I've read Sheldon Brown, and either I'm confused (entirely possible) or I've been sold a load.

I've got an old steel typical Japanese bike- Tange Infinity, Shimano 400EX derailleurs and crank. The Hubs are Exage Sport. I think this is all 1989. Its 52/42 up front, 13-26 HG in the back.

I was wanting to get a little more gearing, I'm an old fogey and a little overweight. On the flat or even small hills, I'm fine, but major hills take it out of me. My companions all have compact front cranks, and that is an option, but I thought I'd try a bigger cassette on the back before I try a compact crank. I'd read that most Shimano RDs might be rated at 28 cogs max, but could probably handle 30 without an issue.

I called around a few local bike shops with varying degrees of success. One place told me flat out it couldn't be done, which of course I knew was false.

One place said sure, no problem, bring it in, quick fix and you can be on your way.

So of course I went there.

He proceeded to tell me that there was an older HG 7 speed and that unless I could find NOS I was out of luck, unless he rebuilt the hub. He explained that if my hub had been one of the higher end hubs like 105/600 or Dura Ace from the same era, I would have been fine. Something about the locking mechanism. Because of the mixup he agreed to charge me for the parts and not the labour.

I had just taken in the wheel not the whole bike, but he was ready to adjust the derailleur for me.

I adjust the limits on the derailleur and upside down, the bike seemed to shift fine.

But on the road, the chain jumped around and the bike was unrideable.

Went to a nearby LBS near where I was trying to ride, a Cervelo dealer. He was baffled, adjusted the tension, suggested it might need a new chain.

I went back to the original LBS with the whole bike. He found he needed to put a washed on the hub or the cassette was too close to the dropout. We also put on a new HG chain.

I took it for a spin around the neighbourhood and all was well.

I'm still confused by the "two" different kinds of 7 speed HG though. I've not seen any of that in the readings I've done.

Shimano had HG "Hyper Glide and also IG "Interactive Glide". Cogs are thicker on the IG cassettes, but the spacing is the same. If you have a standard Shimano freewheel, nearly any 7-speed cassette should work. It could require a small shim spacer here or there depending on individual fitment.

Or maybe you have something very specific to a certain few years in the 1980s. The Shimano HG pattern is pretty much the standard now, but there was a transition period around about the time your bike was built when even Shimano weren't quite sure what they really wanted to do...

The old cassette and new are both clearly stamped HG. But he did have to change the dish on the wheel when changing the freehub, and added a spacer.

Maybe he put on an 8/9/10 speed freehub (which he had in stock) instead of the 7 speed one (which are more rare)? That would explain the need to redish the wheel and add a spacer (washer) to the freehub. Just a crazy random guess.

Sigh.... I you'd dropped into our local bike Co-op while I was wrenching, I would have done the following:

1. Pull apart your existing 7-speed cassette and split it into the spacers and cogs.
2. Then reach into our bin of loose HG cogs, and access a 30. Cost: $1.
3. Then replace your current 26 with the 30.
4. Put everything back together.
5. Replace your chain: because statistically every bike that comes through our doors needs a new chain. Don't even need to measure.
   

Total cost: $14 in parts and 15 minutes of shop time. Our Co-op charges out time at $5-15 per hour, based on your ability to pay.

Any chance the rear hub was originally UniGlide? HyperGlide came out in 1989, and perhaps it had not "trickled down" to Exage by the time the OP's bike was put together. A UG hub would require a freehub body swap to use HG cassettes, which is pretty easy.

[MENTION=367511]JamesRL[/MENTION], a 39T inner ring can be installed on your Exage Sport crank instead of the 42T, if you want to drop your lowest gear a little more.

Possible. If the freehub was Uniglide, this would have added to the 15 minute upgrade job, in that I would have had to allocate an extra 5 minutes to grind the single wide Hyperglide spline off of the new 30-tooth HG cog.

You can actually go down to 38t chainring on 130 PCD crank. It'll have to be a round non-Shimano ring though.

Good call, I've used a 38T Sugino with much success. I debated whether to recommend that or a 39T, since it's much easier to find cheap used 39T rings in good shape.

That is a good point, and I'm not in love with BioPace.