Lansdowne Damper Top Nut removal!

New owner of a '72 750cc Interstate which has Lansdowne dampers fitted. I need to replace the cast alloy speedo holder so the top nut of the damper (and brass adjuster) need to be removed to carry outage swap. Excuse my lack of technical knowledge but is it just a matter of unscrewing the top nut or is there some other action I need to take to remove it as it unscrews so far but does not release, just keeps turning!

Is your front wheel off the floor?

it wasn't off the floor Storm42, Should it be off the ground?

There is a balance to be sort after, if the bike is on a stand and the wheel is off the floor, then the weight of the wheel tends to hold the tops against the top of the forks. If the weight of the bike is on the wheel, then when the threads clear, the suspension collapses and the top shoot up, this can be detrimental to the threads on the tops as they are alloy and can deform as they get to the end of the thread.

I tend to support the bike on a jack stand with the wheel just touching the floor, when I feel the top has cleared the threads in the forks I lower the jack to raise the tops and get at the nut to release them. I also remove the long rods (brass adjuster, take note of which side they come from) from the damper and don't put them back until it is all buttoned up.

None of which seems to be your problem, When you are sure the threads have cleared, try pushing down on the forks.

Thanks for the advice, now all sorted, a little pressure down on the fork popped up the locking nut which allowed the release of the top nut.

It is still attached the the damper assembly. I put my bike on the center stand and loosen the top nut. I then lift on the back of the bike and wedge a 2 x 4 piece of wood under the rear tire. That will cause the spring and top nut to pop out of the fork tube enough to seperate the top nut from the “locking“ nut on the assembly. DO NOT do both forks at once. It is a tricky balancing act so be careful. I believe that “locking” nut is also the spring pre-load so don’t move it too much. Once the spring is exposed you will see how it is put together.
The top nut will come out with the metering rod still attached.
This is from memory, I think I covered everything.