OK so Ive got a bit of a problem. I had an issue were the material around the cylinder head studs had become soft so the studs pulled. I realised what was happening when one of the cylinder head nuts came loose so ended up puling the head off and helicoiling the stud and putting everything back. The bike seized after about 20-30 min of not very hard running.

I figured the problem was the weakened material around the studs letting loose and radiator fluid getting into the cylinder. So I rebuilt the engine and figured nothing of it. Nice new engine and 20-30 min later seized up again once again not being ridden hard. There were gargling sounds form the lower cylinder both times it seized so figure that might be a symptom that I didnt take into account the first time? Not very happy now so any help appreciated.

So before I give it another go what do people thing the problem might be or does anyone have any advice?
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94 mc28 SE