I recently bought a 1999 Honda XR250R that had been mildly modified by the prior owner(s). It came mounted with an oversized, aftermarket tank but the seller threw in a red XR400R tank he had in the garage. I wanted to use a smaller, OEM sized tank for day to day riding so I decided to try to mount the 400 tank as I could not find an OEM 1996+ XR250R tank within my budget.

After I used the red 400 tank, someone on TT sold me a white 400 tank and I ended up using the white tank as it matched my plastic. I don't know what vintage either tank is.

I asked on TT, searched online, etc. to determine if an XR400R tank would fit on an XR250R. The findings were diverse, ranging from absolutely not to it would fit fine. Some findings were based, in part, on the fact that some aftermarket tank makers used the same tank for both models. So, here are my own findings:
Does the tank fit and function? Yes.
Is it perfect and precise? No.
Does it look like it fits? Yes.
Is it secure and solidly mounted? Yes.
Did it mount right on without work? No.
Was it alot of work? No.
Any issues with the petcock, seat front attachment, securing hardware, etc.? See below.

Yes, some people will still say that you absolutely cannot mount a 400 tank on a 250, etc., etc. I won't debate the issue, I'll simply share my own experiences and you can decide for yourself if it works.

Here's what my bike looked like with the red 400 tank mounted.



Here is how it finally ended up, with the white 400 tank mounted.



The XR250R uses this "collar" (PN 83119-GT4-000) to attach the retention band. It may also be used to lock in the front part of the seat:



The XR400R uses this "stay" (PN 17525-KCY-670) to attach the retention band and seat:



The retention band (PN 17516-MN1-670) is used to snug down the rear of the tank, and the cable loop may be intended as a leash to keep the tank from moving too much in a crash. What appears to be a wrapping of electrical tape is the way it comes new:



The front of my seat has a recess that helps lock it to the back of the tank:



When I tried to use the "collar" on the white tank, the seat would not lock into the collar (# 1) and the rubber of the retention band was very tight (# 2). It would have worked, but I thought I could improve on it:



The seat recess fit very well on to the "mushroom" on the back of the stay so I decided to use it instead of the collar. The stay bolted right on to the tank and fit into the recess molded into the tank. I liked the stay but the band seemed a little too loose:



On both the red and white 400 tanks, I tried the 250 forward mounting hardware and they bolted right up to the tanks. The forward mounting plates (PN 17522-KCE-670) are the same on both the 1999 250 and 400. The front of the tank mounts perfectly to the attachment points on the frame:



With the front of the tank firmly mounted to the frame and sitting on the XR250R's rubber tank bumpers I noticed the bottom of the petcock was very close to the top of the engine. The 250 and 400 use very different rubber mounting bumpers between the tank and the frame. I took some rubber and padded the area between the top of the frame and the tank. This tightened up the retention band (# 1), cushioned the area between tank and frame (# 2) and moved the petcock well clear of the engine:



Is it a perfect fit? No, there is a slight gap between the tank and seat front:

http://img.photobuck...torial/Gap1.jpg

However, the gap is not that noticeable IMHO and everything seems tight and secure:

http://img.photobuck...torial/Gap2.jpg

So, I would say that the 400 tank fits fine and is secure as shown. I can live with the gap. If a 1996+ XR250R tank became available at a great price I might consider getting it but I can't see that it would be a huge improvement over what I have.

http://img.photobuck...les/MyBike2.jpg

I am very happy with the look, fit and feel of my Honda XR250R with an XR400R tank sitting on it!

Yes, I know this is something of a duplicate thread, but I wanted to show the procedure for the benefit of anyone thinking about using a 400 tank but unsure if it could be done.

Additional info: No problems with switching out petcocks. When I looked at the petcock, I noticed the tube-shaped filter was disintegrating and the gasket was dried out, so I ordered replacements. Bolt holes lined up and bolts fit fine but the fuel line had to be replaced with a slightly longer one (but this may be because the fuel line was for the larger IMS tank which put the petcock a little lower and off to the side). I used some extra fuel line as the vent hose for the tank, easy way to keep some spare hose handy. Be sure to check the functioning of the petcock while you have it off, good time to clean it up and be sure you get full flow. If you replace the tube filter, be sure to carefully place the little black grommet (comes with the filter) on the top of the brass filler tube.

An online check shows that a 2000 Honda XR400R tank has a 2.5 gallon capacity including a 0.4 gallon reserve. I do not know what year my tanks are from, so YMMV.