My new (to me) '03 XR100 sounds like a lawn mower.  I needed it to sound like a real bike so an exhaust mod was in order.  The easiest mod is to simply remove the exhaust baffle, however it needs a better look so it will stand out in a crowd.  What I installed for an exhaust tip is called a 1-1/2" stainless steel pipe stub end.  I got this one from an industrial supply store for $7.00 CAD.  This one is schedule 10 (schedule refers to how thick the steel is). A schedule 5 would be better (lighter) but I couldn't locate one.  It comes pre-polished so all you need to do is drill the 3 mounting holes (see the .pdf template) and find some shiny 6mm bolts. Look and sound are excellent and the performance (with the correct rejetting of course) is comparable to some aftermarket systems (no dyno but held it's own in a few drag races).  The tip length increases the exhaust volume and adds a bit of backpressure (over no baffle) both of which are known to help out the XR.  It is lighter than the stock baffle, but definitely not as light as an aluminum billet end.  For optimum performance the hole should about 1-1/8" to 1-1/4" (to create more backpressure) but the smaller stub ends don't have a flange large enough to fit over the mounting holes. Works and looks cool, what could be better.

Click Here:  Drilling Templete.pdf