NURBURGRING, Germany - Formula 1 drivers have threatened to pull out of the German Formula 1 GP this weekend if further tyre blow-outs like those at Silverstone the previous weekend occur.Drivers sounded confident during media conferences that tyre provider Pirelli had sorted out the problem by using Kevlar-reinforced instead of steel-back rubber for the Nurburgring race but, after a meeting of the drivers' union, the tone hardened.A statement read: "The drivers of the Grand Prix Drivers' Association wish to express their deepest concern about the events that took place at Silverstone."We trust the changes made to the tyres will have the desired results and that similar problems will not occur during the German GP weekend. We are ready to drive our cars to the limit, as we always do, and as it is expected by our teams, sponsors and fans."However, the drivers have decided that, if similar problems should manifest themselves during the German GP, we shall immediately withdraw from the event as this avoidable problem endangers again the lives of drivers, marshals and fans."NOT THE FIRSTFive drivers suffered tyre blowouts at Silverstone on Sunday – each on the left rear wheel - and a piece of tyre narrowly missed Fernando Alonso’s (Ferrari) head. Team partner Felipe Massa was badly hurt when his helmet was hit by a piece of flying metal during a 2009 GP.Massa talked of a possible boycott immediately after the British race, won by Mercedes’ Nico Rosberg.This would not be the first F1 boycott: there have been several walkouts by teams and drivers - some of them involving South Africa's racial laws - in an often turbulent history...In 2005 sven teams pulled out of the American GP at Indianapolis because of tyre safety fears.In 1985, the F rench Ligier and Renault teams shunned the South African GP - the penultimate race of the season - amid mounting international pressure against racial segregation.In 1982 only 14 cars started the San Marino GP at Imola after a boycott by teams - led by current commercial supremo Bernie Ecclestone - fighting the governing body in a political battle dubbed the 'Fisa-Foca war' within the sport.KEVLAR, NOT STEELThat same year the drivers organised a strike before the South African race at Kyalami over restricting clauses in their superlicence agreements. The race went ahead after a compromise was reached.In 1976 the Austrian GP was run without Ferrari, whose Austrian driver Niki Lauda was in hospital after a fiery crash at the Nurburgring and who were incensed by McLaren's James Hunt having his Spanish GP win reinstated.Pirelli, after the intervention of the governing International Automobile Federation, has brought upgraded rear tyres to the Nurburgring for the 2013 German GP, its inner belt made of the synthetic fibre Kevlar rather than steel.Spaniard Alonso, second in the standings behind Red Bull's triple World champion Sebastian Vettel, said earlier on Thursday that he was reluctant to take part in a test at Silverstone later in July 2013 because he feared a repeat of the blowouts.'NOT VERY SAFE...'F1's governing body responded to the tyre blowouts by deciding on Monday to change the rules and allow race drivers to test revised Pirelli tyres instead of using young drivers."I don't intend to go," Alonso told reporters. "It's not a very safe thing racing on the same track with the same tyres. I don't have the feeling I want to go. But if the team wants me to go..."He said in an earlier statement: "The most important thing is to make it for dinner at home on Sunday. I was lucky... it (the piece of tyre) could have hit my helmet. It would have been like a bullet."Pirelli has put some of the blame for the Silverstone blowouts on the teams for the way they managed its tyres. It conducted a test with Mercedes in Barcelona in May 2013 for which the German team were punished by the FIA.Mercedes has been ruled out of the Silverstone tyre trial this month.Stay with Wheels24 for the 2013 German GP weekend.