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Is there a difference of spacing between cogs on a 7 speed cassette vs a 8 speed cassette?

When I got a new wheel (upgrade from freewheel hub), I tried to get a 7 speed cassette (SRAM) for it, but the local shop didn't have the spacers for the hub. Instead I swapped it out for a 8 speed cassette (SRAM), but kept the 7 speed shifters (shimano EF-51). I have a few gears in the lower and middle gears where the chain constantly tries to jump between. The higher gears are fine. The issue mostly went away when the tension on the cable was adjusted, but not completely. I noticed it may have moved the issue to another part of the cassette.

Update Soon after posting this I replaced the cassette with a proper 7 speed and all the issues went away.

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    I'd probably just go for the 8 speed shifters; I used to run 7 speed, but when the EF51's were the only 7 speed shifters on the market, I switched to 8 speed (Alivio), because the EF51's didn't last very long. But this doesn't really answer your question, so...
    – Batman
    May 16, 2017 at 16:35
  • @Batman, I tried to keep it matching. Shifters were a Christmas present for the old wheel. Old wheel went bust before shifters installed. So I figure I would get a 7spd for the new wheel and knew I needed a spacer. He didn't have a spacer and said I didn't need one. I came back in showing him otherwise, so he swapped my 7spd for an 8spd cassette. I was going to go with 8spd shifters after that, but decided to save the money and stick with the 7spds I had. I may just Amazon the 7spd cassette and spacer and call it a day and dream for a new bike.
    – BPugh
    May 16, 2017 at 19:42
  • Nothing wrong with 7 or 8 speed. You could go friction shifting for cheaper than a new cassette, probably too ;)
    – Batman
    May 16, 2017 at 20:45

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Yes, there is. Shimano/SRAM & compatible seven speed is 5mm cog-to-cog, and eight is 4.8mm. Generally what you're trying to do here can't be made to work very well. There are some tricks that without any additional parts can increase the movement of the derailer, but not decrease it. Also if you have a true 7-speed-only chain (not common anymore but they exist, KMC Z50 is one example) it won't work well on 8.

A shop not having anything to offer to make the conversion work is pretty unfathomable. The "correct" spacer is a 4.5mm one, but the only thing that makes it correct is that's the number that locates the cassette exactly where an actual Shimano 7-speed hub would, so you could theoretically swap wheels without adjusting the derailer. Any other stack of correct internal diameter spacers in the 4-5mm or so range will still work, so if this shop was too new-stuff-oriented to have a 7->8 spacer, they should just sell you a 2.5mm bottom bracket spacer (ubiquitous modern shop item) plus a 1.8mm 10->11 hub spacer (ubiquitous modern shop item). That will work perfectly.

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    My experience is that many shops are not very good with things that aren't done "by the book". Using a bottom bracket spacer on a cassette is something that many mechanics may not even think of. I'm a self taught home bike tinkerer, and I'm often amazed how little some shop mechanics know when it comes to older or less expensive parts, or doing things in ways that aren't officially correct.
    – Kibbee
    May 16, 2017 at 15:22
  • @Kibbee - True. "I want a 8 speed freewheel with w-x range and a 7 speed cassette with 7-z range" invariably ends up with the person trying to give you a 8 speed cassette and a 7 speed freewheel.
    – Batman
    May 16, 2017 at 16:39
  • "can't be made to work very well" means that if you adjust the derailleur for center cog and set the limit screws right, the adjustment will be 0.4mm off at worst. In practice it works quite well.
    – ojs
    May 16, 2017 at 18:11
  • @ojs It's true that it can be made to shift. My view on it is that most or many people in practice have a hard time keeping their shifting dialed even with the .5mm of upper pulley float on a normal derailer, and this pushes the issue that much further. I do concede I haven't tried going through life for any length of time with this particular mismatch.
    – Nathan Knutson
    May 16, 2017 at 22:43
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all chains up to 8 speed are the same width as is the distance between the cogs, regardless of make or actual number of gears until you go to a 9 speed cassette. your problem is probably not due to the number of gears, but due to wear, damage or mis-adjustment.

I am a BBI certified bicycle mechanic who has been working in shops since the 1990's.

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