Here is my standard chain blurb FWIW.
Chains wear and get longer as they do, and often wrongly referred to as stretch, which implies some form of elastic recovery, whatever.
A good wear indicator is pulling on the chain, away from the rear of the rear sprocket. If you expose a half tooth or so, it’s probably time to spend some $s
Chains can have tight spots, or parts that have worn less, but it can also be an out of round sprocket that is the problem. Adjust when tightest, but if the tight spot moves, its a sprocket problem.
Alternatively, or the same depending on your terminology and perception, a chain can have loose spots. They're usually from a link or more wearing more due to a loss of lubrication. These are the weak links that will encourage a chain to break when the pins wear/stress excessively.
Tensile strength is a chain spec, but not necessarily and indicator of life. Other steel properties like ductility and hardness are more relevant, and not readily quoted in specs.
Sealed, O ring type chains are not completely sealed. Chains wear firstly between the pins and bush, which is sealed by the O ring, but then the roller spins on the bush and it’s not sealed. The roller then contacts the sprocket and it’s obviously not sealed either.
When lubing a chain, lube it on the inside of the run aiming towards the side plates each side to encourage lube to get down between the roller and pin. Then lube is centrifuged out between the side plate and rollers, and then some gets into the roller bush area. Lubing on the outside means some lube gets between the roller and sprocket teeth and the rest gets flung off.
If you don’t over lube your chain, cleaning them is unnecessary.
And even if you think cleaning a chain is a good idea, more often than not, and depending on technique, it promotes wear by forcing crud down past the side plates and between the rollers and pin, and can damage the O rings too.
Changing out a counter shaft sprocket at least 3 times to a chain and rear sprocket is a cost effective way of increasing the total life. Cs sprockets wear faster because they have about 1/3 the number of teeth to deal with the same transmission loads as the rear sprocket. They are in effect the weakest link.
Use the biggest cs sprocket you can, more teeth and bigger diameter equals less wear.
Over tightening of chains is chronic in my experience. Regular tightening of your chain likely means you have it too tight to start with. An over tight chain wears it and the sprockets prematurely, and puts extra loads and therefore wear on the wheel bearings, countershaft and countershaft bearing, and compromises the rear suspension operation as the loads are transferred to the engine cases instead of the spring and damper.
Chain adjustment and tension is not dependant on load, just the geometry above. Chains should be adjusted when all 3 pivot points, ie the countershaft, swingarm pivot and rear axle are in line because that’s when the distance from the axle to the countershaft is greatest.
Modern chains rarely need adjusting. Once they do, its normally time to spend $s