We use 219HTM chain.
the number is an arbitrary number assigned to chains, so you have to look up the details in google
eg: http://www.did-oltd.com/english/products/pdf/s_102-103.pdf
There you will find that the pitch, the distance between the centres of a 219 chain is 7.774 mm.
Also, you will find that the plate height, the height of the plate at the side of the link through a pin centre is 6.5mm
So, to find out how many links in a chain
hold the chain with a middle link horizontal at the top (since chains are even number of links, this will give you a horizontal link at the bottom too)
then measure from the top of the top link to the bottom of the bottom link.
subtract 6.5mm and divide by 7.774
I have just measured a chain and it is 410 mm long
subtract 6.5 mm = 404mm ( to get the distance between pin centres
divide by 7.774 =51.96= 52 multiply by 2 because there are 2 sides= 104 add in the 2 horizontal links= 106
The chain is 106 links long ( I counted).
You can measure the distance between pins but I found it more difficult than measuring top to bottom and subtracting 6.5.
somewhere else there's a figure that says a chain is worn out when it has extended 1% . That's 8.25mm on a 106 link chain or 4mm on a side.
So if your chain measured 415mm, not 410 then it is ready to be changed.
Just for reference a 104 link chain would measure 403 mm (402.974 but I haven't met anyone who can measure that accurately with a tape measure) and a 108 chain would be 418.5 mm
You'll see that there is only 3 mm between a worn 106 at 415mm and a new 108 at 418.5mm which is why I measure over the top of the link rather than try to measure the pin centres.
Does that help?
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