Coolant tank was showing slightly low, not sure why ?

On trying to remove the cap it became tighter and tighter, to the point I needed to use two hands and a lot of force, to finally remove it.

Checking and topping up the tank I replaced the cap, without any problems.

Today, I checked the water level, it had not changed. I then tried to remove the cap, and again I experienced the earlier problem.
I have applied a smear of 'rubber oil' to the O ring within the cap and to the threaded portion of the cap and tank. I screwed the cap back on. Then removed the cap several times, it is now quite free.

Will check on the cap over the next few weeks. Anyone experienced this issue.

A lot of people have had the same problem, myself included, if it gets really bad the inner section can part company with the top making it awkward to get it out
Oil/greasing it is good but in the end i replaced the cap, still quite tight though. Common sense isn't very common.
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2007 Land Rover Freelander2 I6 hse Auto

so do you think I should change the cap ..... and the header tank ?

could the thread on the cap and tank become distorted ?

Try the cap first unless the tank is leaking, seem to remember a post on a leak from the back of the tank that was almost impossible to see. Common sense isn't very common.
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2007 Land Rover Freelander2 I6 hse Auto