THE chairman's office at Weir Group's turn-of-the century headquarters in Glasgow looks more like a grandfather's study.

The fabric is coming off the sofa, the telephones on the desk are 1960's vintage and the hearth rug in front of the electric bar fire is thread-bare. You almost expect a cat to be sleeping on it.

This shabby, comfortable den hardly looks like the powerhouse of an international engineering company with a market capitalisation of #410m and pre-tax profits of over #60m a year. And the chances are that it won't stay this way for much longer.

For Lord Weir, the great-grandson of the company's founder, retires this week after 42 years with the company - 25 of them as chairman.

Dressed in a check suit with a red hankie stuffed into the breast pocket, he surveys the wood-panelled room on his last working day in the office. ''We tarted this place up in about 1956,'' he says. ''It was very dark before then.''

Lord Weir, who turned 65 last year, looks a bit of an old-fashioned fuddy-duddy. His classic aristocratic education at Eton and Cambridge, his love of golf, shooting and fishing, and his die-hard opposition to devolution all serve to reinforce the image.

But appearances are deceptive. For this was the man who took a traditional engineering company, associated with the shipbuilders of the Clyde, and turned it into a modern world-wide business that has carved out a specialist niche in sophisticated pumps and valves.

The turning point came in the early 1970s when he took over the running of the company. Weir Group made a successful bid for Harland Engineering, a specialised pump and valve manufacturer with factories in Manchester and Alloa.

This was followed by a series of acquisitions to reinforce the company's expertise in the sector. It now designs and manufactures specialist pumps and valves for nuclear power stations, water works, oil refineries and any other large engineering project you care to mention.

The company's international expansion was consolidated with the acquisition of Envirotech Pump Systems in the US in 1994. This gave Weir Group a major presence in both North and South America. Nearly three quarters of its #600m-plus turnover now comes from overseas.

But it was not all plain sailing. Lord Weir nearly came unstuck when he let the company over-reach itself 20 years ago. ''My biggest mistake was not to cut back and rationalise some of our operations much more quickly in the late 1970s,'' he says.

''We had allowed ourselves to become over-extended''.

That mistake exposed Weir Group to the risk of takeover, and Lord Weir was forced to vacate the chairmanship for a couple of years while a company doctor, Sir Francis Tombs, was brought in to clear up the mess. Fortunately two of Lord Weir's friends - the financier Lord Rothschild and the American investor Gerald Ruttenberg - stepped in to mop up 80% of an unpopular rights issue.

That kept Weir Group afloat, and in 1983 Lord Weir resumed control of the company.

''If they had not been around I am sure the firm would not have remained independent,'' he says.

Weir Group now does lots of other things besides valves and pumps. Its Strachan & Henshaw subsidiary is big in materials handling, and the group forms part of the consortium that runs Devonport naval dockyard.

But Lord Weir muses that his company nearly ended up making helicopters instead.

His grandfather became a great aviation enthusiast after presiding over the birth of the Royal Air Force as a member of Lloyd George's War Cabinet in 1918. And his great-uncle Jimmy was a keen pilot who used to commute to work in Cathcart by autogyro.

So it was that Weir Group started to develop flying machines in the 1920s and 30s, first autogyros and then helicopters. The company tested its first helicopter in Glasgow in 1933 (workmen held the clattering machine close to the ground with ropes), and continued developing them during the Second World War.

But the Air Ministry wasn't that interested, so Weir sold its helicopter interests to an engineering company called Westland in 1945.

The rest is history. But Lord Weir recalls that one of Westland's first helicopters to see military service - the Skeeter - was actually designed in Glasgow.

Looking ahead, his Lordship does not plan a leisurely retirement at his country home in Mauchline in Ayrshire, near the farm of Robert Burns from whom he claims descent.

He remains chairman of the BICC construction and engineering group, and a non-executive director of Canadian Pacific and St James Place Capital. And on Sunday he is off to Turkey at the head of a trade mission organised by the Water Association.

But the active involvement of the Weir family in the company it founded in 1871 has finally come to an end.

Control has passed into the hands of institutional investors, and the family now owns less than 1% of the shares.

Lord Weir's successor as chairman is Sir Ron Garrick, the group's long-serving chief executive.