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On 4/17/2017 at 1:56 PM, grayracer513 said:

You have your old chain, right? Roll that into place and see how much longer/shorter it needs to be.

This may, or may not, be such a good way to judge the length of the new chain. If the old chain was really worn out, or there was a big change in the number of teeth in the sprockets this technique may not work. 

If the new sprockets are the same as what came off just count the links on the old chain and go with that. This is assuming that the old chain never had a link removed to compensate for wear/stretch. That should never be done, but people do it in ignorance. 

What do is to mount the new sprockets, run the rear axel adjustment all the in, and run the chain around the drive sprocket and through all the chain guides and stuff. Have one end of the new chain engaged with the teeth of the rear sprocket and bring the other end around and decide how many links need to be removed to have the chain fit right. Remember to err on the side of of a bit to much slack on the chain, because you can correct that by adjusting the rear axle once the chain is on.

TL/DR. Using an old chain to judge the length, not link count, of a new one is not a good idea. A worn chain easily be a couple inches longer than a new one with the same number of links.  I've seen more than a foot difference in length on some of the industrial equipment that I repair professionally. 

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4 hours ago, HappyAndy said:

This may, or may not, be such a good way to judge the length of the new chain. If the old chain was really worn out, or there was a big change in the number of teeth in the sprockets this technique may not work. [...] A worn chain easily be a couple inches longer than a new one with the same number of links.  I've seen more than a foot difference in length on some of the industrial equipment that I repair professionally. 

That's true to an extent, but consider that there will be roughly something about 60" of chain involved, and the chain should be replaced at or before being worn 2% longer than new.  That would be 1.2".  If we allow for deferred maintenance on the order of negligence and use 4%, that's 2.4" and with a 428 chain (1/2" pitch), that really could be over two full links longer than it should be.  Most of the time, you run out of axle slot before a chain wears that much, but you never know. 

If you own a chain breaker, the simplest approach is to buy the longest available chain in that pitch, fit it on the new sprockets, and cut the excess off, or mark it and have a shop cut it.

If you know the original number of links in the chain, you can calculate the length delta by using the chain pitch and the difference in the tooth count of the sprockets to determine the necessary size pretty accurately.  This can be done by measuring the linear run from one sprocket to the other (top and bottom distance may be a little different), Multiply that length by 0.5" (for the 428 chain), then count the number of roller cutouts (between the teeth) on both sprockets that the chain will run over, and that's your pin count.

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