ORIGINAL: Kman1971
its an os 91 surpass II its hitting where the muffler threads into the engine and then there is a nut that tightens against the engine...that nut is touching and there is not a way to get it to not short of a hole there is actually about 1/16 of an inch between the cowl and the nut but it gets hot enough so that it is causing a hole.
4 strokes exhausts run substantially hotter than 2 stroke exhausts. You need a MINIMUM of 3/16" clearance (1/4" is better ) from ANY part of the exhaust to the cowl. Less than that and heat and vibration will melt and/or crack the cowl.
If the cylinder head sticks out through the cowl you need a MINIMUM of 1/4" for engine cooling. If the engine cylinder/head is over 50% covered by the cowl you need additional cooling to let the hot air out. Use a 1:3 ratio for this. For every 1" of air INLET (air coming in ) you need 3" of opening for the hot air to get out. Without this there is an extremely high probability of the engine overheating. I realize that in a lot of cases this does ruin the appearance of the plane, but that is the price we pay to fly certain types of planes.
To minimize any cracking and reenforce the opening, after you enlarge the opening, take some 2 oz fiberglass cloth, and using THIN CA, glue it to the inside of the cowl. Trim any excess after the CA dries. I also suggest doing this around the outer edge where it attaches to the fuselage (minimizes/prevents the attaching screws from cracking the cowl.
Hope this helps.