due to rushed timing, I wound up ordering parts from multiple sources. I wouldn't recommend it to the next guy trying to pull off this upgrade. I have so far confirmed:
> FORKS & TRIPLES - the XR400 forks (43 mm diameter) do not fit XR250 triple trees (41 mm diameter). the unknown triple trees already on the bike when I bought it would also accept the 43 mm forks, but I changed to the known XR400 triple trees anyway. The frame/neck accepts either set of triple trees just fine.
> WHEEL - I read on one of the forums that the XR250 and XR400 wheels were the same except that the larger diameter axle of the XR400 requires you to change out the bearings & seals. This is not entirely accurate. After a survey of the market for used front wheels, I found that the XR250 wheels are much less expensive than the XR400's, so I bought a used XR250 wheel from one source (less than $100), a separate XR400 axle, odometer and spacers from a second source, and then some new replacement XR400 bearings & seals from a third source. The bearings & seals fit the XR250 hub just fine, but I didn't realize until pulling the bearings that I'd forgotten to get the XR400 spacer that goes inside the hub (between the bearings). The larger diameter XR400 axle won't fit through the XR250 spacer. A new XR400 spacer is due in to my local Honda dealer tomorrow (fourth source).
I test fit everything I could last night. The wheel looks offset to the brake side - the wheel sits closer to the fork on the side the disc brakes are on. Also, the spacing didn't look right for the odometer assembly; in order to get the odometer housing to seat flush against the hub, the axle has to thread in all the way (flush with the fork clamps - can't get a wrench on it). That made me suspect that the hubs are different on the two wheels; when I checked the OEM
parts list this morning, I see that the XR250 front wheel hub (44601-KV6-860) is a different part number from the XR400 hub (44601-KAE-870). I assume that the XR400 hub is wider, but I have no idea by how much. So now I'm suspicious that the internal hub spacer that's coming in tomorrow will be too long, and that will mean that the XR400 bearings wont seat completely in the XR250 hub.
If those assumptions are correct, then if I stick with the XR400 fork conversion I think my options are to either buy an XR400 front hub (at least $175) and relace the wheel, or save the aggravation and buy a whole XR400 wheel (about $300 seems to be the current price for used). Or I could keep the XR250 wheel and change to stock forks & triple trees. The price is about the same either way, so I guess I'll go ahead with the conversion...
> DISC BRAKES - I bought used calipers and master cylinder from one source, and used brake line, banjo bolts and clamps from a second source. After everything arrived I realized that I didn't have the bolts to attach the calipers to the fork...did I mention that I have parts coming into the Honda dealer tomorrow?
Edited by scrape88, 24 May 2012 - 07:00 AM.