Hole in piston

Thats interesting, I think you are right , a thin top, perhaps cracked first then burnt thro the crack.What make pistons?, Look as if they have been worked on a lathe ,maybe skimmed off the top?.

Never heard good stories about JP pistons, the one I have direct experience of had a missed operation, the clearance area at the ring area had been missed and was at the same diameter as the skirt. I witnessed the result, a quiet tickover on first start-up followed by seizure 20 yards later. And those appalling stiff rings that draw blood of removed by hand.

I had almost exactly the same happen to my Atlas engine. I was trundling off to work down the M27 at a steady 65 mph when bang, followed by loads of white smoke. An engine strip revealed a similar looking piston top to Dave's. A radial crack with a chunk missing on the crack line.  The sides of this piston had just a little scoring, suggesting that a seizure had not been the main reason for the failure.

What surprised me was this engine (& piston) had run with no problems for 23,000 miles of mixed motoring. Not too far away, was an AE component supplier so I headed off to see them to ask an opinion. They had a demised piston chart but none of the pictures matched any of those shown with my example.  The salesman suggested that my piston had perhaps been cast with impurities in the metal mix which caused a weak spot. This is possible but why did the Big Bang not occur much sooner?  

My own theory is a combination of weather conditions, ignition timing and carburation were the cause.  That morning the weather was heavy, over-cast and humid conditions. The bike had twin carbs and magneto igintion; both well known for supplying varying fuel mixes and firing points. My money would now go on pre-ignition for causing the piston failure. I fitted a single carb and electronic igintion and never had the problem return.

I have had 3 similar failures over the years, 88, 99 and Atlas. With the 88 and 99, a piston cracked and then holed. The cylinder kept firing and the combustion flame torched the small end of the rod which opened out and let go. The cause on the 99 was an asymetric mag cam ring and over advanced ignition on one side. With son's 88, it was cheapest low octane unleaded, 2 up on a very hot day on the autobahn so the engine was cooked and both piston crowns collapsed then the rod let go. Pre-ignition probably. With the Atlas, no obvious cause. A quick rebore and all was once again well.   

Dave.......there is a 99 left-hand piston for sale on eBay this week. It is 68mm + 30"in very good condition with a £30 price tag. The downside being it looks like the make is GPM.