw assembly (belleville washer and cap screw). Depending on the application there are 4, 5 or 6 of each, for 8, 10 or 12 total.
3. Keep the lengths straight. The shorter screw always gets the dark gray belleville washer.
4. Thread locker. Apply high-temperature thread locking fluid to every M5 cap screw.
5. Torque. With the hat fully assembled to the ring, torque all M5 cap screws to 60 in-lb (7 N-m). GiroDisc warns not to over-tighten. These are small screws in aluminum threads, so this is an inch-pound wrench job, not a foot-pound one.

If you would rather not assemble them yourself, GiroDisc offers to reassemble and turn the rotors when you send in the hats, for the cost of shipping, and says that service is available for the life of the rotors.

Once the rotors are back on the car, the rest is a normal pad and rotor job: clean the caliper brackets, lubricate the slide pins, torque the caliper and wheel fasteners to the factory service manual values, and bleed the brakes if any line was opened.

What to Replace at the Same Time

• Brake pads. New rings need a fresh friction surface to bed against. Old pads carry the wear pattern and material from the old rings. Shop performance and street brake pads for the compound that suits how you drive.
• Pad hardware. Abutment clips and shims lose tension from heat. A brake hardware kit keeps the new pads from rattling or dragging.
• Brake fluid. Cars that see enough heat to wear out two-piece rotors are hard on fluid too. Flush with the fluid type your manufacturer specifies.
• Lines. If the flexible hoses are cracked or soft, braided stainless brake lines give a firmer pedal under repeated hard stops.

Bed in the new rings and pads before hard use. GiroDisc notes that any time different friction materials or discs are introduced, a bedding procedure should be carried out. Follow the procedure published by your pad manufacturer, and avoid sitting with the brakes clamped on hot rotors.

If your car runs one-piece rotors and you are simply due for a brake job, our brake rotors collection covers direct replacements for most vehicles.

Frequently Asked Questions

Can I reuse the hardware when I replace GiroDisc rings?

No. GiroDisc's instructions state the hardware kit must be replaced any time a rotor ring is replaced. Replacement ring sets include new hardware, and kits are sold separately.

Will GiroDisc rings fit my factory two-piece rotor hats?

GiroDisc says its replacement rings are designed for GiroDisc hats and are not meant for other brands or for hats from cars that came with two-piece rotors from the factory. Some specific 380mm sizes are sold as a direct fit for certain Brembo and StopTech big brake kit hats, so check the exact listing.

What torque do the ring-to-hat screws take?

GiroDisc specifies 60 in-lb (7 N-m) on the M5 cap screws, with high-temperature thread locker applied, after the hat is fully assembled to the ring. Do not over-tighten.

Do I have to replace both rings on an axle?

Yes. Rings are sold in pairs, and Brembo's guidance is to replace discs on both sides even if only one is below the wear limit, so both sides brake evenly.

Can the aluminum hat wear out?

The hat has no pad contact, so it does not wear like the ring. Inspect it at each ring change for damaged threads, bent mounting surfaces or corrosion on the hub face. A hat with damaged threads or a distorted flange should be replaced.

Can I resurface worn two-piece rings?

Only if they stay above the minimum thickness after cutting and show no cracks. GiroDisc's own service includes turning the rotors during reassembly. Rings with radial cracks or blue heat spots should be replaced, not machined.

Sources

1. GiroDisc Disc Assembly, Service and Replacement Instructions - hardware replaced with every ring change, parts list, screw layout, thread locker, 60 in-lb (7 N-m) torque
2. GiroDisc Technical Info - 6061-T6 aluminum hats, floating mount and self-centering, weight-reduction purpose, bedding after new discs
3. GiroDisc 380x34 Replacement Rotor Rings - rings sold in pairs with hardware, hats are the only reused part, GiroDisc hats only
4. GiroDisc 380x34mm Replacement Rings (D62) - reassembly and turning service for the cost of shipping, for the life of the rotors
5. GiroDisc 380x32mm Replacement Rotor Rings (D52) - US-poured iron machined in house, fit with Brembo or StopTech big brake kit hats
6. Brembo Floating Disc - elastic expansion of hat and braking surface, fixing bushes, up to 30% weight reduction
7. Brembo Parts: Minimum Brake Rotor Thickness - MIN TH marking, measuring in four points, effects of wear, replacing both sides
8. Brembo Parts: When to Replace Rotors - scoring, radial cracks, blue spots and deformation as replacement signs