Thomas Thomas, 46, of Kingston upon Thames, was found guilty by a majority verdict at Woolwich Crown Court of conspiracy to rob.
Thomas had used blocking vehicles after a heist at Graff Diamonds in Mayfair, central London, last August, in which 40 million worth of gems were stolen at gunpoint. Two other men accused of conspiracy to rob were found not guilty.
Courtney Lawrence, 31, of west London and David Joseph, 23, of south east London, were cleared by the jury after 43 hours of deliberations.
The jury was still deliberating on charges against Craig Calderwood, 27, of no fixed address; Solomun Beyene, 25, of Marylebone, north west London; and Clinton Mogg, 43, of Bournemouth, Dorset.
On Friday, Aman Kassaye, 25, of no fixed address, was also convicted over the robbery. He was found guilty of conspiracy to rob, kidnap and possession of a forearm.
Kassaye forced a young shop assistant to help steal the gems, which have never been recovered.
Kassaye used a professional make-up artist to disguise his appearance and a series of getaway cars to escape across the capital. The raiders shot at anyone who got in their way, the court was told.
Footage was shown to the jury of Kassaye firing a shot in the street outside the store as passers-by began to realise what was happening, sending them running for cover.
Kassaye's defence rested on his assertion that the raid was carried out "by someone remarkably like him".
But Petra Ehnar, the shop assistant held at gunpoint during the raid, identified him.