The need to check valve springs.
June 28, 2018I have a simple question for you: When was the last time you gave valve springs any thought? That quizzical look on your face is shared by most. No one thinks about the valve springs.
The valve spring’s primary task is to close the valve after the camshaft via the lifter, push rod and rocker arm opens it. The other job is to maintain proper clearance (lash) in the valve train.
If the spring pressure is weak the lash cannot be maintained.
When the spring pressure is low, the valve can bounce on the seat during closing. This can result in noise, a lack of power, and eventual engine damage.
At high rpm the spring is responsible for controlling float, a condition that allows the valve to travel beyond its designed lift opening, with the possibility of destroying the engine.
The spring also keeps at bay an effect identified as surge. This is an undesirable harmonic that can cause the spring to break, along with valve seat recession and rocker arm failure.
Installed height and pressure
A valve spring is designed to have a specific pressure at the height it is installed at (valve closed) and a higher pressure when the valve is fully open. The pressure is identified as open and closed.
Spring pressure is rated in pounds (lbs) of force on the coils and is not read in pounds per square inch (PSI) as a hydraulic system would be.
The installed height is the dimension from the top of the spring surface where the retainer sits, to the surface of the bottom coil where it rests on the spring pad of the cylinder head.
Every engine will have a specification for installed height, closed and open pressure.
As an engine gets old, the spring experiences tension relax. This term is commonly used for the change in pressure when a new spring breaks-in but can be attached to the condition of a worn and tired spring.
The correct installed height along with the spring design will result in the proper pressure, open and closed. Wear in the cylinder head, valve seat and valve stem can alter the installed height, which can then be adjusted with a specially designed shim between the spring and the spring seat. Shims come in three thicknesses: 0.060 inch (Type A), 0.030 inch (Type B), and 0.015 inch (Type C).
The uses are as follows:
Type A: To keep weakened springs in service
Type B: To set new springs properly
Type C: For balancing pressure
An engine machine shop can easily check valve spring open and closed pressures once the installed height is determined.
The Goodson Company (www.goodson.com) offers all of the tools to test valve springs. The lead photo shows a Rimac spring tester with the Goodson VST-25 installed height indicator attached.
If the valve springs were changed without proper measuring and the pressure is too great, then the lifter will wear the cam lobe excessively, the valve train will flex, and the rocker arm and pushrod will deflect.
Low operating rpm make a farm engine more forgiving than a car or truck power plant. Still, the installed height and pressures are critical for long engine life and as designed performance.
How are your engine’s valve springs?
