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

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Related, superseded, cross reference or alternative numbers for comparison.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

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