THE BIG medical breakthrough of the '80s will be the development of mechanical parts for the human body. We've seen the success of the pacemaker, plastic joints and hips, steel replacements for legs and arms, and artificial organs which do the work that was formerly left to human ones.

Perhaps I'm being too optimistic, but I predict that by 1989, half our bodies will either come from DuPont, U.S. Steel or Reynolds Aluminum.

It follows, then, that when our bodies are not working well we shall all have to go to a garage, rather than a hospital, to have repairs done.

A man walks in to George's Human Body Repair Shop.

"Can I help you sir?"

"Yes, I'd like a tuneup and a grease job."

George takes out a repair slip and starts filling it in.

"Mind getting up on the rack, so I can examine you?"

The man stands on a rack and George jacks him up a few feet. "How are the knees?"

"They seem a little sluggish when I jog."

"I better change the bearings and linkage," George says, as he writes on his clipboard. "When did you have these leg muscles checked?"

"About a year ago. Why?"

"They're starting to fray. We have a new steel-belted radial muscle which is guaranteed for five years."

"Do I really need them?"

"Depends if you ever want to walk again."

The man nods his assent and George scibbles on the sheet. Then he says, "How are the arms?"

"I have tennis elbow in the right one."

"We'll have to put in a new shock absorber. Are your finger joints okay?"

"They seem to be. I can still make a fist."

George examines the hands. "I'll have to put new joints in your fingers. Your nails seem to be rusting."

"Look, I just came in for a tuneup. My body's 50 years old, and I don't want to put a lot of money into it."

"You notice anything else bothering you?"

"Well, my back hurts when I do situps. I hear this click, click, click every time I touch my toes."

George runs his hand along the man's back. "I was afraid of that," he says. "Your spine is all out of whack. We're going to have to replace it. We have a sale on back shafts that will last a lifetime, or we replace it for you absolutely free. How's the head?"

"Fine. I have no trouble keeping it together."

"You're losing a lot of hair. We better give you a transplant. We have a Lincoln Boy special on new all-weather Orlon that is so strong you never have to wear a hat."

George kept writing on the pad. Then he said, "I don't think you're getting enough air into your lungs. I'll put in new valves. You're American aren't you?"

"Of course I am. Why do you ask?"

"Because if you were a foreigner, we'd have to order the parts from overseas, and it would take two months."

"Are you finished?" the man said.

"That should do it," George said.

"When can I have the work done?"

George looked at his schedule. "If you come on Thursday morning at 8, I should have you out by 5 p.m. unless my mechanics find something I've overlooked."

"If you do everything you want to, can you guarantee that my body will be good for at least two more years?" the man asks.

"How can I do that?" George replies. "I'm not a doctor."