Igor_M

The issue was already covered many times, what confuses me is that different sources claim different spacer's thicknesses.
for instance on Sheldonbrown.com and on this forum it is said that it has to be 4.5mm

on an Italian website they say it's 3.5mm

and this shop sells a Shimano spacer specifically made for that purpose that is 3mm thick.


So, which dimension should I go for?

ThermionicScott 

The spacer on the German site is an 8-speed spacer (note the little "8S" molded into it), meant to go between cogs rather than to occupy extra space between the hub and cassette. I think they found themselves with leftover stock, and are not being completely honest in how they get rid of them...

3.5mm ought to be enough to do what you want, but 4.5mm makes sure that there is enough overhang so that the lockring can torque everything together properly -- I'm pretty sure that's why Sheldon and others sell the thicker one for that purpose. There can be slight differences in dimensions between hub makers.

Hope this helps.

dabac

+1
Not only hub MAKERS, but even in variation from the same maker. I think I've got a 4 mm spacer on mine.

Igor_M

It sure does, although I would still wish to get a satisfactory explanation to the "German way", mostly on practical grounds, it seems it's not so easy to get those 4.5mm spacers here on the Old continent.

There are many shops here in Germany that sell that item as a 7to8 spacer, so I doubt that one shop is being dishonest (maybe it's a national stereotype kind of thing -> all German shops are dishonest about that )

See here for instance.
+
here
That's probably how they mean to use them

RoadGuy

Although it seems like there's a lot of difference between 3, 3.5, 4, or 4.5mm spacer, there really isn't.

Depending on the components on your bike, any of them may work, or all of them may work.

I've been playing with Shimano 7-speed cassettes on 8,9,10 Shimano wheel hubs, and what is working for me is 4mm spacers that have been notched for the screw or rivet heads on the back of the low gear (that hold the cassettes together). The 4mm spacer would have worked okay, BUT I didn't like the way that the three screw heads (or rivet heads) would be the only points of contact for the cassette against the wheel hub unless the notches were made.

I notched the spacer in a couple of minutes (spacer I used is aluminum) using a dremel tool with a cutting/grinding disc.

The three screws (or rivets) are not equally spaced apart, so if you're doing the notching yourself, you are going to want to set the spacer against the low gear, and mark the places that need to be notched to clear the screw/rivet heads. Also heads during assembly if you mark the spacer so you can tell how it lines up with the screw/river heads (maybe mark the spacer with a index point like the cassette cogs are).

fietsbob

Or get 8 speed cassettes and only use 7 of them [ H limit screw will do that ]



sideways slop on top pulley may help it be 'close enough'..

or learn to use friction shifting - skills vs hardware.

ThermionicScott 

Perhaps 3mm is the bare minimum with Shimano 8/9/10-speed hubs. You could try it and see. If that isn't enough, you can always add another 1mm spacer (aka "bottom bracket spacer").

FastJake

But the spacing is slightly different, and a 7 speed shifter may not index an 8 speed cassette quite right. I've tried it in the past. Sometimes it seemed to work just fine, other times it was too finicky.

cyccommute 

No. The spacing between cogs is the same for 6,7 and 8. That's why the same chain will work on all 3.

fietsbob

7,8 is similar 5, 6 a bit wider between cogs..

Lazyass

I've been using a 4.5mm spacer on my Paramount for years now. Original 7 speed 105sc group with Ultegra 6600 hubs. Shifts perfectly.

Cassette Spacer > Components > Maintenance and Service Parts > Misc. Small Parts | Jenson USA

dabac

No. Shimano 7-speed is 5.0 mm while Shimano 8-speed is 4.8 mm. 6-speed usually 5.5 mm.
Several more variations exist.

FastJake

What dabac said. It's close enough that the same chain can be used, but not always close enough for the indexing to work. See here: Sheldon Brown's Bicycle Frame and Cassette Spacing Crib Sheet

70sSanO

I've been using 7 speed cassettes on 8-10 speed hubs, as well as re-spaced 7 speed cassettes (8 speed 3.0 vs 7 speed 3.15). Respaced 7's run really nice with 8s shifters and make the overall cassette width a scosche narrower. It does let me easily run all 7 cogs with both chainrings on my mtb's without crossover issues.

For me it all comes down to chainline. I want to optimize the chainline or slightly tweak it, to one side or the other of optimum, depending on whether I want to reduce cross chaining in one direction or the other.

I may run a narrow spacer on the front side of the cassette between the smallest cog and the lock ring if necessary.

John