STAR apprentice Simon 'Hong Kong' Yim breaks through to the big time on Sunday when he rides Northern Fire Ball for local handler Wong Tang-ping in the St Andrew's Challenge Quaich. Yim, plus his valuable five-pound allowance, is rubbing shoulders with the likes of Darren Gauci and Tony Cruz on the 10-winner mark in the jockey standings. It is the most remarkable of transformations as the young claimer has, by dint of sheer graft, turned himself from someone who, in the words of his trainer Patrick Biancone, 'could hardly sit on' to a rider of immense promise. Northern Fire Ball will be a real test for Yim as he is a talented performer but with a very light mouth. Even a jockey so sympathetic as Julie Krone couldn't stop him hanging violently when riding him to victory on his start before last. Only one of the world's truly great jockeys - Gerald Mosse - has been able to get Northern Fire Ball to travel gun-barrel straight to the line. Last time out Northern Fire Ball did not enjoy the run of the race in the National Panasonic Cup where the pace was far too slow for such a confirmed back runner as himself. In the circumstances, he was far from disgraced behind Sterling Town and yesterday looked on good terms with himself as he flowed up the home straight with his improving stablemate, Miss Piggy, in a comfortable last quarter in 24.8 seconds. Champion River Verdon makes his eagerly-awaited return to the fray in the St Andrew's Challenge Quaich where he has been set 140 lbs. A combination of two factors make this the toughest challenge he has ever faced on the local calendar. First, there is the weight. He is conceding lumps of weight all round against some of the very best 1,800-metre horses in the territory. Weight is the great equaliser. Secondly, there are the after-effects of his Melbourne Cup and W. S. Cox Plate exertions. They looked to be in evidence on Tuesday morning when he galloped on the grass with Lionheart. River Verdon moved alongside and seemed poised to kick clear before Lionheart rallied and was going better on the line. Returning to yesterday's work and there was a most pleasing piece from the barriers at the top of the back straight from Lam Hung-fie's Real Perfect. Given the way he caught the eye of virtually every professional punter in town with his run last time, he is unlikely to start at particularly remunerative odds when next seen out. But he should take some beating. Yesterday he went better than his speedy stablemate, Auto Focus, with Happy Birthday running on strongly to join level at the end of the hitout. Happy Birthday accomplished little in nine starts last season but has since joined Lam. His one run at the beginning of this campaign was an inauspicious effort, too, but the break from racing seems to have done him good.