How many extra links do I need to add to chain if I go from a 42t cassette to a 46t?
I'd try it without adding any first, unless your chain was already borderline too short.
4, obvs. The teeth are the same as the chain links, if you think about it...
Well that is if you consider the distance between each pin to be one link. You might consider that half a link.
None. Always fit a new chain when you fit a new cassette.
I am going to install new chain, but was going to use old chain as length guide, then just go the amount of links extra.
Size the chain to the new set up. Disregard how long the old one was.
Size the chain to the new set up. Disregard how long the old one was.
This. Every time.
Size the chain to the new set up. Disregard how long the old one was.
Seconded
4, obvs. The teeth are the same as the chain links, if you think about it…
Not obvs to me. The chain doesn't go round the whole cog, only about half, unless you add 4t to the chainring as well. So 2 extra half-links, one full link. Unless your bigger cog needs a longer mech as well.