Engine Head Studs Cap Nut Porsche 911 1970-89  ❭  93010117201

Dilavar Head Studs.
Normally 24 studs per engine. Sold EACH



Fits:
Porsche 911 1965-89
Porsche 964 1989-94
Porsche 993 1994-98

This product supersedes Porsche's old part numbers:
- 99310117051

Solves the problems of loose head studs, The exhaust leak you hear in the 911 may actually be the cylinder head leaking where the studs have pulled loose. As Porsche increased displacement of the 911 engine, the different rates of thermal expansion have caused head studs to break or pull out from the engine case. These latest Dilavar steel head studs have a thermal expansion rate of approximately the same as the alloy case.

Click 'Zoom in' for large parts diagram.
Diagram Ref No 2.

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Related, superseded, cross reference or alternative numbers for comparison.

99310117054

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Crankcase Through Bolt O-Ring 
Sold each - 24 required per engine

The Porsche Crankcase Through Bolt O-Ring is a small but critically important sealing component used in Porsche 911 engines from 1965 to 1989. Fitted around the crankcase through bolts that secure the engine case halves together, this O-ring helps prevent oil leakage from one of the most common sealing points found on classic air-cooled Porsche engines.

Fits:

• Porsche 911 1965-1968 2.0L / 912 SWB (F)
• Porsche 911 1968-1973 2.2L / 2.4L / 2.7L RS LWB (F)
• Porsche 911 1974-1977 2.7L / 1976-77 3.0 Carrera
• Porsche 911 1978-1983 3.0L / SC
• Porsche 911 1984-1986 3.2L
• Porsche 911 1987-1989 3.2L G50
• Porsche 911 1975-1977 3.0L Turbo (930)
• Porsche 911 1978-1989 3.3L Turbo (930)

Diagram ref no 22

Although inexpensive, these O-rings play a vital role in maintaining the integrity of the engine lubrication system. When properly installed, they create a reliable seal around the through bolts, preventing pressurised engine oil from escaping the crankcase and helping to keep the engine clean and leak-free.

Whether you're performing a complete engine rebuild, repairing oil leaks, or restoring a classic Porsche 911, replacing the crankcase through bolt O-rings is considered essential preventative maintenance.

What Does the Crankcase Through Bolt O-Ring Do?

The through bolt O-ring seals the openings where the crankcase through bolts pass through the engine case.

Its primary functions include:

• Preventing oil leaks from the crankcase
• Maintaining crankcase sealing integrity
• Supporting proper oil system operation
• Helping preserve engine cleanliness
• Reducing oil loss and contamination
• Protecting engine components from oil seepage

Because Porsche air-cooled engines operate with a high-capacity dry-sump lubrication system, maintaining effective sealing throughout the crankcase is crucial for long-term reliability.

Why Does the Through Bolt O-Ring Fail?

Over time, the O-ring is exposed to extreme engine heat, oil, pressure fluctuations, and age-related deterioration.

Common causes of failure include:

• Rubber hardening due to heat cycles
• Age-related shrinkage
• Loss of elasticity
• Oil exposure over decades of use
• Improper installation
• Damage during assembly
• Long-term storage and inactivity

As the O-rings age, they gradually lose their sealing ability and become one of the most common sources of oil leaks on classic Porsche 911 engines.

Common Symptoms of a Failing Through Bolt O-Ring

Signs that the O-rings may require replacement include:

• Oil leaks around the crankcase
• Oil residue underneath the engine
• Persistent oil drips despite replacing other seals
• Oil staining around through bolt locations
• Excessive oil mist accumulation on the engine case
• Leaks becoming worse as the engine reaches operating temperature

These leaks are often mistaken for rocker cover, timing chain housing, or crankshaft seal leaks due to the way oil travels across the engine casing.

When Should You Replace It?

Replacement is strongly recommended whenever the engine is disassembled.

The through bolt O-rings should always be replaced:

• During complete engine rebuilds
• During crankcase splitting procedures
• When repairing major oil leaks
• During full engine restorations
• On high-mileage engines undergoing overhaul
• Whenever the crankcase through bolts are removed
• As preventative maintenance during engine refurbishment

Because access requires significant engine disassembly, replacing these O-rings during rebuild work is considered standard Porsche engine-building practice.

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

99970100640

The product you are viewing cross references to these numbers

Crankcase through bolt washer
24 per engine.



Fits:
Porsche 911 1965-89

Click picture for large parts diagram.
Diagram Ref No 23.

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

90110116103

The product you are viewing cross references to these numbers