RD400 front end on my XS?

[mouser]

I was given an '77 RD400 and had the notion the front end would fit my XS. It is slightly shorter and uses an 18" front wheel and has air assisted dampening. I found that the steering head is 1" shorter on the 400 than the 650 but is the same diameter. Is it possible to remove the shaft from the lower triple tree and replace it with the shaft from a 650?

 

[gggGary]

I have swapped stems between an XS750 and an XS650 grind out the weld and beat the stem out swap and weld it in. Careful of the depth you set it to, check clearances seals fork stops etc. before welding it together. The smaller? RD tubes may be rather "whippy" on the heavy XS650.

 

[XSLeo]

If the fork upper tubes are less than 34 mm it wouldn't really be a better fork. If it was 35 mm or larger then yes the swap might be worth it. The larger the fork tubes the stiffer the frorks. Stiffer forks = better handling.
Leo