First of all, I would not use hardware store bolts on anything. For metric use at least grade 8.8 or better if you can find them. for SAE bolts use grade 8 or above. As for screwing bolts into aluminum, you have to be careful about cross threading them, and over tightening them. But a stronger bolt will not do any more damage than a weak bolt. A high grade bolt cross threaded or over tightened into aluminum will strip out the threads in the aluminum. With a low grade bolt, it will strip out both the aluminum and the threads on the bolt. You just have to be careful with aluminum. My preferred way of threading bolts into aluminum in high stress locations is to use steel inserts if there is room. Drill out and rethread the hole, then install a HeliCoil or TimeSert. Then you have steel threads to screw the steel bolt into. This is especially important if the bolt is going to be removed and reinstalled frequently. Aluminum threads are not only easy to strip out, but they also wear down a bit every time you remove and reinstall a bolt.
As for those "tensioners" that come with the kits, they are pure mung. From a design and metallurgy viewpoint they are extremely dangerous. First of all, a chainstay is round. It does not matter how tight you clamp something to it, it can still turn, right up to the point where it is tight enough to crush the frame. But crushing the frame is unlikely, as those tensioners are made of such soft material (it is magnetic, but I would hesitate to call it metal) that they would be destroyed first. Also, chainstays come in a variety of sizes, those tensioners only come in one size. I have heard people say to drill a hole in the frame and put a screw through the tensioner and the frame. That will dangerously weaken the frame. You could weld it, if the soft stuff the tensioner is made of is weldable, but it is so weak that the weld would probably break loose. Even if it didn't, the tensioner is so weak that it will bend if any force is applied to it (like a motor driven chain) And even if the soft tensioner bracket didn't bend, the cheap white plastic roller will break. It may have bearings, but the plastic is brittle, and will not hold up long with the chain beating on it. Next, that pulley is not lined up correctly with the chain. The chain runs straight from the engine sprocket to the rear wheel sprocket. But the chainstay (and anything mounted to it) will be at an angle to the chain. And next (yes, there is still more) the tensioner is not spring loaded, so the roller cannot move up and down with the chain. That causes the chain to "whip" on the cheap plastic roller) It does not matter how straight the sprockets are, there are always going to be tight spots and loose spots in the chain. Even if the sprockets were perfect, and they are not, the chain itself is not perfect. The tensioner needs to be spring loaded to maintain constant tension on the chain as it turns. Just watch a spring loaded tensioner move up and down as the wheel turns and the chain moves.
Those crappy chain tensioners, or idlers, or whatever you call them WILL fail, they have to. They were designed to fail, whether the designer knew that or not. It's not a question of if they will fail, it's a matter of when. That can vary with speed, how well they are lined up with the chain, how tight they are on the frame, how tight they are against the chain, and how uneven (tight spots and loose spots) the chain is. But one thing is fairly constant. WHEN they do fail, they almost always wind up in the rear wheel, locking it up, and causing a crash. I know this from experience because it happened to me. In a really bad spot. It was after this that I really started looking at this part, and realized just how many problems it has.
If I built another chain drive bike, and I'm planning one right now, not because I need it, I have a perfectly good friction drive bike. But after seeing all of the problems with these kits, I want to build one to see if I can solve most of those problems without having to "rig" things. It will likely be a 4 stroke, as nobody builds a quality 2 stroke. I want a bike I can ride 100 miles at the time, at 20 mph, with no problems, nothing breaking and nothing falling off. It will also have to be strong enough to support my 240 pounds while riding on the rough shoulders of rural 2 lane roads without breaking spokes and bending wheels.
