Simply, do not bother. To remove the damper, you must remove the springs; to remove the springs, you must remove the plug in the top of the stanchion. Once they are removed, the damper simply pushes out. I have owned a T160 from brand new, I have never removed the nuts from the bottom of the stanchions.
To remove the plug in the top of a stanchion, do not use an Allen key, the size is expensive and you cannot push down on the plug. I use a short length (1") of hex bar in the plug's socket and then a standard socket on a short extension on a T bar.
You must push down on the plug both when removing and fitting, it is soft alloy and the threads are fine; if you do not push down on the plug when removing (and fitting), when only a few threads are engaged in the stanchion, the spring pressure can rip the threads off the plug.
Plug removed, if the spring does not extend about 1" above the stanchion, the spring is faulty or wrong.
Unless fitting better than the standard dampers, T140 progressive springs specifically from Progressive Suspension.
The standard damper head O ring (97-4003) is the cause of much stiction in the forks, I replace with
Damper valve seal conversion 1971 on (pair) for much reduced stiction.
The stanchions must be polished inside (some are not
); if not, this will also cause much stiction whichever damper seals are fitted and will wear either seal type rapidly.
The standard 97-4001 fork seals are poor. I have used the later Leak Proof seals, as Meriden fitted 78 onwards, since 1983, never required replacement. If you make the change, you will require 97-7010, includes the seals retaining rings.
L.P. Williams also have the Ariete version of 97-4001 (97-4001A) but I have never used them.
Progressive Suspension fitting instructions advise setting fork oil level by height, as on modern bikes,
not volume. Ime, setting by volume, the oil heights can differ significantly between legs, which affects the volume of the air spring between the two legs.
I have used Bel-Ray fork oil since 1983, 7.5 is about equivalent to the Triumph recommended ATF.
If you mean part 31 97-4004, it is not an O ring, it should be a 5/16" Dowty washer, that specifically fits in the recess in the bottom of the slider (parts catalogue "Outer member").
However, unless the 14-1019 Allen screw is not threaded where it ends up through the Dowty washer, damping fluid can leak down the thread past the Dowty;
I assemble with Hylomar (ensuring all parts are degreased thoroughly).
Before refitting the alloy plug in the stanchion, degrease and clean both threads throroughly - slivers of alloy can come off the plug, will jam it in the stanchion.
When refitting the alloy plug, because the spring protrudes above the stanchion, you must be able to push down absolutely centrally on the plug and you must be able to screw the plug well into the stanchion without releasing the downward pressure.
However, because of the spring pressure, be aware it is difficult to know if the plug cross threads; if you do cross thread it, sometimes it can be rescued after unscrewing and another thorough clean of both threads; but sometimes you need a new plug.
I seal the top threads of a plug to prevent damping fluid leaks.
