Maybe I can clear up the aluminum bolt deal a little. It is not done for weight reasons. Those bolts shave maybe an ounce off the overall weight. It's done to help stabilize the thermal expansion stresses on the engine. When the engine heats up it obviously expands. With steel bolts the stress on the threaded holes holding the cylinder to the case is higher. Steel bolts will also somewhat limit the expansion process of the cylinder but the rod and piston assembly expands to full potential thus causing an increase in compression that could lead to bad things. When cold the lower rod bearings are a tad loose, this is to allow for heat expansion as well.That's the theory at least. How ip plays out in reality I couldn't say, all my DA engines had aluminum bolts their whole life as well as the two 3W engines I have owned. To the best of my knowledge all DA engines still have aluminum bolts. I have heard of a few cases of them breaking, can't comment to why as I don't have any info. Richard is correct about the muffler. A stock muffler does not fit everything so by purchasing your exhaust separately gives you the option of what suits the airplane best. Again we can go back to the fact most IMAC competitors use DA engines and also use canister exhaust systems. As the saying goes, what counts is when the rubber hits the road so here is a video of my 14 year old worn out DA 150



https://www.youtube.com/watch?v=GQTLS6l28fo