It's 4am here, ive just been called to attend a job, just had a quick read of this and I'm amazed!
So you're basically saying it fits?? Bar stem length right?? I'm going to have to read this properly after work!
I really can't recall ever seeing more than a couple of XV250's in this country ,but they appear to be common down there.
What I said was that the upper and lower bearing I.D.'s are the same, making it a likely candidate.
The stem length and location of the upper bearing could still be a dealbreaker. You'll need to physically get your hands on one to verify this. But this is one way you can at least narrow things down with no wasted investment.
Suppose you found a front end which used a 27mm i.d. top bearing and a 29mm i.d. lower bearing. In order for it to fit your TW you'd first need to find out if a 48x29x12 and 43x27x11 bearings exist. If they do you have a potentially viable candidate because the "48" and "43" dimensions are the diameter of the TW's races. Those dimensions are pretty much set in stone, but the hole in the center of the bearing can be a number of different sizes which are pretty much standardized in the bearing industry. If no bearings exist, move on.
Then there's the matter of stem length and the placement of the upper bearing. The lower bearing is rarely an issue with front end swaps. As long as a bearing and race exist in the proper dimemensions the lower triple tree will usually fit.
The upper bearing has a shoulder on it which places the bearing at a given height range. If this height is too tall your bearing will protrude above the top race in the steering head and you won't be able to tighten down your upper bearing. If the shoulder for the bearing is the same height or slightly lower than that of the TW you can usually get the top bearing to seat fully in the race.
But you're not totally out of the woods yet.
If the steering stem is too long you may not be able to tighten down your upper stem bearing nut, which holds the bearings in proper tension. In some cases you may not be able to tighten down the upper triple clamp, as well. Generally the cure for this is either cutting deeper threads on the steering stem or putting a bearing spacer between the stem nut and the bearing. Relatively inexpensive machine work and it usally looks fine as long as the spacer is less than 3/4" or so.
We're talking bolt-on or nearly bolt-on here. Other options exist including pressing the steering stem from your TW into the lower triple of your donor front end (not always possible) or having a complete new stem turned which duplicates the TW dimensions, but they can become prohibitively costly and the process of pressing stems in or out of a steel lower tree like ours often only results in a bent and unusable stem. The upper bearing shoulder can sometimes also be lowered on a lathe, but it usually requires that the stem be pressed out in order to do the machining.
I've had this stuff go well and I've had it go badly due to an overlooked minor detail, but sometimes everything just clicks.