Reed K. Martin of the Cabin John section of Bethesda has a half-dozen 100-mile-per-hour race cars parked on his desk top. They are finely detailed relics of an era 40 years ago when these racers, powered by single spark plug ignition engines, sped around concrete circles tethered to a post.

There were tracks in Annapolis and on Chillium Road then, says Martin, 33. Speed killed the sport. The 10-pound cars were noisy. They went so fast they broke their tethers and flew into spectators.

Martin, a transplanted Hoosier, was once a racer himself. "Placed 11th among 400 in the Indianapolis Soap Box Derby," he recalls. Now a Montgomery County employe, he is an incurable swapper who "just hangs on to things."

Several months ago he heard of a widow with 65 model airplane engines. He bought them "because I knew they were worth buying. Now, I had something to trade but didn't know how to do it," Martin says.

He soon learned at flea markets, garage sales and swap meets. In time he got the cars, their engines, and their tiny rubber tires. "None of these things are made today," he says.

"Sure someone with money could buy everything. But that's not the fun of it."

Some of the fun is "trashing," a Martin specialty. "Since I have no money, I examine any discards I see. Fact is, the stuff is being thrown out faster than I get it. I think I'm the only one around here interested in these cars," said Martin.

That's why Martin wants to hear from anyone who has anything or knows anything about the little racers. But don't call him a "collector."

"I just get interested in things," Martin states. That explains the water rams, steam engines and farm puzzles in his house and the three Fords, none newer than 1931, and the 1901 Oldsmobile in his garage.

Just a tick separates the sportsman stock cars, fastest of the Saturday Night Specials racing at Old Dominion Speedway, Manassas, and the track's own late model sedan class. Morgan Shepherd won the pole for the three-eighth-mile pavement. Bob McElwee of Fairfax clocked 17.16 seconds, 78.7 miles per hour, in his late model. He then won his division's feature race, his first win in his comback since quitting the sport two years ago.

Noting the three racing weekends at the Watkins Glen, N.Y., circuit produce $23 million in tourist revenue, the state put together a $1.2 million package of loans and grants to finance improvements there. They are needed to keep the U.S. Grand Prix at the Glen. Almost all the updating will be in amenities for competitors and the press. Very little will be spent on the track itself.

Once again there is talk the Indianapolis Motor Speedway will have a stock car race. The new track president, John Cooper, is receptive to the idea of a Grand National race in August 1981.A summer speed week might be fitted into the NASCAR schedule, says its chief promoter, Bill France sr.

With the season half over, Dale Earnhardt clings to a narrow lead in the Winston Cup Grand National stock car standings. He has 2,387 points to Richard Petty's 2,374 with Cale Yarborough at 2,349, Darrell Waltrip, 2,244 and Benny Parsons, 2,149. All drive Chevrolets and all have won more than $200,000 his season.

Trucker Bob Motz, Cuyahoga Falls, Ohio, really "hauls" but not freight.

He has a Kenworth tractor complete with sleeper cab aiming at 200 miles per hour for a quarter-mile dash. Motz has gone 145 in the rig powered by a jet engine from a B-58 bomber.