MARYLAND FIRE MUSEUM 1301 York Road, Lutherville, (Exit 26 on the Baltimore Beltway). On Sundays 1 to 5; group tours on Fridays by appointment. 301/321-7500.

Stephen G. Heaver, casting about for something to do with his weekends, decided it would be fun to restore an antique Ahrens-Fox fire engine.

"That was in 1962," Heaver told a visitor recently. "It took my son and me six years, and it was fun, although it wasn't easy. Looking for parts and advice I ran into a lot of other fire-engine freaks and, well, one thing led to another . . ."

Now, three-quarters of a million dollars later, Heaver and a score of friends spend night and weekends restoring and maintaining five dozen vehiclesrepresenting four centuries of American firefighting. To house them he has built a giant firehouse in the Baltimore suburb of Lutherville. Heaver's organization has grown into the Fire Museum of Maryland, which could put on a mile-long Fourth of July parade all by itself.

"I guess you could say that our hobby has grown a little out of hand," Heaver said as he showed a visiting family around. "But, as much time and money as this place has cost me, my only regret is that we can't be open to the public every day. The people who do all this work are volunteers, and they just can't squeeze out any more time."

Which means that visitors must squeeze in between 1 and 5 p.m. on Sundays; if you come early and stay late you can almost see everything. The apparatus, from colonial hand-drawn pumps to massive, brass-bright steam engines, is an endless delight to the eye and a boggle to the mind.

Heaver and his friends stand by to explain and expand on any piece of equipment or fi-refighting in general. Kids are encouraged to turn in false alarms, which fill the 18,000-square foot building with their clangor and flash lights in the working alarm center.

"A firebox is an awful temptation to kids," a volunteer said. "We hope this helps them get it out of their systems, and also to understand how the system works." After the children in one Arlington family had had their fill of false alarms their mother went on working the handles like a slot-machine addict. "All my life I have wanted to pull these things," she said. "I'm not leaving until my arm get tired."

The volunteers understand and encourage this sort of thing. Many of them can date their own involvement in the museum from some Sunday afternoon spent innocently admiring, say, the thundering great 19th-century horse-drawn steamer converted in 1916 to trundle behind a massive Christie tractor. The engines are so ornate, and so beautiful in a way that machinery never will be again, that it's almost physically painful to learn that Baltimore scrapped five of them in 1944 to make bombs and bullets for World War II.

"That's the kind of thing that keeps this hobby completely out of control," Heaver said. "You see some beautiful old piece of equipment rusting in a field and you want to save it. Before you finish with that you come across another piece. These things are important, but they're so big and complicated nobody knows what to do with them. And nobody wants to spend the money."

Heaver doesn't want to spend the money, either, at least not at the rate he has been subsidizing the museum. He is a developer who could be rich and retired, but his sense of responsibility won't let him quit. "We started out intending to stick to automotive equipment, but then one day we were visiting this fire museum in Baldwin, up on Long Island, and learned that the owner wanted to retire and sell out. He didn't know what would become of his collection of hand- and horse-drawn apparatus, which was one of the finest in the country. We didn't think it was right to let something like that be broken up, so here it all is."

This sense of responsibility extends to authenticity. Nothing is faked in the restorations. Surely not one visitor in 10,000 would know that the antique truss-type ladders on the 1908 American LaFrance horse-drawn ladder truck are not "in period," if the sign on it didn't say so. Nearly all the equipment is in full working order - the museum even has its own licensed steam operating engineer - and it isn't very hard to persuade the volunteers to roll out a piece or two for a municipal parade.

It is hard, however, to pull oneself away from the museum. There is a 19th-century hand-drawn Philadelphia pumper, fancy as a French funeral coach with its silver fittings and cut-glass lanterns, that by itself is worth the trip. CAPTION: Picture, A 19TH CENTURY STEAM PUMPER, "MODERNIZED" BY CONVERSION FROM HORSEPOWER TO A TRACTOR IN 1916, STILL IS READY TO ANSWER THE CALL, IF ONLY FOR PARADES.