You're right, as your turning the nut your also turning the danper rod assembly that the nut/bolt threads into. Thats why you need a damper holding tool to stick down inside the tube and hold the damper rod assembly from turning while you unscrew the bottom nut.. or....you can use an impact gun ..air or electric will work. You have to take the fork off the bike so you can put your weight on the fork,compressing it and zipping the nut off with the gun. Compressing the fork will cause the spring to keep the damper rod from turning.
On the RM I'm currently working on, I had to go to extremes and compress the fork with a pair of tie downs to keep the damper rod from turning. If you have to do this, DO NOT! get your face or upper body anywhere near the compressed tubes. If the tie downs were to slip or give the results could be very bad!
Thats why it would be best to get the damper holding tool from your dealer,if you plan on keeping the bike. Unlike me I'm fixing this one up for resale...gotta save up for some knee braces.
So just try and use the weight of your body first to compress the fork. If this doesn't work I'll explain how I did it with the tie downs.
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Vester
96 RM250
"Let's see if an "A" rider tries it first.."