This is a very small collar that fits around the valve stem to centralize the valve spring for situations where the spring needs to sit directly on the head to obtain the correct fitted length. Normally a lower collar is fitted in all dual spring installations to prevent the inner spring from moving around at high RPM, where it could come in contact with the outer spring and lead to catastrophic failure. This one is the Special Tuning alternative to the stock lower collar AEA0403. Sold each.
Some valve springs are designed to not need a lower collar such as our SEN0092 and SEN0093 spring sets.