7 and 8 speed Indexing query

[eBay_special]

A small conundrum I'm sure some wise old owl on here will know?

Bike has 1992 ST-MO71 Deore DX rapidfire push-button shifters without any friction option.
My question relates to the RH shifter assembly.
As you click, the detente/ball bearing within the shifter dictates the amount of indexed movement at the rear mech. The bike has an 8 speed cassette instead of a 7 and whilst trying to set this up, it appears that the indexation will never match the sprocket spacing on that cassette?
Am I right or am I missing something here?
Can this system; 7 spd with 8 rear sprockets ever work correctly and if 'NO', what are the required changes; cassette, chain, both?

All help and advice gratefully received!

A thousand 'thank you's',

ebay_special

 

[fagin]

The indexing is the same for 7 and 8 speed, so the indexing will work, but you can only use 7 speeds.

 

[drystonepaul]

As above. The spacing for 7 and 8 speed cassettes will be the same so you'd be best off getting a 7 speed block rather than carting around an extra sprocket that you can't get to with your shifter.

 

[marc two tone]

8 speed

with 6 speed -you sometimes gained an extra or 'secret' notch that made a 7speed poss-proper 7speed i dont think has an extra notch :?

you're best getting a 7sp cassette or 8speed rh

 

[Splatter Paint]

Set the mech to run from the largest to the 7th smallest and leave it at that. How often do you ever use the smallest cog anyway? The cog-spacing and indexing is exactly the same on both cassettes.

SP

 

[hamster]

The other posts aren't correct. 7 speed has a sprocket pitch of 5.0mm, 8 speed has 4.8mm. So they are not (strictly) compatible.

HOWEVER, because of the floaty top jockey wheel, you can usually get an 8 speed cassette to index satisfactorily with a 7s thumbshifter - the so-called 'hidden click'.

BUT: you need cables in good condition (especially outers) and also it's best set up from gear 3 being dead centre rather than gear 7.

 

[elite504]

The other posts aren't correct. 7 speed has a sprocket pitch of 5.0mm, 8 speed has 4.8mm. So they are not (strictly) compatible.

HOWEVER, because of the floaty top jockey wheel, you can usually get an 8 speed cassette to index satisfactorily with a 7s thumbshifter - the so-called 'hidden click'.

BUT: you need cables in good condition (especially outers) and also it's best set up from gear 3 being dead centre rather than gear 7.

Ditto. Absolutely. Perfect post.