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Porsche > 911 & 912 (1965-1989) > 911 1984-1986 3.2L > Engine Rebuild Parts > Engine Bolts / Studs >

Crankcase through bolt o-ring. Porsche 911 1965-89  ❭  99970100640

Crankcase through bolt o-ring. Porsche 911 1965-89 - 19805, 99970100640, 999.701.006.40, 917.079
2
Related to Engine Bolts / Studs for 911 1984-1986 3.2L
99970100640/1
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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
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Dilavar head studs. Porsche 911 / 964 / 993 1965-98 - 99310117051, 99310117053, 99310117054
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Dilavar head studs. Porsche 911 / 964 / 993 1965-98
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Dilavar Head Studs.
Normally 24 studs per engine. Sold EACH

porsche

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

This product supersedes Porsche's old part numbers:
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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.

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Upper Head / Barrel Stud – Sold Each
12 required except 2.7 Carrera RS where 24 are required (upper and lower)

The Porsche Upper Head / Barrel Stud is a critical engine fastening component used in Porsche 911 models produced from 1973 to 1989. This stud forms part of the cylinder head retention system that secures the engine cylinders (barrels) and cylinder heads firmly to the crankcase, maintaining the precise clamping force required for reliable engine operation.

Fits:

  • Porsche 911 1973 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 2

Porsche's air-cooled flat-six engines rely on these studs to withstand enormous combustion pressures and continuous thermal expansion cycles. Without adequate clamping force, cylinder sealing can be compromised, leading to compression loss, oil leaks, exhaust gas leakage, and costly engine damage.

For most Porsche 911 models within this period, 12 upper head studs are required. The notable exception is the legendary Porsche 2.7 Carrera RS, which requires a complete set of 24 studs.

Whether you're carrying out a top-end rebuild, complete engine overhaul, or concours restoration, replacing ageing head studs is an important step in maintaining long-term engine reliability.

What Does the Upper Head / Barrel Stud Do?

The upper head stud provides the clamping force that secures the cylinder head and cylinder barrel assembly to the engine crankcase.

Its primary functions include:

  • Securing the cylinder heads to the engine case
  • Maintaining correct cylinder sealing pressure
  • Preserving combustion chamber integrity
  • Preventing movement between cylinders and heads
  • Supporting stable engine compression
  • Maintaining long-term engine reliability

The stud is constantly under tension and plays a crucial role in preserving the structural integrity of the entire engine assembly.

Why Does the Head Stud Fail?

Head studs operate under extreme conditions, being subjected to continuous heat cycles, combustion forces, vibration, and material expansion and contraction.

Common causes of failure include:

  • Metal fatigue
  • Corrosion and oxidation
  • Age-related material degradation
  • Thermal cycling stress
  • Engine overheating
  • Improper torque procedures during previous rebuilds
  • High-mileage wear Long-term mechanical loading

Even on well-maintained engines, decades of operation can weaken original studs and reduce their ability to maintain the required clamping force.

Common Symptoms of a Failing Head Stud

A damaged or broken head stud can result in:

  • Compression leaks
  • Exhaust gas leakage between the cylinder and head
  • Ticking noises from the cylinder heads
  • Oil seepage around the cylinder base
  • Reduced engine performance
  • Uneven cylinder sealing
  • Failed leak-down test results
  • Visible broken studs during valve cover inspection

In some cases, a broken head stud may go unnoticed until routine valve adjustment or engine rebuild work is carried out.

When Should You Replace Head Studs?

Head studs should always be inspected during major engine work and replaced whenever wear, corrosion, or damage is identified.

Replacement is highly recommended:

  • During top-end rebuilds
  • During complete engine overhauls
  • When one or more studs have broken
  • During cylinder head refurbishment
  • On high-mileage engines
  • During preventative restoration projects
  • When upgrading engine performance
  • Whenever the cylinder heads are removed

Because replacing head studs requires significant engine disassembly, many Porsche specialists recommend replacement while access is available.



Related reference numbers
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Crankcase through bolt washer Porsche 911 1965-89
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Crankcase through bolt washer
24 per engine.

porsche

Fits:
Porsche 911 1965-89

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

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