1985 DT 85 Hole in 3rd Piston

[strutman]

My Dt 85 , for the second time in 3 years has developed a 1/2 inch hole directly below the plug.Last time we rebuilt it ,we put in a new piston and rings in that pot and all went well until last week. Pulled head off ,same hole same spot.Here in Australia we use 98 Octane Unleaded, and always use fresh fuel.I have read here that a pin hole in fuel pump diaphram can cause gas to build up.
Any help would be Great

 

[seahorse5]

Re: 1985 DT 85 Hole in 3rd Piston

Did the hole in the piston look like someone used a blow-torch to make it (smooth melted edges) or did it look like someone used a small hammer (ragged edges)?

What is the top rpm on the motor when you have a tank of gas and a normal load in it?