There appear to be diverse causes for guide problems. In my own case, there were two rounds of difficulty:
- The ET122 lock ring had loosened, and wore away the threads in the head as it vibrated. The guide itself wasn't loose. I fixed this by making and installing the alloy spacer unit as described by TT.
- Later, the piston crown separated from the gudgeon pin, striking and bending the exhaust valve. This also forced the entire guide sideways, enlarging the bore and cracking the surrounding alloy. This is the current problem, and due to the cracks I don't think it can be fixed with a simple oversize guide.
The one-piece bronze guide with extended cylinder, as Lintman posts, is a nice solution. I think in all cases, though, one needs an interference fit between the guide and its bore to insure rigidity.
Cheers,
Dave