I have a 21 speed low end MTB (Mongoose/MGX S20) which serves my needs perfectly but now the chain slips when I am in the largest chain ring and climbing a slight incline. I thought that the chain was stretched and reduced it by 1 link and it made a very small but negligible improvement. Am I looking for a new crankset? I measured the chain and the pics are attached. In 6" there seems to be exactly 12 links to/from the edge of the pins. The middle and smaller chain rings work fine. The teeth on the large rig do look worn.

If your chain is worn removing links will do almost nothing to reduce slippage. Find a tutorial explaining how to measure chain wear with a ruler (an easy Google search) which will tell you if yours is actually worn. The chainring teeth look to have some wear but if the chain is done for a cheap new one may solve your problem. There is also the possibility that the rear sprockets may be worn and will also slip with a new chain needing replacement as well but first determine if your chain is worn. If you have a Co-op nearby probably worth a visit to sort everything out without spending a load of cash.

Make very certain that you have not shortened your chain too much; it must still be long enough to engage the big chain ring/big rear cog combination without strain. Check this by gently turning the cranks by hand and stop if anything gets tight.

new chainrings chain and freewheel/cassette all at once?

Chainrings have to be seriously worn before they will cause skipping. Yours doesn't look that bad to me yet. The teeth look real pointy when badly worn.

Since you've been messing with the chain, I'd check for a tight link if, for no other reason, it doesn't cost anything.
Prop your bike up and turn the crank backwards with your hand while watching where the chain exits the derailleur. If you have a tight link you will see the derailleur arm hiccup as the tight link exits. Loosen up that link and you'll be good-to-go.

If you haven't just put a chain on it, then get one. If it still skips, then get a new chainwheel or entire cranksets. Might be a good excuse for a new bike. But still get the old one fixed. Never can have too many.

It's on the cogs in the back where the skipping happens, most often on the smallest cogs,

but skipping can happen from just the derailler being out of adjustment, so that's the first thing to check.

Replacing the cassette or freewheel is next, but mismatched wear between chain & cogs frequently causes skipping,

so replace chain as well. Chainrings last a long time- unlikely that's the issue.

The freewheel or cassette hub can also gum up and cause skipping although that would probably happen on all chainrings.

I checked the chain for stiff links and there are none. The chain length seems to be correct too, there are 12 links in exactly 6 inches indicating not much stretch.
I rode the bike yesterday and it felt like the freewheel pawls may be slipping under strain and not the chain. What I mean by chain slipping is that when the bike is pedaled hard, in the largest chain ring and in almost any rear sprocket and on a slight incline, there is loud cracking noise and the pedal moves forward without any resistance for a few inches and engages again. I am clarifying so that the symptom is not mistaken for unintentional sideways movement of the chain which is not the case. Is it possible for the pawls to give way against force beyond a certain limit, which may be happening with the largest chain ring engaged?

I guess I will have to verify by swapping the rear wheel.

Looking at the last picture, your measurement looks like it is stretched. Your zero is not nearly on center with the pin, nor is the 6, or pin edge to pin edge. A new chain is relatively cheap. I'd also say that your chainrings ARE worn. Just replace the big ring.

I am going to do exactly this on my commuter when I get a chance as I also experience skipping, and also have had the chain completely slip off the front a few times. I just need to look for a new 48t 110bcd ring, which shouldn't be hard to find as they were pretty standard.

I agree with this. You should have exactly 6" between 12 pins on the chain. Your 6" end on the ruler is lined up pretty good on one pin, but the 12th pin, counting down the ruler, falls way off the end of the ruler. It should line up exactly with the 0" end of your ruler. You can see by 3" or even 4" on your ruler, your pins are not exactly in line with the mark. Unless I'm interpreting your photograph incorrectly, I'd say that chain is significantly worn.

