How to Rebuild a 911 Cylinder Head. Lesson 3, Valve Springs

Valve Spring in the head

Air-Cooled 911 Valve Spring

Valve springs control the valvetrain. It is valve spring pressure that forces the valves to stay in contact with the rocker arm. Without the proper spring pressure, the valvetrain becomes uncontrollable. Not enough pressure and you can get valve float when the valvetrain gear loses contact with the camshaft lobe. Too much pressure and you can get premature valvetrain wear. Causing excess wear on the rocker arm, camshaft, and the valve face and seat.

Lesson Three, Valve Springs

Testing a Valve Spring

The 911 valve spring is a duel spring. There is an innerspring and outer spring. They are also directional on the outer spring, meaning there is a top and a bottom to the spring. This is a performance set up because dual springs allow for high load pressures. Dual springs also allow for frequency dampening, which means the engine can rev faster with fewer concerns about bounce.

An inner and outer spring make up a 911 valve spring assembly
Inner and Outer Valve Spring

You can see that the best spring pressure is the spring pressure the factory designed. Over time the material in the spring loses its temper. Heat is the major cause of temper loss. The 911 cylinder head runs hot by nature and the spring life is finite. This means that every spring should be tested. And unless you know the complete history of the head, things could be out of specification. What’s been done to the head? How many miles are on the spring set? Most of the time it it best just to replace the valve springs.

Assemble the spring set to measure
Set up the spring assembly with seat and retainer to measure.

If you want to know the spring pressure of your spring sets you need to test the springs. When you test the valve spring assembly, you must set them up correctly. Assemble the inner and outer spring, the valve spring seat, and the valve spring retainer is also in place.

Using a sping tester on the valve springs
The Spring Tester

The assembly should be tested at Seat Pressure, which is the valve closed height. And also tested at Nose Pressure, which is the valve completely open. The springs used in the intake and exhaust valves are all the same part number. The intake and exhaust springs are just set at different pressures. Springs should be within +/- 5% of the specification.

Valve Spring Pressures

Intake Seat Pressure              44 PSI @ 42 mm +/- 5%

Intake Full Load Pressure      177.6 PSI @ 30.5 mm +/- 5%

Exhaust Seat pressure          44 PSI @ 42 mm +/- 5%

Exhaust Full load pressure  165.3 PSI @ 31.5 mm +/- 5%

Tools you may need

The spring compression tester that we use is old, but it does the job. There are many choices out there. We test new springs as well as old. Testing springs makes you informed, if you don’t know your spring pressure, you don’t know.

A modern spring tester.
Any spring tester you purchase must have the ability to control the height. This spring tester can be wound into the correct height for set and full pressure.
DIGI-Science Accumatic Digital Caliper Measuring Tool | 6 Inch Stainless Steel Electronic Vernier Calipers Measures up to 0-6”/0-150mm | Large LCD Display Screen | 2 Batteries Included (50-10150)
LSM Racing Products SM-1000 Bench Top Valve Spring Tester
DIGI-Science Accumatic Digital Caliper Measuring Tool | 6 Inch Stainless Steel Electronic Vernier Calipers Measures up to 0-6”/0-150mm | Large LCD Display Screen | 2 Batteries Included (50-10150)
LSM Racing Products SM-1000 Bench Top Valve Spring Tester
$39.99
$439.51
DIGI-Science Accumatic Digital Caliper Measuring Tool | 6 Inch Stainless Steel Electronic Vernier Calipers Measures up to 0-6”/0-150mm | Large LCD Display Screen | 2 Batteries Included (50-10150)
DIGI-Science Accumatic Digital Caliper Measuring Tool | 6 Inch Stainless Steel Electronic Vernier Calipers Measures up to 0-6”/0-150mm | Large LCD Display Screen | 2 Batteries Included (50-10150)
$39.99
LSM Racing Products SM-1000 Bench Top Valve Spring Tester
LSM Racing Products SM-1000 Bench Top Valve Spring Tester
$439.51

How to rebuild a 911 Cylinder Head. Lesson 4, Valve Guides

In the next lesson, we cover the Valve Guides. How to inspect them, getting precise measurements, and how to replace them.

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