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Porsche > Cayenne 92A (958) 2010-2017 > 958 Cayenne V6 3.0L Diesel 245HP 2010-17 > Engine Rebuild Parts > Rocker Arms & Tappets >

Hexagon nut M8x1 for valve control adjustment screw, left. Porsche 911 / 964 / 914 ❭ 99903400500

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OE Match

99903400500/1

Product Information

Hexagon nut M8x1 for valve control adjustment screw, left.
This locking nut is critical in maintaining the correct clearance setting between the rocker-arm or cam follower and valve stem, ensuring reliable valvetrain performance and correct engine behaviour.
Sold each.

Fits:
Porsche 911 1973 - 1989
Porsche 911 Turbo 1975 - 1989
Porsche 964 C2 / C4 / RS 1989 - 1994
Porsche 964 Turbo 1991 - 1994
Porsche 914 1970 - 1976
Porsche 914/6 1970 - 1972

Diagram ref no 50

• Position on Vehicle: Engine cylinder head, at the valve adjusting screw area - used to lock the adjuster once clearance is set.

•  Function: Locks the valve-adjusting screw in place, ensures the correct clearance remains unchanged under engine operation, prevents loosening of adjuster and ensures valvetrain reliability.

•  Specification: M8 × 1 thread, correct height and socket size, treated or coated for durability - direct-fit replacement of Porsche part number 99903400500.

•  Applications: Ideal when your original nut is damaged, the adjuster is loose, you are doing a valve-clearance service, or you require reliable hardware for engine rebuilds.

While small, this locking nut performs a vital role: without it, the adjuster can drift, valve clearance can open up or close down, leading to noisy operation, performance loss, premature wear or even engine damage. Symptoms of a failing/incorrect nut include adjusting screw backing out, noisy valvetrain, incorrect clearance and poor idle or performance.

When to replace:
• The adjuster locking nut is missing, corroded, rounded or damaged.
• After valve-clearance adjustment and you want guaranteed lock-in of the setting.
• During engine rebuild or cylinder-head service when valvetrain hardware is exposed.
• To replace the original with fresh hardware to maintain service integrity and reliability.

Related reference numbers
Related, superseded, cross reference or alternative numbers for comparison.

99903400500

99903400500

The product you are viewing cross references to these numbers

Related products

Hydraulic Tappet / Valve Lifter for Porsche 957 / 958 Cayenne / 95B Macan / 970 / 971 Panamera Diesel

958109521

Product Information

Diesel Engine Hydraulic Tappet / Valve Lifter

This engine hydraulic tappet (also known as a hydraulic valve lifter) is a precision component used in Porsche diesel engines to maintain correct valve clearance automatically. By using engine oil pressure, the tappet ensures smooth valve operation, quiet running and consistent performance across all driving conditions.

Fits:

Porsche 957 Cayenne 3.0L Diesel 2007-10
Porsche 958 Cayenne V6 3.0L Diesel 245HP 2010-17
Porsche 958 Cayenne S V8 4.2L Diesel 382HP 2010-17
Porsche 970.1 Panamera Diesel V6 3.0L 2011-13
Porsche 970.2 Panamera Diesel V6 3.0L (250Hp) 2014-16
Porsche 971.1 Panamera 4S Diesel V8 4.0L 4WD (422 Hp) 2017-18
Porsche 971.1 Panamera 4S Diesel 4.0L Twin-Turbo V8 Sport Turismo 2017-18
Porsche 95B.1 Macan S Diesel 3.0L V6 258Bhp 2014-18

Diagram ref no 11

In modern Porsche diesel engines, hydraulic tappets play a crucial role in durability, refinement and emissions control—making them an essential service and repair component.

What Does an Engine Hydraulic Tappet Do?

Hydraulic tappets sit between the camshaft and valves, using oil pressure to compensate for thermal expansion and component wear. This allows the valve train to operate with:

Correct valve timing and lift
Reduced mechanical noise
Improved combustion efficiency
Lower internal engine wear

By automatically adjusting clearance, hydraulic tappets eliminate the need for manual valve adjustment and help maintain optimal engine performance throughout the rev range.

Why Do Hydraulic Tappets Fail on Porsche Diesel Engines?

Over time, hydraulic tappets are subject to heat, oil contamination and mechanical stress, particularly in high-torque diesel engines.
Common causes of failure include:

Oil sludge or poor lubrication
Wear to internal check valves
Loss of hydraulic pressure retention
Extended oil change intervals
Contaminated or incorrect oil grades

Symptoms of worn or failing tappets may include:

Ticking or tapping noises from the cylinder head Rough idle or misfiring Reduced engine efficiency Increased valve train wear

Why Replace Worn Hydraulic Tappets?

Replacing worn hydraulic tappets restores:

Quiet and smooth engine operation
Correct valve timing and combustion
Reduced wear on camshafts and valves
Long-term engine reliability

Ignoring tappet wear can lead to accelerated damage to the camshaft and valve train, resulting in significantly higher repair costs.

Related reference numbers
Related, superseded, cross reference or alternative numbers for comparison.

958109521

958109521

95810542310

95510542310

The product you are viewing cross references to these numbers

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Hexagon nut M8x1 for valve control adjustment screw, left. Porsche 911 / 964 / 914 ❭ 99903400500