Hello all,
I've just joined as I have the same Bearing issue. I am changing the forks (as I don't have the top clamp and stem) to USD Derbi forks. The method I am employing is this;
Measure stem width. Mine is 25 mm at bottom and top. (RS 125 stem is NOT 25mm top and bottom but it is same as
Measure Honda RS 125 NF4 steering tube, this is 55 mm bottom and 47 mm top
the old ball races depth is 12 mm at bottom and 8 mm top.
The problem is Honda doesn't make/supply these parts for RTL or RS respectively
https://www.cmsnl.com/honda-rtl250s-1985-1986-hrc_model35475/partslist/0092.html#.XC5Yglz7S00
http://tso.us.com/parts-book/honda/0125rs125/1989/3_part_pages/f_02.asp
What i'm in the process of doing is first find bearing to suit stem then to suit outside diameter.
https://uk.rs-online.com/web/c/pneumatics-hydraulics-power-transmission/power-transmission-rotary-bearings/ball-bearings/?applied-dimensions=4294845448,4294845461
gives some alternative bearings for my 25 mm stem with/without seals. NOTICE these are ball bearings for the bottom and is 12 mm thick.
https://www.ebay.co.uk/i/321858810549?chn=ps 15mm thick
this gives a ball bearing for the top.
these bearings are too thick for standard replacement. This issue was discussed by Kiwi Biker
https://www.kiwibiker.co.nz/forums/showthread.php/181289-NF4-steering-head-bearings(RS125-88-94)
for me im in the process of using taper bearings as they fill in the gap between the Derbi Forks and the Honda steering tube.
https://www.bearingstation.co.uk/products/Bearings/Taper_Roller_Bearing/Metric_Tapered_Roller
To get original ball bearings in it is a case of sourcing the correct size of bearing and get them skimmed/ground down.
Ed