John Munro and sisters June Aitken, Margaret Popenhagan and Gwen Henderson offered on an online auction site a hand-made piston which had been cast, turned and fitted by Burt Munro for his stream-lined motorcycle which set a 185.586mph (295.453kmh) speed record on the Bonneville salt flats in August 1967.
The feat led to the film World's Fastest Indian.
John Munro said the successful bidder was an Auckland man who had bid throughout the auction.
"He seemed to be determined to get it," Mr Munro said.
The bidding reached $6000 in $50 increments, not long before the auction closed, the Auckland buyer then bidding another $1000 to take it to the final price.
Mr Munro has sent details to the bidder, but had not yet heard back by late yesterday.
As well as the piston, the buyer will get a certificate of authentication and some books on Burt Munro.
The auctioned piston was originally one of hundreds of items which sat on shelves in Burt Munro's shed as "offerings to the God of Speed".
The family is considering something else from the Burt Munro collection to benefit earthquake relief as well.
