HG stands for Hyper Glide, IG for InterGlide. Shimano first came out with HG chains in '91. In '94, Shimano went from 7 speed to 8 speed and in '95 they introduced the IG drivetrain. The new IG replaced the old HG on the upper end Shimano lines. IG chains were designed to run specifically with IG chainrings and cassettes. In '99, Shimano introduced the 9 speed drivetrain (or Mega 9). This included a new narrower chain, chainrings and cassette and they brought back the HG chain prefix. Got all that?
The main difference between IG and HG is the chain width. IG chain is 7.1 mm wide, HG 6.6 mm wide. (The old HG is 7.4 mm wide.)
Today, HG means 9 speed drivetrain. IG is pretty much for 8 speed drivetrains. As for the numbers that follow, the bigger the number, the better the chain (supposedly). For example, the HG-53 is Deore, HG-73 is LX and HG-93 is XT. The XTR chains are completely different. They start with CN-7700.
According to Shimano, you can run any IG chain with any HG cassette. But, as for the chainrings, " The 9-speed chainring teeth are reprofiled for narrower chains, therefore you can not use an 8-speed chain [on Mega 9 chainrings]".
Also something to consider: The mega 9 front derailler cage is narrower than an 8 speed cage. An 8-speed chain will not fit in a Mega 9 front derailler cage and a 9 speed chain is too narrow for an 8 speed front derailler, causing poor shifting between chainrings.
I will assume you are replacing your 8-speed cassette with another 8-speed cassette? If so, you will want to use an IG chain. If you go with a 9-speed cassette, of course you will need a 9-speed rear derailler and shifters if you want to use all nine. And then, to run a 9-speed chain, you will need 9-speed chainrings and a 9-speed front derailler. It's pretty much all or nothing when upgrading to 9-speed.
Of course, this is all Shimano talk. There are other companies that make chains that work with Shimano parts and vice versa.
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