I bought a Park chain checker to see if my 1998 Trek 720 chain needed replacement. It isn't showing any signs of wear. When I checked it at the .75 measurement it came out like a new chain( I do good maintenance). When I checked my 2009 Trek 820's chain it was way past the scale and it(the checker) completely sunk in. According to the instructions, that chain is shot. I know that can't be. What gives? Thanks in advance

Use a tape measure, measure 12 inches of chain from center of pin to center of pin. Is the distance between pins exactly 12 inches? how far off is it?

If center of pin to center of pin is more than 1/16th of an inch off from the 12 inch mark then it is time to replace the chain.

How old are the chains, how much use, what kind of use. The make and model of the bikes, and even the year of the bikes is irrelevant. The chain can wear out quickly or slowly depending on usage. The ruler method is the "definitive" method of checking chain wear, but I don't know if the chain-checker gives false positives or false negatives. It will be interesting to see how your measurement compares to the chain checker.

I have one of the chain checkers and it works fine but I also check the chain with a ruler. I also measure between the 1' and 2' marks since my metal tape has a little play in the tab and I am not sure how far off the measurement will be.

Trust the tool, not ones impressions.

FYI - metal tapes tabs are supposed to be loose. it should move the thickness of the tab. This way when you hook the tab over an edge it will measure the same as if you but the tape into a corner.

Park Tool make a couple different chain checkers and I`ve actually checked one against the other just for fun. Personally I prefer the simpler 75% / 100% model. Basicly they both measure the distance between a number of links and if either checker indicates your chain is past the 100% wear mark - you better believe it.

There are two ways to accurately measure chain wear:
a) Shimano TL-CN41. (overpriced)
b) A ruler.

Other chain checkers are junk.
Explanation here: https://pardo.net/bike/pic/fail-004/000.html

+1 This is a problem everywhere. It seems like such mundane knowledge that people who know don't mention it much, and people who don't never find out.

Great link! Thanks

Read that article and personally found no practical information in it. The concept of seperating one kind of wear from another exists only in the lab. In the real world its impossible that only one kind of wear happens in a chain and due to the way a chain operates pin/bushing wear is far more pronounced.

Tools are by definition `labor saving devices` and have and always will be partly dependent on operator skill to be most effective. I regularly use a Park Tool chain checker to visually demonstrate to a customer that their chain is finished. When the tool will easily drop into the chain at the 100% wear mark it really doesn`t matter if there`s a 2% error in the tool. If the bike is in for a tune up and its the beginning og the riding season it would be stupid to suggest that they come back in a few more weeks.

But if you want to consider chain tools inaccurate - suggest you have another look at rulers too. Having spent over 15 years in high tech labs - = can tell you for a fact that there isn`t a `ruler` on the market that would be accepted by any lab as an accurate measuring device either. All commercially available `rulers` have manufacturing errors `built in` and materials can also add additional errors depending on material and ambient temperature and humidity.

Its fairly common to use a Vernier instead of a ruler in the shop because its more accurate. However, as far as chains are concerned - the most common practice (since there is a large amount of new chain available in bulk) is to use a length of new chain as a reference. The most common chain seen in the shops here is still 5/6/7/8 speed width and retails for $19.99. A freewheel or cassette is more expensive and any time evaluating wear and any possible need for replacement is usually spent there.

That same logic would also apply to a 9 speed or 10 speed drive chain.

Author's claim that chain checkers are innacurate is based on the assumption that roller wear is not relevant; and thus adds innacuracy to checkers which measure based on roller position.
On what do we base the assumption that roller wear is not relevant?

Seeing as how the rollers are the primary(only) interface between the chain and the sprokets, I suggest that roller wear is in fact quite important, and that there is nothing wrong with chain checkers which use the rollers as their contact points.
In fact, they take into account wear which a ruler measuring the pins from an external-only view does not.

Is it a good assumption that the roller wear uniform for each roller? If roller wear is uniform, and if the pin-to-pin distance is correct & not 'stretched', then the roller distance will be correct too. The driving force is applied or imparted in only one direction per cog & the rollers will all ride on the same side of the (worn) shafts for each cog.

Now, modern chains don't have roller shafts. The rollers ride on indents of the outside side plates. Not sure how this affects things in a practical way.

Don't you think Park Tool has more expertise than any tinkerer posting on these forums? Trust their tool. Do you have an 8 speed chain on one bike and 9 speed on the other? If these are mtn bikes used in variably nasty conditions, you are not going to get the consistent wear you would on a road bike.

If you haven't been checking the chain some other way regularly, then do you really have a reason to doubt the tool? I've found the Park checker to not be terribly accurate, but it works for my purposes, which is to quickly check whether a chain is worn out. I don't care how worn out it is, it's just a yes or no question. If the chain isn't obviously fine on the Park tool, then I can assume it is worn out. The actual error in the tool is so small as to be insignificant unless you are trying to put a precise number on the wear.

Wow, lotta ruler haters in the house.

The way I see it a roller checker primarily tells you how noisy your chain is likely to be. Measuring the length of a tensioned chain over 24 pins will tell you how far apart those rollers are allowed to sit when tensioned. The longer this distance is the greater the likelihood of greater lateral flex in the chain which is often a culprit in poor shifting and the increased distance between pins on a tensioned chain can lead to increased chainring and cog wear.

If you're worried about the calibration or dimensional stability of your ruler get a good 15" metal rule with 1/64" increments for part of its length and measure your new chain under tension at a certain temperature. Make a note of your measurement and ambient temperature. Make future measurements under tension, at the same temperature and compare to your new chain findings.

Those chain roller dealies probably work just fine also. Seems like it would be tough to totally bore out your rollers without incurring a certain amount of pin-to-plate interface wear. I'd suggest testing new chains to establish a baseline when using roller checkers also as I've heard reports that certain makes or models of chain seem to test as partially worn even when new.

I reckon the OP would have a lot more faith in the roller checker tool's results if he'd had a baseline measurement for his 2009 Trek's chain when new.