How to Open a Shimano Nexus Hub Gear

It's actually relatively easy to get into one of these hubs.

The 'maintenance manual' images accompanying my description here are copied from the truly excellent Sheldon Brown's web site. He has the full service manual online. It's actually for an older, non-premium, model (SG-8R20), but at least the first few steps follow the same dismantling sequence. Obviously, I can't guarantee that any particular hub you have will be exactly the same, but steps 1 to 10 work the same for both the SG-8R25 and the SG-8R36 that I have tried (shown here), and I've been told they work for an SG-8R30 as well.

Dismantling

1. Remove the wheel from the bike, setting aside the wheelnuts and lugged washers (remember which colour washer goes on which side, though there's a table on these service instructions from Shimano if you forget..
2. Remove the shifter connector assembly - put the hub in lowest gear, then twist the lock-ring so the yellow dots line up and lift it off. Then lift off the changer assembly thing. It's the reverse of steps 3 and 2 in 'Installation of the cassette joint to the hub' in the same service instructions.
3. Remove the hub brake by undoing the ordinary nut on the axle and set nut and brake aside (beware that the brake innards are typically very dirty - if you've used Shimano grease, it's full of very thin very black grease).
4. For a proper job, prise the plastic ring that's just outside the sprocket off. It's just a push fit, and isn't too tight. Make note of what way round it is, remove and set aside. This is step 1 on the same section in the service instructions.
5. The sprocket is restrained by a tight snap-ring in a groove. Again, prise it out, then the sprocket simply lifts off.
6. On the non-drive side, undo the lock-nut (circular with two flats)and screw the cone (four flats) off. The cone is a larger diameter thread, so you only unscrew it a bit then it lifts off. Steps 2 and 3 from the maintenance manual page 12.
7. Remove the balls in cage. I think they are captive, but don't lose any. Step 4 from the maintenance manual page 13.
8. Now you simply push everything out. I position the hub with the axle vertical, drive (sprocket) side at the top, and push down on the hub shell. If the hub is in a wheel, you can push down on the rim. It's a firm push, but it doesn't need too much force - you don't need to strain. The innards unit will pop the push-fit plastic cap off and then you lift it out. In the maintenance manual, you simply lift it off (step 5 on page 13) but it's not actually that loose.

This is as far as you need to go if the innards are OK. You can keep going following these instructions from the service manual if you must. I've never gone past step 10 of page 15, because soon past this point is tight metal to metal push-fit so you start hammering things using a special tool (see step 16 on page 16). At step 10 you can get grease well into the internals, so that's enough for me.

• page 13
• page 14
• page 15
• page 16

Reassembly

Just do it all backwards.

Actually, it's fairly obvious except the routing of the shifting cable. Fortunately, page 9 of the maintenance manual covers that (steps 7 to 9). Pay particular note to the bit about the 2mm allen key (in the box near the middle of the page) - this makes it much, much easier to reassemble.

Shortcut

The reason I refer to a 'proper job' is that actually, the easiest way to re-lubricate is by dipping the internals in an oil bath. You only need to do about half as much to achieve that - take the wheel out, shifter connector thing off, brake off, undo the locknut and take cone and bearings out of the left side. Now you can pop the innards out leaving the sprocket in place. This assembly can then be dipped, keeping the plastic cover clear.

There's a Shimano dipping kit, with special oil, and a plastic tub with a line on it so that when you dip the innards in the oil comes up to the right level. The down side is price - I paid 32 quid in December 2008 and thought that was almost implausibly expensive for one litre of oil. As at spring 2012, however, it was often listed over 100 pounds (!) (for one litre of oil!), though by 2020 the going rate was about 85 pounds and in 2024 it's under 80 pounds (£71.22 on ebay is the cheapest I've seen).

I bought mine from a bike shop that now seems to be out of business. In principle, any Shimano stockist should presumbaly be able to get it, but my experience is that many become incoherent if you quote part numbers at them. Also, because I bought the 'proper' Shimano stuff, I have no idea what would be a suitable alternative, sorry.

The official illustration (shown here) has the sprocket and plastic cap off, but I can't see any reason why you'd need to do that - you're not supposed to submerge it that deep anyway, according to the pictures on the can.

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