hi everyone,
im new to guitar building and need to buy a truss rod for my guitar neck and looked on this website and they only sell a 17" length truss rod however after researching i read that the anchor of the truss rod should be under the 21st fret but after measuring a few of my guitars from the 21st fret i found they were longer than 17"
ps i am making a les paul style guitar

any recommendaions and tips would be really helpfull
thanks

Truss rods with anchors have a single rod and work by compressing the back of the neck. I've never used one of that type (and probably never will), so I don't know how important the anchor position is.
The truss rod you saw here has two rods (and no anchors), and in action it's the truss rod itself that bends.
As long as the end of truss rod is in the neck-body junction area, it should be fine.

The Tonetech truss rods are 2 piece 2 way rods which require a straight channel - they work both ways enabling you to push the neck forward and backward as required.

Gibson and older Fender style truss rods are 1 piece 1 way. The rod sits in a curved channel which has a fixed anchored point at one end with a threaded section through an anchored point at the other - when you tighten the nut the rod tries to straighten forcing the neck back.

The Tonetech rods will be fine for what you want - but you will have to make sure there is enough room at the headstock end to house the rod adjuster as the 2 piece rods sit deeper in the neck.

I've started installing the two ways truss rods upside down. The adjuster rod is therefore closest to the fret board. This means that the allen key adjustment channel is not too wide (nor deep) to weaken the neck at the nut. Simply turn the adjuster the other way from normal (anticlockwise instead of clockwise to force the neck away from the strings etc). It works fine and I have built several this way.
However, you do have to route about 0.5mm in the back of the fret board to clear the head of the adjuster.
As the T/T truss rods are 11mm deep max you can go to about 14-15mm neck thickness at the nut. If your fingerboard is about 5mm thick you can easily get a total of around 19-20mm thickness if you desire.

I personally go no thinner than around 21mm at the nut end, tapering to about 24mm at 16th fret as it seems to suit most people.

Thanks everyone this info has helped a lot, ive just finished the pao ferro fretboard with a 10" radius should be installing the truss rod within a couple of weeks fingers crossed it all goes well