Edit: you normally measure 24 pins across 12" to get a more accurate measurement of chain wear. You can see (again, if I'm interpreting your photo correctly) significant wear across only 6". It looks like 12 of your chain pins measure about 6-1/16" or so (or maybe even 6-1/8"). 24 pins would probably measure between 12-1/8" and 12-1/4", which is way beyond the acceptable measurement. Bike Radar (below) says that any chain with a measurement of longer than 12-1/16" should be replaced, and anything beyond 12-1/8" is worn to death (their words), and a new cassette or freewheel is likely needed as well.

https://www.bikeradar.com/us/road/ge...plained-46015/

Bike shops around me are always willing to let the mechanic look and give advice for free, in hopes you'll let them fix it and just for the good will of keeping you coming to the shop. Will your shops around you do that? They'll check the chain and slap a new one on pretty cheap I'd bet if that's all you need. You still might need a cog too, but if that is also worn then the chains definitely gone if it hadn't recently been replaced.

Your issue sounds a lot like what i had a few months back. Going up a hill with added tension, the chain felt like it was slipping. i too thought it was my chain, and like you i did all of the measurements with a ruler, ( i even used a chain gauge tool), but they all indicated no chain stretch. I finally brought it in to the shop and they gave the derailleurs a slight adjustment. The problem wasn't chain slipping, it was chain moving down a gear. My problem went away immediately. Other than the "slippage" i experienced on hills, the gears shifted with no problem whatsoever, so there was no reason to think the gears were out of adjustment. But that was definitely the issue, because it didn't happen anymore after the adjustment.

Not saying this is your problem, and i agree with others, your chain looks a little stretched, but it's something worth looking at if you haven't already.

After a thorough cleaning of the rear derailleur and chain and reinstalling them, the problem has minimized. It must be mainly due to the RD pulley ceasing plus a couple of stiff links.

Now, I tried to replace the front shifting cable as it was frayed at the pinch point and find that the inside of the TDQ-PRO HS-220 grip shifter seems to be wearing out as there was black powdery stuff inside. The front D. did not shift well, too. If I adjust the cable so that it shifts to the largest ring, it would not go down to the lowest ring and vice versa. Looks like the cable is not pulled along its intended helical path but along a new worn out path in the grip shifter. Is this a known issue in plastic bodies shifters and need a replacement now? In that case is this Microshift Grip Shifter a good replacement ? I cant find TDQ-PRO brand anywhere, seems to be out of business.

Are you adjusting the DR correctly? You need to have your high adjusted so the jockey wheel is in line with the high gear before attaching the cable. It's not a final adjustment, but it assures you'll get the cable attached at a point where all other adjustments can usually be accomplished with the range of adjustment allowed by them.

If you aren't certain about how all the pieces parts work together and how the adjustments affect all the others then you really need to get the manual for your model and go through all the steps in order.

I replaced the front shifter yesterday and it rides mush better in the largest chain ring without almost any slipping. Backpaddling still shows issues with the chain or pulley and will be looking at that.

The chain is stretched, more than 1/8" over 12".

1. The reason it slips on the large chainring is usually because it overall has higher gears than the other two, requiring more pedal pressure at a given speed. Your description of a large amount of travel before catching does make the freewheel/freehub suspect. Those chainring teeth are nowhere near worn enough to cause skipping.
2. 6" is not long enough to determine wear, especially if the chain is not under tension. 12" is the norm.
3. Not a good idea to be on the large chainring on a slight incline unless you are in one of the largest rear cogs, which I doubt is the case.
4. I would suggest you plan on a chain and freewheel if you have any further slipping under pressure. Once skipping starts usually wear on the small cog(s) is sufficient to cause skipping with a new chain.
5. It's unclear what you mean by "issues" with backpedaling, but if the chain is drooping when you backpedal that is often due to excess friction in the freehub/freewheel. Coupled with possible slipping when engaged I would say you need to flush it out.

You can see from his photograph that it's stretched almost the width of a pin over six inches, and the pins appear to be about 1/8" across. At least that's what it looks like to me, and it would mean that his chain is stretched enough to warrant replacing.

+100 Chain is worn out. Freewheel will wear out next. Crank chain rings will go last. I buy 5/6/7 speed new chain at a nearby co-op for a whopping $4.