Australian left-hander Head hit 175 off 139 balls while Leaning finished unbeaten on 131 off 110 deliveries as Yorkshire recorded their highest List A total against another first class county, beating the 352-6 made against Nottinghamshire in Scarborough in 2001.
Their highest total, 411-4, was made against Devon in 2004.
Leaning, whose century was his first in List A cricket, said it had been a pleasure to bat with Head, describing the 21-year-old as “a huge talent who will play a big part for Australia in the future.”
“It was also good to keep up our current form. Wins always breed confidence, and we can take this into a busy period of games,” Leaning added.
Having been asked to bowl first, Leicestershire might have thought fortune was going to favour them after Yorkshire opener Adam Lyth, who scored centuries in his two previous List A games, was run out for just two following a mix-up with fellow opener Alex Lees.
Lees himself was stumped for 32 off the bowling of Foxes’ off-spinner Rob Sayer, coming down the wicket and missing with an ugly have across the line, but 51-2 was as good as it got for the Foxes. On a flat track Head looked in fine touch from the start, while Leaning hit the third ball he faced, from off-spinner Rob Sayer, high over long-off for six.
Head brought up his half-century from a relatively sedate 60 balls, but accelerated thereafter, reaching his hundred off 99 deliveries. He might have been dismissed on 116, when he lofted a delivery from Mark Cosgrove to long-off, but although Sayer held the ball one-handed above his head, he did so in the act of stepping over the boundary and was forced to throw the ball back inside the rope.
Leaning also had an escape, a more straightforward chance dropped by Robson at deep mid-wicket off Sayer when he was on 83, and the pair had passed Yorkshire’s previous List A record partnership of 242, between Martyn Moxon and Ashley Metcalfe against Warwickshire in 1990, by the time Head passed 150 and Leaning 100 (96 balls, five fours and three sixes).
Head eventually top-edged a simple catch behind the wicket to be dismissed for 175 (remarkably, still 25 short of his best List A score of 200 made for South Australia last October), but Leaning finished unbeaten as Yorkshire continued to pile on the runs.
Leicestershire’s reply began poorly, when captain Mark Pettini was bowled by a superb in-swinging delivery from David Willey, and though Angus Robson could consider himself unlucky to glove an attempted hook at Tim Bresnan and be caught by the wicket-keeper down the leg-side, Kevin O’Brien can only have been disappointed with the shot that saw him give mid-wicket a simple catch.
Cosgrove was caught off a leading edge, and though Lewis Hill played well in going to 50, the rest of the Leicestershire batsmen had no answer to the spin of Azeem Rafiq and Adil Rashid.