Koyo HH060643N N-Flo Radiator for 1989-1992 Mazda RX-7 FC3S Series 5 13B MT

Vehicle Fitment

• 1989-1992 Mazda RX-7 FC3S Series 5 - Naturally Aspirated (13B-DEI 1.3L Twin-Rotor Wankel, RWD, MT)
• 1989-1992 Mazda RX-7 FC3S Series 5 - Turbo II (13B-DEI Turbo 1.3L Twin-Rotor Wankel, RWD, MT)

Fitment Notes

• Fitment is specific to the post-facelift FC3S Series 5 chassis (1989-1992) ONLY. This radiator does NOT fit the pre-facelift Series 4 FC3S (1986-1988) - the Series 4 side mounting brackets are a different design and are not compatible.
• OEM Series 5 side brackets must be used. Series 4 side brackets will not work with this radiator.
• The N-Flo dual-pass crossflow configuration is NOT compatible with the OEM intake snorkel - an aftermarket intake or modified intake routing is required.
• The OEM fan shroud requires minor trimming for clearance due to the dual-pass tank design. No other chassis modifications are required.
• The OEM factory radiator cap will NOT fit this radiator. A deep plunger-style cap is required - Koyorad SK-C13 or equivalent (sold separately).
• Retains the OEM coolant level sensor provision on the top tank.
• Manual Transmission (MT) primary application. Automatic transmission vehicles require a separate external transmission cooler.

Features

• N-Flo dual-pass technology uses internally welded aluminum tank partitions to route coolant through the core twice before returning to the engine, delivering 30-40% greater cooling capacity over stock and approximately 10% more than a standard single-pass R-Core radiator.
• 48mm HH Series hyper core with ultra-high fin and tube density - over 40% thicker than the OEM radiator core - providing dramatically increased coolant volume and heat rejection capacity.
• Bolt-in fit with OE-specific mounting locations - retains all factory mounting provisions, coolant hose positions, and the OEM coolant level sensor provision. No drilling, cutting, or chassis modification required.
• Nocolok-R furnace brazing bonds all tubes, fins, headers, and tank brackets for long-term structural integrity under high-temperature cycling, vibration, and track use.
• Hand-crafted Heliarc (TIG) welds on every radiator create the strongest possible aluminum-to-aluminum bond at all tank and core junctions.
• Triple pressure tested during manufacturing to verify structural integrity and confirm zero leaks before leaving the factory.
• Precision-machined billet aluminum filler neck provides superior sealing and long-term durability over the OEM plastic filler neck.
• Integrated 1/8" NPT female accessory fitting (supplied with threaded plug) accepts coolant temperature sensors from most aftermarket temperature gauge brands.
• Manufactured in Koyorad's privately owned, ISO 9001-certified Japanese facilities with in-house oversight at every stage of production.

Specifications

• Part Number: HH060643N
• Core Thickness: 48mm (HH Series Hyper Core)
• Core Depth: 1.889" (approximately 48mm)
• Core Type: N-Flo Dual-Pass Crossflow
• Material: Full aluminum construction - tanks, core, filler neck
• Filler Neck: Billet aluminum, precision-machined
• Accessory Port: 1/8" NPT female fitting (threaded plug included)
• Radiator Cap Interface: Deep plunger-style required (OEM cap will not fit)
• Compatible Radiator Cap: Koyorad SK-C13 or equivalent (sold separately)
• Brazing Method: Nocolok-R furnace brazed
• Welding Method: Hand TIG (Heliarc) welded
• Pressure Testing: Triple pressure tested
• Transmission Compatibility: Manual Transmission (MT) primary; AT requires external cooler
• Drivetrain: RWD
• Platform: Mazda FC3S Series 5 (1989-1992)
• Engine Compatibility: 13B-DEI NA and 13B-DEI Turbo II 1.3L Twin-Rotor Wankel

Included Components

• 1x Koyo HH060643N Full Aluminum N-Flo Dual-Pass Radiator
• 1x 1/8" NPT Threaded Plug (for accessory port)

Product Description

The Koyo HH060643N is a 48mm full-aluminum HH Series N-Flo dual-pass racing radiator engineered specifically for the 1989-1992 Mazda RX-7 FC3S Series 5 - the post-facelift second-generation RX-7 equipped with the 13B twin-rotor Wankel engine in either naturally aspirated or Turbo II form. This radiator is a Series 5-specific application and will not fit the earlier Series 4 FC3S (1986-1988), which uses different side mounting brackets. The HH060643N bolts directly into the Series 5 factory radiator location using the OEM Series 5 side brackets, retaining the stock coolant hose positions and the factory coolant level sensor provision on the top tank - making it a straightforward upgrade for any FC3S owner looking to eliminate the OEM plastic-end-tank radiator's chronic cooling limitations, particularly on boosted or track-driven builds.

The defining performance feature of the HH060643N is Koyorad's N-Flo dual-pass technology. Unlike a conventional single-pass radiator, the N-Flo design uses internally welded aluminum partitions in the upper and lower tanks to route coolant through the core twice before exiting back to the engine. This extended coolant dwell time translates to 30-40% greater heat rejection over the OEM radiator and approximately 10% more than a standard single-pass aluminum R-Core unit - a meaningful margin for a rotary engine that is inherently heat-sensitive, particularly when pushed under track or spirited street conditions. The 48mm hyper core, with its increased tube and fin density, is over 40% thicker than the factory radiator, nearly doubling coolant capacity within the same physical mounting envelope.

Two installation notes must be addressed before fitment. First, the N-Flo crossflow dual-pass tank design occupies slightly more tank volume than a traditional single-pass layout, which means the OEM factory intake snorkel is not compatible - an aftermarket intake or modified air routing is required. Second, the OEM fan shroud requires minor trimming along its edges for clearance around the wider tank; no other chassis modifications are needed. The factory radiator cap is also incompatible with the billet aluminum filler neck's deep plunger interface - a Koyorad SK-C13 cap or equivalent deep plunger-style cap must be sourced separately. Every HH060643N is Nocolok-R furnace brazed, hand Heliarc TIG welded, and triple pressure tested at Koyorad's ISO 9001-certified facilities in Japan before leaving the factory, ensuring a radiator built to withstand the sustained high-RPM thermal demands of rotary-powered FC3S ownership.