I just bought an FMF Fatty Pipe and Power Core II silencer for my 86 250R. The silencer slips on to the pipe. Do I need any type of seal, or will it not leak? Thankls

 

Yes,you need a seal.The most common way to go is a clamp type seal. It uses rubber o-rings inside and it tightens down over the joint.You can purchase one through Nac's racing.I think they are around 40 bucks.
You could also get one from your dealer that slips over the joint and is held tight by springs.
This type is considerably cheaper,but will not last as long.Using zippy ties along with the springs helps,but the rubber is still prone to ripping,resulting in leaks.I don't recommend the putty type sealants available,because they tend to get hard and difficult to remove when it's time to replace your silencer packing.

 

High Temp(orange) silicon works great for a cheap seal. If you try this, put it on the MALE pipe(exhaust pipe usually). If you put it on the Female side it tends to be pushed into silencer inlet pipe and is blown out. Wipe off any excess and allow it to fully cure for 24 hours and you have a seal that is cheap and works. I've used it a year before I went to a clamp, however I STILL seal the pipe with silicon(under the clamp) as described.