Genuine Mazda Rear Differential Oil Filler Plug – 18mm Hex • 9951-11-800

Genuine Mazda 9951-11-800 Rear Differential Oil Filler Plug

Replacement Genuine Mazda differential oil filler plug for all Mazda MX-5 generations, including Mk1 NA, Mk2 and Mk2.5 NB, Mk3 NC and Mk4 ND/RF models.

The plug is installed in the differential housing’s filler and oil-level inspection opening. It provides access when filling the differential with lubricant and when confirming that the oil is at the specified level.

The plug has an 18mm external hex head. It may require replacement if the original head has become rounded, the threads are damaged or corroded, or the sealing face has been marked during previous servicing.

A damaged or incorrectly installed filler plug can become difficult to remove and may allow differential oil to leak from the housing. There is typically one filler plug fitted per differential assembly.

• Part type: Rear differential oil filler / level plug
• Primary use: Closes the differential oil filling and level-inspection opening
• Location: Rear differential housing
• Hex head size: 18mm
• Quantity per differential: 1
• Quantity supplied: 1 filler plug
• Sealing washer included: No
• Brand: Genuine Mazda
• Part No.: 9951-11-800
• Also referenced as: 995111800
• Fitment note: Confirmed for all Mazda MX-5 generations

Important: This listing is for the filler plug only. The sealing washer is not included and should normally be replaced whenever the plug is removed.

Fitment advice: Although Mazda uses this plug in several differential, axle and transfer assemblies, its confirmed position on Mazda MX-5 models is the rear differential filler and oil-level opening. Where a replacement or non-standard differential has been fitted, compare the original plug and thread before ordering.

Other items to consider: Consider purchasing a new sealing washer and the correct specification and quantity of differential oil. Inspect the differential drain plug, casing threads, breather and input and output oil seals for damage or leakage during servicing.

Useful tools / consumables: 18mm socket or ring spanner, ratchet or breaker bar, torque wrench, drain container, fluid pump, cleaning brush, lint-free cloths and manufacturer-specified differential oil.

Difficulty guide: DIY Difficulty: Easy to Moderate. The vehicle must be raised and supported safely while remaining level for an accurate oil-level check. Always confirm that the filler plug can be removed before opening the differential drain plug.

Why Genuine Mazda? Genuine Mazda differential plugs are manufactured to match the original thread, sealing face, head dimensions and housing arrangement. Correct fit helps protect the differential casing threads and maintain a reliable oil seal when used with the appropriate washer.

What’s in the box: One Genuine Mazda rear differential oil filler plug, part number 9951-11-800. Sealing washer not included.

Installation notes: Clean the plug opening and surrounding differential housing before removal. Inspect the casing threads and fit a new sealing washer to the replacement plug. Refill the differential with the Mazda-specified lubricant until the correct level is reached, then tighten the plug to the torque specified for the applicable differential. Clean away any spilled oil and inspect for leakage after installation.