All of the above steps have been followed. I even went to the length of replacing a good thermostat with the version that has the support lugs underneath. I even had to return one to the supplier as it didn't have the lugs as their ebay photo showed.
This was fitted last June/July with the front inlet manifold removed, all pipe bores well cleaned, using Viton O-rings and a generous smear of molykote 111. Clips were pushed out as far as possible and the bolt torqued correctly. The system was well flushed and refilled with non OAT coolant. All good after that.
I was put back on to OAT after a cam belt and (2) waterpump changes in September, and everythings been fine until the first signs of a leak yesterday.
I'm not sure what more I can do. If I rub back the banana pipe bore any more I'll need a bigger o-ring and a way to get the metal filings out of the block.
