I found a few helpful pieces of info that might help clear up the issue of drilling the stock tip. This if from a post here on TT... 1) Drill (well, grind weld) at 1 1/4" and removed 4" long insert 2) Drilled to 2" and removed 1 1/4" insert 3) Same as 2 but with stand off baffle snipped off 4) Same as 3 but with 1 to 1 1/4" hole through remaining baffle and stainless mesh bolted over this hole (to retain S.A. function) ...and this is from the FAQ's on the Pig Pen... http://www.xr650r.us/xr650r_faq/#1a #4 Q: What can I do with the stock exhaust? That depends on where you are. It seems that the ED (Euro spec) bikes come with the HRC competition exhaust tip, either installed, or in a package with the bike. These have about a 1.5" opening and very little in terms of internal baffling, other than the stainless steel mesh for spark arrestor function. The NA bikes have an exhaust tip with a tiny 3/4" exit and 2" baffles that line up with plates welded in the pipe so that they can pass the 82dB federal noise regulation. If you have one of these, your choice is to either modify the one you have, or buy the HRC tip (part number 18317-MBN-640, approx $110 USD). For a slight increase in flow (and sound level) a machinist can use a lathe to cut away the weld holding the spacer which in turn holds the 3/4" pipe. This leaves the 1 3/8" pipe as an exit, which is already bigger than the area for flow around the internal 2" baffles. Taking things to the next level involves drilling out the whole 1 3/8" section with a 2" holesaw. This is what you will typically see recommended on most of the sites that describe the uncorking process. You can also remove the tip assy. and cut the last baffle off from it, leaving the spark arrestor intact. You get tremendous gains by doing this, a true "thumper" sound, without breaking the windows of houses and having little kids tremble in fear as you ride by. Finally, if you are after full flow HRC-like closed course mode, you will need to add a 1 1/4" hole in the remaining 2" baffle and use four small screws to fit a 2" disc of stainless mesh (similar to what Honda uses) over this new hole to retain spark arrestor function. This may be too loud for most circumstances, but a 2" disc of stainless steel can be exchanged with the mesh to revert back to the previous step. It looks like I'm at #2. This must be what you were talking about abby. I'll be getting out the snips this weekend. I don't think I'll be drilling a hole in the end of the spark arrestor plate though. Sorry for the confusion earlier!