Typewriter Olympics: trying to beat 166 words/min.

By definition, business equipment expositions are probably meant to be prosaic affairs. After all, how many computer terminals and phone systems can you look at in three days? Even the attendants at ''Starve a Thief with the Anchor Pad'' booth look bored by the time Wednesday afternoon rolls around.

But up the aisles laced with blase business types, a little hive of secretaries has formed around two IMB Selectric typewriters. Overhead hangs a black and gold felt sign reading MEET THE CHAMP. Two middle-aged women are pounding away on the IBMs as if their lives depended on it. The observing secretaries look nervous, edgy. Craning their necks to get better views of the competing typers, they throw suspicious glances at one another.

The event? The annual Boston ''type-off'' with a purse of $600. The contestants are worried.

''Do I have to beat 166 words?'' asks a woman in a pink ski jacket. ''Who can beat that?''

Precious few people, if the Guinness Book of Records is to be believed. One woman hit 170 words on a manual typewriter back in 1918. Another reached into the typer's never-never land with 216 words in 1946. But that was on an electric typewriter and for only a minute - not a sustained pace. Most people type a pedestrian, if respectable, 50 to 60 words a minute.

Who is the currently reigning Horowitz of the typewriter? According to Western Temporary Services Inc., the sponsor of this successful public relations ploy, it's Betty Baird of Charleston, W.Va. She types a sizzling 166 words a minute on an electric. Or at least she did at one time.

''May of 1980 is when I hit my top speed,'' says Betty, who insists she's really just a homemaker and mother of two. ''I had just come from working two months with the legislature and I knew my time was faster. I certainly don't hit that speed all the time.''

What does she normally clock? ''Oh, only about 140,'' she adds modestly in her soft Southern twang. ''But that's under ideal office conditions, not like this,'' she says gesturing at the gawking contestants. As if on cue, a tall woman belonging to the sponsoring company politely shoos the waiting typers away from the entry table. ''Excuse us, we're in the middle of a test, please.''

Western Temporary Services has sponsored 20 such typing contests over the years. The purpose: ''to let women know it's a benefit to be a good typist.'' Maybe, maybe not. At this point, the unofficial winner of this year's Boston type-off is a man - John Orenberg of nearby Cambridge, Mass., who sits on a tenuous lead of 117 words per minute. (Since no one is expected to beat Mrs. Baird's time of 166, the winner is simply the one with the fastest time.) Like Mrs. Baird, Mr. Orenberg plays the piano, a skill they both say adds to ''finger dexterity.''

A typing tip from the oracle, or at least pinnacle, of keyboard speed: ''Concentration.'' Mrs. Baird insists this is key. Without it, too many errors occur, a flaw which brings down even the most speedy secretary. Beyond that, ''typing just comes easy, comes natural,'' she says with a shy grin.

Waiting for their turns at the keyboard, several women crowd close to Betty, seeking advice on this profession that requires one to be efficient, accurate, and unobtrusive. Says one, ''How do you always know how long your letters are gonna come out - I like mine to be exact.'' Betty, who is sipping a cup of hot chocolate, agrees. ''I know, it's tough.'' Another woman whispers: ''You can always type a rough draft.''

Game or not, John Orenberg has just walked away from the winner's circle with his check for $600. The annual type-off is over, despite a snafu with the typing warmup, and a sometimes lengthy waiting line, up to a half hour at times, too long for many of the 90 women here over lunch hour.

Despite Mr. Orenberg's victory, it is clear that Mrs. Baird is still the reigning speed queen. Urged by several spectators, she seats herself at the keyboard. Cautious at first, she gradually picks up speed. She seems fast, although no one is seeing double images of her flying fingers. After about a minute and a half, she suddenly stops and grins at the rapt observers. ''That's it,'' she says of the fastest typing in the land. ''That's all it is.''