Stripped threads for Power Valve Cover.

It could be oil or some other liquid in the hole preventing it from going in all the way, because you can't compress a liquid so you would either break the cylinder or the bolt which ever ones weaker.:think

Before you go any further, I would get a tap exactly the same dimension as the bolt. Keep turning the tap in part way, then removing it until it goes all the way in.
Perhaps the threads are just grunged up.

if the bolt was a factory bolt and had an 8mm head, then the threads are 6mmX1.0. Get a tap this size and pitch. Run it in the hole. It will clean up any threads that have been crossthreaded. If the hole is completely stripped ( doesn't sound like it, otherwise the bolt would just keep turning

Be very careful with the tap. They are very brittle and break easy. They can handle some torque but don't apply any off-center force. Go in a little, back it out, in a little more, etc. Use some light oil. Blow the chips out if you have compressed air. Take your time - if you break it you will become upset.

By the way, there are different kinds of taps - some have a long taper to make them easy to start, but they won't tap all the way to the bottom of the hole. "Bottoming taps" will tap closer to the bottom of the hole but are harder to start.

If the threads are stripped, you can probably install a Heli-coil or other kind of insert. Not a huge deal, but you might want some help if you've never done one before.

FLBob-
good follow up.