A veteran friend of mine had the following problem with his allegedly
restored BMW R75:
The exhaust valve of the right cylinder hang tight for the
third time.
The valve guide was replaced, the valve stem was given almost
double play.
Nothing helped.
Since this was not a material problem but rather a temperature problem,
he came up with the idea to examine its existing exhaust manifold closer.
He ripped the right side of the manifold apart and found the situation of the
exhaust system depicted on the left.
It formed such a strong backlog that the right cylinder was excessively hot and the valve is stuck again and again.
Remember:
Not everything that looks like an exhaust also works reliable.
