Feb 26 (Reuters) - Berkshire Hathaway class B shares gained 5% before the bell on Monday after the $900-billion conglomerate posted its second straight record annual profit and its chairman Warren Buffett reassured investors that it was 'built to last'.

The 93-year-old investing legend, however, toned down expectations for the stock price in his annual letter to shareholders, lamenting that Berkshire did not have many lucrative investment opportunities left.

He said Berkshire would perform slightly better than the "average American corporation", but anything beyond that is "wishful thinking", even as it had a cash pile of $167.6 billion.

Berkshire's Class B stock, which carry higher voting rights and whose value was 1/1,500th of Class A shares, was trading at $435.50. Investors closely watch Berkshire as its results are often seen as a bellwether for the U.S. economy.

"There remain only a handful of companies in this country capable of truly moving the needle at Berkshire, and they have been endlessly picked over by us and by others... All in all, we have no possibility of eye-popping performance," Buffett wrote.

He also mourned the passing of his longtime second-in-command Charlie Munger in his annual letter, while assuring investors that vice Chairman and designated successor Greg Abel was "ready to be CEO of Berkshire tomorrow."

Berkshire's annual operating profit climbed 21% to $37.4 billion on improved underwriting and higher investment income from the insurance segment. Operating profit for the fourth quarter also came in ahead of analysts' expectations.

(Reporting by Niket Nishant in Bengaluru; Editing by Arun Koyyur)