I keep burning pistons with my modified engine

I have a classic Mini and has a 1275 engine modified to 1380 cc using 73.5 pistons.Compression ratio is around 13 : 1 I had gasket problems and now it has started to blow pistons. Earlier one piston was burnt and a friend told me it was running too advance and it was the reason to burn the piston. Then I replaced the piston and Retard the engine to have maximum advance of 35 at maximum RPM of 7500. Again another piston was burnt. I am using 100 octane petrol. What would be the reason to burn the piston. Also the car was not moving fast. I am using NGK Cooler (10) plugs. I have another similar car and it has no problems. The car does not heat up and the temperature is normal.