Alcon 0.875" Pivot-Mount Master Cylinder: Gold Nut vs Red Nut

Short answer: The Gold Nut (MAR5240HM161MBB) uses a 3/8”-24 UNF outlet port, making it the right choice for North American race builds plumbed with standard AN or UNF brake line fittings. The Red Nut (MAR5240HM161MAB) uses an M10×1 metric outlet port, suiting European-spec builds or any car already fitted with metric banjo or bubble-flare lines. Both share the same 0.875” bore and pivot (spherical bearing) mount. Pick the wrong nut color and you are chasing adapters at the worst possible time.

Spec Comparison

Stroke figures are not directly comparable: 25.4 mm is the nominal stroke published for the Gold Nut UNF family across smaller bore sizes; 29.5 mm is the maximum stroke published for the Red Nut metric MAR52 family. Alcon has not published a single document reconciling both figures at the 0.875” bore. Confirm with Alcon or your distributor before building to either number.

Gold Nut (MAR5240HM161MBB): Who It’s For

If your car is built around AN-style plumbing, the Gold Nut is the straightforward choice. The 3/8”-24 UNF outlet thread is the standard that North American fabricators reach for: it works with common -3 AN adapters and the banjo fittings stocked by most domestic race suppliers. The 7/16”-20 UNF inlet thread follows the same logic on the reservoir side.

The 0.875” bore sits at the large end of Alcon’s pivot-mount family. That larger bore displaces more fluid per unit of pedal travel, which suits applications running large-piston calipers or a longer caliper circuit (think rear bias or a combined front circuit on a car with a high pedal ratio). The pivot mount with its spherical bearing keeps side-loading off the piston seals whether your pedal geometry is perfectly aligned or not, which is exactly what you want in a fabricated pedal box where perfect geometry is aspirational.

Honest trade-off: If your existing hard lines or hose ends are already M10×1 metric, the Gold Nut requires either adapters or refabrication of the outlet fitting. Adapters work, but they add potential leak points and length in a space that is usually tight behind a pedal box. Save yourself the hassle and confirm your existing fitting spec before ordering.

Red Nut (MAR5240HM161MAB): Who It’s For

The Red Nut is the metric-world counterpart. Its M10×1 outlet and inlet ports match the fittings found on most European-sourced components: Brembo, AP Racing, and OEM European brake lines all commonly use M10×1. If you are building around a European chassis, converting a road car that retains OEM-style metric hard lines, or sourcing hose ends from a European supplier, the Red Nut means everything threads together without adaptation.

The 0.25” spherical bearing in the pivot mount is confirmed in Alcon’s own published data for this family. That bearing allows the cylinder to rotate with the natural arc of the pedal and to accommodate offset from a balance bar, keeping the load path as close to axial as possible throughout the stroke. Alcon’s installation note for this variant is worth following precisely: install with the ports uppermost, and do not allow the fluid outlet port to drop below horizontal. That keeps air from becoming trapped against the outlet during bleeding.

Honest trade-off: If your fabricator is set up for AN fittings and has none of the M10×1 metric banjo hardware on the shelf, lead time for fittings can slow a build. It is not a reason to avoid this cylinder, but it is worth a conversation with your plumbing supplier before you order.

How to Choose

These two cylinders are functionally the same device in the only specifications that affect braking performance: bore diameter, mount style, and body construction. The nut color is not a performance differentiator. It is a plumbing compatibility signal, and it exists so you can identify the correct variant at a glance once you know the code.

Work through these questions in order:

1. What thread are your existing brake line fittings? If they are 3/8”-24 UNF or AN, order the Gold Nut. If they are M10×1, order the Red Nut. If you do not know, find out before ordering rather than assuming.
2. Are you starting from scratch? Choose the thread standard that matches your caliper fittings and the hose ends most readily available from your preferred supplier. North American builders generally default to UNF/AN; European builds generally default to metric.
3. Is adapter usage acceptable? Adapters can make either variant work with the other thread standard, but they add a potential leak point and length. In a production race car, clean plumbing with native threads is always preferable.

If you are still weighing how bore size affects front-to-rear brake bias in your specific setup, the Alconkits guide Understanding Brake Bias: Why Alcon Engineers Caliper Kits for Proper Balance covers the underlying principles worth reading before you finalize your pedal box spec.

Questions about which variant fits your build? Contact Alcon or reach out to the Alconkits team directly.