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O-Ring Genuine Porsche part 99970108340

Code: 99970108340

Genuine Porsche part

O-Ring

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Additional information:

• This part is brand new and unused.
• Please verify that it is 100% suitable for your specific vehicle before purchase to ensure compatibility.

Before purchasing thoroughly check part to confirm compatibility.

Related products

Crankcase through bolt o-ring. Porsche 911 1965-89

99970100640/1

Product Information

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

19805

99970100640

999.701.006.40

917.079

The product you are viewing cross references to these numbers