hokiefyd 

I, too, shall bump this thread, as I've found it on Google a few times (so I imagine others must see it often as well).

I've used all four of these KMC chains on various bikes, and I've often used multiple model numbers on the same bike with the same equipment. Not because I'm wearing them out, but just to try and see how each of them works. All of my stuff right now is 7/8 speed and my most expensive chain is an X8.99 and my least expensive chain is a Z51. I also have a Z7, a Z72, and an X8.93.

The price difference between them all is minimal, and I will say that I do like the "polished" appearance of the X-series. The X8.93 actually looks a touch better to me because of the contrast between the shiny nickel outer plates with the standard grey inner plates. Appearances aside, I have perceived zero shifting performance benefit. In fact, I would say the cheaper Z-series chains shift at least as good as their more expensive X-series counterparts.

The mushroomed rivets on some of these makes a trivial task of sizing the chain a real bear unless you have a real sturdy chain tool. In fact, I about ruined a portable chain tool I have working on a mushroomed rivet. Wrong tool for the job, yes, and I don't blame either the tool or the chain. I say that only to advise one to have a stout chain tool before buying one of the mushroomed models.

The 7.1mm Missing Link fits the Z51 and Z72 models, and also fits two SRAM 8-series chains I have. The 7.3mm Missing Link fits the Z33, Z50, Z7, Z8, and X8.xx models. I think Shimano chains can use the 7.1mm Missing Links also (but I'm not 100% positive).

ThermionicScott 

The X8.93 and X8.99 claim "stretch-proof treatment" whereas the cheaper Z51 and Z72 chains don't. My hunch is that "stretch-proof treatment" is a reference to hardened pins, which is what separates SRAM's cheapest 8-speed chains (PC-830) from the longer-lasting models (PC-850 and up.) Sometimes you come out ahead on miles/$ by avoiding the very cheapest chains you can find...

hokiefyd 

That is my hunch, too: the "stretch proof" KMCs use chromed or otherwise hardened pins. The KMC X8.93 seems to be the best value going -- it's $10.60 on Amazon, Primed to your door in either 1 or 2 days depending on where you live.

3alarmer 

.
...three years later, and I'm still a fan of the X 8.93 in bulk packages of five.
It's like the glory days, back when you could buy Sedisport chain in bulk rolls for cheap.

canklecat

Over the past several months I've used both the KMC Z51 and Z72 on my older road bike with indexed Suntour 7-speed shifter and freewheel. No problems, can't hear or feel any difference between 'em.

In an earlier post a couple of years ago another user mentioned the mushroom head rivets on the Z72 can be difficult to manage for some chain tools. No problem with my chain tool, but it's a shop grade version. Might be different for the chain tools found in some multitools.

I don't anticipate any problems but do carry a spare Missing Link in my saddle bag. After several disassembly/reassembly cycles, the Missing Link will loosen up. I've worried the Missing Links may pop off during a chain drop -- which happened a few times when I switched from a 42T to 39T chainring before adjusting the front derailleur. But so far the Missing Links have held, even though they're loose enough that I can disassemble them with just my fingers.

kcblair

Until my LBS retired, he only stocked KMC Z51 chains for my 6/7 speeds. I have been using them since about 2000. Never had an issue. At $8 a pop, I usually changed them around 1,800 miles, and usually before they reached 12 1/16 in. KB.