If you are a serious off-road/woods rider, terrain can get tough, and parts can be broken or damaged easily, especially your header pipe, if your on a 2-stroke. Some riders experience new power robbing dents after every ride... and we all know that pipes are expensive.
My riding friends and i have tested pipe repair. If you have an air compressor, a mapp gas torch and a local hardware store, you can do it. Now of course there are going to be some pipes that will be considered irrepairable, but ill leave that judgement up to you.
1. First you need to seal off one end of the pipe. You should probably start with the smaller end, its easier to find smaller plugs and the next steps are easier with the engine side end. Some better hardware stores carry rubber plugs with a bolt and a wing nut through them. These work great, and they stay in air tight. If you cant find them, buy a regular rubber plug (like the ones for jars) of desired diameter, drill a hole in it, get a bolt and some fender washers and make your own.
2. The other end of the pipe also needs to be sealed, but it also needs a way to fill the pipe with air. For this you will need a regular rubber plug, and a valve stem with proper fittings. You have to find a way to fit the valve stem through the plug, while keeping it air tight (we're not talking about alot of PSI here, depending on the dent it will be around 20-25 PSI). At better hardware stores, you can find valve stems with 1/4 pipe thread on the end. One way is to fit this with a barb fitting, drill a hole in the plug so that the barb fits tight, and insert it in the plug. Another way is to fit the valve stem to the plug using a pipe nipple. Lamp nipples and lamp nuts work well also because they are pipe thread and they are threaded all the way so you can really clamp it onto the plug. Of course i cant really explain this step too much more because i dont know the availability of materials in your area, or the extent of your mechanical ability.
3. Now you have to secure this rigged up plug into the engine side of the pipe. This may be tricky because you dont have a definite way of clamping it in there. If you get a plug that is a tight fit, you may not have a problem because you can have somebody hold it in there while they are filling it with air(again, not much PSI). But, you CAN wrap wire or duct tape around it if you dont think its tight enough. Be creative, use your brain.
4. Now for the fun part. Fill your compressor up to about 18 psi to begin with. Have a friend hold onto the plug with the valve through it and fill it with air. Use a torch (Mapp Gas, propane, OXY-Acetalene will work but you have to be VERY careful not to burn through the pipe) to evenly heat up the dented area until it is red hot. If the pipe is hot enough and there is enough air in the pipe, you will see the dent begin to pop out. If not, boost up the PSI a little at a time. If the dent is in a large section of the pipe, it will be fairly easy. Keep working at it until the dent is rounded off. It may be difficult to get it PERFECT, but sometimes it is possible. Dents on a bend or a weld are the most difficult, but are still possible to repair.
Use your better judgement, dont try this on a flattened pipe, its a waste of time. Also, the more times your pipe is dented and repaired, the weaker it will be. This is mearly a method for REPAIR, not renewal... although in some cases this can work VERY well and can save money.
The instructions in this tech tip are SUGGESTIONS, use your own ability and common sense in the materials and method of doing this repair. Its impossible to explain EXACTLY what to do unless im there watching you.