I have run a 53 Irvine for several months with the standard muffler and a fuel mix of 18% synthetic, 2% castor oil and 6% nitro. Ran beautifully. Recently fitted a Yusa tuned pipe (350mm x 13mm id inlet) and a 160mm x 13mm id header. Used same fuel mix and needle valve setting (+/- 1 1/2 turns), but started wrecking glo plugs (OS8) after only a few runs each. Someone riched out the fuel mixture by one full turn on the needle valve, which then appeared to solve the problem. Yesterday (1 March 2003) the motor seized comprehensively during mid-flight, without any warning. Mystery: Piston appears fine (no score marks), but liner has 'shrunk' at top end, causing piston to jam at about 3/4 stroke. What has gone wrong?