Another article on this site covers maintenance issues that apply to 4-, 7-, 8- and 11-speed Shimano Nexus and Alfine hubs:

• Anti-rotation Washers
• Brake Types
• Cable Adjustment
• Cable Installation
• General Advice
• Hubs Covered
• Links
• Lubrication
• Sprockets
• Upgrading Existing Bikes
• Vertical Dropouts

There are also articles on this site more generally about

• Internal-gear hubs
• Internal gear theory
• Shimano product lines and buzzwords

Read those articles along with this one. For more-detailed advice on installation of sprockets and setting the gear range, see the section on sprockets in the main article on internal-gear hubs.

Nexus is a family of parts, of which the Nexus Inter 8 eight speed hub is one. The Alfine 8-speed hub is a very similar, higher-end model, with dual roller clutches and needle-bearing pinions.

The overall range of the Shimano 8-speed hubs is 307%:

For comparison, the Nexus 7-speed hub as a range of 244%:

The table below lists models in each series of Shimano 8-speed hubs in order from newest to oldest.

These hubs have a overlocknut spacing ranging of 132 to 135 mm, as sold; -- see Shimano's overlocknut spacing chart. It is possible to reduce the spacing by leaving off the Rollerbrake and using narrower washers.

These hubs have gone through several minor design changes, resulting in new model numbers and parts incompatibilities. Model numbers are in the form SG-8Rxx --: example, SG-8R20. The first of the two digits after the "R" represents the series, reflecting the number of roller clutches and whether the hub has cable shifting or electronic shifting. The greatest compatibility is within each series.

The right-hand column in the table holds links to available information about these hubs. Shimano has rebuilding instructions available only for one model, though others are similar. Several models are documented only by compatiblity information in parts lists for other models.

The internal mechanism of these hubs is complicated, and the usual repair is to replace it in its entirety. It can usually be replaced with a newer version as long as the hub is in the same series (with or without coaster brake; same number of roller clutches contacting the shell -- but also be aware of other minordifferences, which may be reflected in the design of the shell). Shimano indicates little compatibility with hub shells of different models -- see the last two pages of this document. I haven't had the opportunity to experiment, but I suspect that Alfine shells are largely compatible with parts for Nexus hubs that have two roller clutches contacting the shell, and vice versa. See this Shimano document for description of some internal differences. It shows a third roller clutch inside the mechanism of premium models, and having no effect on shell compatibility.

The right-hand hub bearing cup is vulnerable to damage from water contamination. Oil lubrication is recommended to forestall this problem.and to make cleaning and rebuilding easier. The bearing cup is listed as a replaceable part in some parts lists. The old one would have to be extracted and a new one pressed in. If you succeed with this, please send advice, because that repair can save a wheel!

Most parts lists include some compatibility information, and many are formatted similarly so that you can compare them visually (like an astronomer looking at two sky photos in a "blink comparison" when searching for an exploding star or an asteroid).

Ideally, the frame should have a drop-out spacing of 127 to 135 mm to fit one of the Nexus hubs. Older bicycles are typically narrower than that. In the case of steel (Cromoly) frames, it is usually possible to spread the rear triangle to the needed width. Any good bicycle mechanic will be able to do this, or you can do it yourself.

If you leave off the Rollerbrake ®, you can get the spacing down to about 126 mm. This is what I did on my Nexus bike. The Rollerbrake ® is a separate module, and when you buy the hub it is not even installed. (The brake unit would get in the way of fitting the spokes through the hub flange, so it has to be installed after the wheel has been built.)