This instant timing knob is truly revolutionary! The knob allows you to change your timing without having to loosen and re-tighten the distributor hold-down clamp. It is great for quick pit stop tuning. Plus, your crew chief will be able to avoid those nasty hand burns that happen while rotating a distributor on a hot racing engine. How does it work? The instant timing knob protrudes from the side of the distributor. When you need to advance or retard the timing, simply turn the knob (1/2 revolution equals 1 degree of timing, a full revolution equals 2 degrees of timing). It's so accurate that no timing light is needed to verify it! THESE WILL ONLY WORK IN STOCK GM VACUUM ADVANCE DISTRIBUTORS NOT MSD DISTRIBUTORS!