As a special request, hereby a page out of the Honda Zoomer-X parts catalog, this is page F-12 which is for the front suspension (forks).

See attachment to download....

For the Zoomer-X Honda is using standard Showa cartridge type forks, so you probably can get muych more information is you search for Showa Inverted Cartiridge type forks...

The layout is like this:

Hmm I never had that question before, I know that the steering stem nut is the same for the Honda CBR250R, MSX125 and Yamaha WR450F....

But what the thread size is at the top of the forks is completely new. Also I'm not sure if the aluminum-alloy from the inverted fork is strong enough for a 30mm extension... I have serious doubts that the housing can handle the extra stress... with a regular fork the material would likely be steel and much stronger.

But I will try to find out the thread size, as I actually would be interested to know if it is possible...

I am Mr. Squirrel's friend.

In the photo of my existing forks (of unknown origin) you can see the reduced diameter extension that I would like to fit to the Zoomer forks.

I agree with the concerns about the structural integrity of doing this, but considering that the existing forks are very similar to the Zoomer forks and have, in the past, suffered an impact strong enough to bend one of them without dislodging or damaging the union between the upper outer fork tube and the steel extension piece, I am quite confident that there is not much to worry about!

The plan therefore is to try to fit these extenders to the Zoomer forks and extend the damper rod to suit the new overall length. This should be possible by one of two methods:

Use some threaded bar to thread into the Zoomer fork's existing bolt hole and, if the threads match, thread that bar into the threaded recess of the cap you can see on the top of the extenders, using lock nuts to keep it in place.

Or:

Drill a hole in the cap of the existing extenders and use a longer bolt to thread into the damper rod (this will also require a lock nut against the damper rod and inside the extender below the cap, or some form of tube around the bolt to allow it to tighten onto the damper rod).

So, I actually ideally need two thread sizes, one (the most important) is the large cap thread of the fork tube cap itself, the other is the thread of the bolt which retains the damper rod inside the cap.

Once again, thanks for your help so far.