Genuine Mazda Six-Speed Gearbox Mainshaft Gear Locking Nut • P501-17-629B

Genuine Mazda Six-Speed Gearbox Mainshaft Gear Locking Nut

Replacement Genuine Mazda internal transmission locking nut for Mazda MX-5 Mk3, Mk3.5 and Mk3.75 NC 2.0 Sport models fitted with the factory six-speed manual gearbox.

This locking nut is installed inside the six-speed manual transmission on the main gear shaft. It retains the internal gear and shaft assembly in its specified position during gearbox operation.

Correct retention of the mainshaft gear assembly is essential for maintaining the intended shaft preload, gear position and engagement relationship. A damaged or previously disturbed locking nut should not be reused where replacement is required by the Mazda overhaul procedure.

One mainshaft locking nut is fitted per applicable gearbox, and this listing supplies one Genuine Mazda component.

Transmission restriction: This component is for the Mazda six-speed manual transmission. It is not listed for five-speed manual or automatic MX-5 NC gearboxes.

Related component: P501-17-629B is the mainshaft locking nut. The separate countershaft / clutch-hub locking nut is Mazda reference P601-17-628.

• Part type: Internal manual-transmission locking nut
• Common names: Mainshaft lock nut / main gear shaft nut / gear locking nut
• Mazda catalogue description: Nut, Lock-Gear
• Installation area: Inside the six-speed manual transmission
• Installation position: Main gear shaft / mainshaft assembly
• Function: Retains the internal gear assembly on the transmission mainshaft
• Transmission application: Six-speed manual
• Quantity fitted per gearbox: 1
• Quantity supplied: 1 locking nut
• Mainshaft included: No
• Gears or bearings included: No
• Synchroniser components included: No
• Countershaft locking nut included: No
• Brand: Genuine Mazda
• Part No.: P501-17-629B
• Previous Mazda reference: P501-17-629A
• MX-5 fitment: Mk3, Mk3.5 and Mk3.75 NC 2.0 Sport six-speed manual models

Typical reasons for replacement:

• Mainshaft dismantled during gearbox rebuilding
• Third/fourth selector-fork or clutch-hub repair
• Internal gears, synchronisers or bearings being replaced
• Original locking nut removed during shaft disassembly
• Locking or staking feature disturbed during removal
• Nut drive faces damaged or rounded
• Threads worn, damaged or cross-threaded
• Incorrect non-standard fastener previously installed

Quantity advice: One nut is required per applicable six-speed gearbox. This listing supplies one locking nut.

Fitment advice: Confirm that the vehicle is equipped with the factory six-speed manual transmission. The five-speed manual and automatic transmissions use different internal shaft assemblies.

Replacement advice: Renew the locking nut whenever removal, staking or the applicable Mazda gearbox-overhaul procedure requires replacement. A previously disturbed locking feature may not provide reliable retention when reused.

Mainshaft inspection: Inspect the mainshaft threads, splines, bearing surfaces, gear-location shoulders and adjoining components before reassembly. A replacement nut will not correct damaged threads or wear elsewhere in the shaft assembly.

Gearbox inspection: Where the gearbox has been dismantled because of difficult selection, gear disengagement or selector damage, inspect the clutch hubs, synchroniser rings, selector forks, bearings and gear engagement teeth.

Tightening advice: Install, tighten and secure the nut using the specified Mazda procedure for the applicable six-speed transmission. Correct shaft support, tightening specification and locking method are essential.

Cleanliness advice: Internal gearbox components must be assembled in a clean working environment. Prevent dirt, abrasive material and lint from entering the bearings, synchronisers or lubricant passages.

Other items to inspect: Check the mainshaft bearings, gears, synchroniser components, selector forks, spacers, retaining rings, oil seals and the corresponding countershaft locking nut while the gearbox is dismantled.

Useful tools / consumables: Transmission stand or suitable workbench, gearbox pullers and press equipment, correct sockets, calibrated torque wrench, staking tools where required, circlip pliers, assembly lubricant and the specified manual-transmission oil.

Difficulty guide: DIY Difficulty: Advanced / Specialist. Installation requires transmission removal, complete or partial gearbox dismantling and accurate shaft reassembly using the Mazda overhaul procedure.

Why Genuine Mazda? Genuine Mazda internal transmission hardware matches the original thread, dimensions and locking arrangement used on the six-speed mainshaft. Correct retention hardware is essential for maintaining the intended gear and shaft assembly.

What’s in the box: One Genuine Mazda mainshaft gear locking nut, part number P501-17-629B.

Installation notes: Remove and dismantle the transmission according to the applicable Mazda workshop procedure. Support the shaft assembly correctly before releasing the original locking nut. Inspect and clean the mainshaft threads and adjoining components, then install the replacement nut in the specified orientation. Tighten and secure it using the prescribed method before completing shaft and gearbox reassembly. Refill the gearbox with the specified lubricant and confirm normal operation before road use.