Valve spring inserts (shims) are designed to __.
a. Increase installed height of the valve
b. Decrease installed height of the valve
c. Adjust the correct valve spring installed height
d. Decrease valve spring pressure to compensate for decreased installed height
The correct answer and explanation is :
Correct Answer: c. Adjust the correct valve spring installed height
Explanation:
Valve spring inserts, commonly known as shims, play a vital role in the valve train of an internal combustion engine. These thin washers or discs are placed under the valve spring to fine-tune the installed height of the spring — which is the distance from the valve spring seat to the bottom of the spring retainer when the valve is fully closed.
The installed height of a valve spring is critical because it directly affects the spring’s pressure. Valve springs must exert the proper amount of pressure to keep the valve securely closed during engine operation, especially at high speeds, while also allowing for proper valve movement. If the installed height is too great, the spring pressure will be too low, potentially causing valve float or poor sealing. If it’s too small, the spring pressure might be excessive, leading to premature wear of the camshaft and other valve train components.
This is where valve spring inserts (shims) come into play. By inserting shims under the valve spring, the installed height is reduced, thereby increasing spring pressure to bring it within the manufacturer’s specifications. On the contrary, if less spring pressure is needed (for example, after machining the valve seat, which may reduce the installed height), the shims can be removed or thinner shims can be used.
So while it’s technically true that shims decrease the installed height (which corresponds to answer b), the most complete and functionally correct answer is c — because the purpose of the shim is not merely to reduce the height, but to adjust it accurately to meet specific engine requirements. This ensures optimal valve operation, engine performance, and longevity of the valve train components.