How often have you had to put a chain back on in the middle of the road? If never, this article isn't for you. As the owner of an Alpha moped, I've had to do it more than once — and not only on this moped, though there were other reasons then. Talking to other owners of similar mopeds, I realized it's not unusual. I want to share how I dealt with the problem — maybe it'll be useful to beginners. In my case, the chain coming off went hand in hand with the thread being stripped on the tensioner on the chain side.
I changed the tensioners and tightened the nut on the rear wheel axle. Nothing helped. As soon as there was any load (hard acceleration, a steep climb), it all happened again. After a careful look at how the rear wheel is built, it became clear that neither the wheel axle nor, especially, the tensioner has much effect on chain tension. The axle keeps the wheel from radial play, and that's not enough to stop the wheel from moving back and forth under chain load, changing the chain tension.
The tensioners can't take that load either: they're only there to align the wheel and tension the chain when there's no load. All the load is taken by the rear sprocket half-shaft, or the rear wheel bushing. It holds the sprocket hub in one position and keeps it from shifting. It's the large nut on the left side of the wheel. If it isn't tight, all the load falls on the tensioners, especially the left one. Naturally, such thin tensioners can't withstand that kind of pulling load, and the thread gets stripped. The whole problem lies in these bushings. Most often, they're made of plain steel at the factory — I've even come across aluminum alloy ones.
Add to that a fine thread pitch, and you get a very low tightening torque. Given the length of the wrench for this nut, stripping the thread is all too easy. Some may think it's no big deal — just tighten the axle nut harder and everything will be fine. But as experience has shown, that won't help. So it's better to simply replace this bushing. It's best to buy hardened steel ones — they're black. They've been tested on different mopeds and have proven themselves well. This way, you'll protect yourself from the chain suddenly coming off on the road and getting damaged, and most importantly, you'll save your time.