Older style headsets attached to threaded steerers need to be serviced once a year. This is quite a big job that involves quite a number of steps, but will ensure that it lasts for a very long time. This is in contrast to new style cartridge bearings that can be left alone for quite a long time, but have to be thrown out once they are worn.
This is a long guide, so you can choose to either watch the entire video above, or follow it in smaller chunks by clicking on the individual steps below.
Steps to Overhaul The Headset Attached To A Threaded Steerer:
The bike headset is the hinge that allows the handlebars and front wheel to turn relative to the back wheel. There are two basic types. The new version is simple and light but - once set-up - doesn't allow much adjustment of the handlebar height. The old version is more complicated and heavier and requires special tools to adjust but usually allows more adustment of the handlebar height.
A bike headset on a threaded steerer needs to be serviced once a year. This involves taking the assembly apart, cleaning, inspecting, relubricating and reassembling. If your bike has sealed cartridge bearings you don't need to service it until it stops working correctly but you may need to take it apart once to find out what kind of bearings it has. If the bike is used in dirty conditions, gets washed a lot or has no front-mudguard underneath the fork it may need to be serviced and regreased more often.
It's simple to adjust the height of the handlebars with an old-style threaded steerer-tube as the handlebar stem telescopes into the steerer tube and expands to lock there. In this system the handlebar stem is a completely separate system from the headset bearing that allows the front wheel to 'steer'.
The headset on a bike is the most important bearing on the bike. It allows you to balance the forces while you're riding so you don't fall over. If it's too loose or too tight the bike may be uncomfortable to ride and the headset will wear out faster than if it's correctly adjusted