42t --> 39t: # Chain links reduced?
 

I am going to replace my 42t with a 39t. Just wondering if someone can provide me a calculation or to determine how many chain links should be reduced to compensate?

The rear cluster is 12-25.

should be zero.
only around half the teeth of the cogs are ever engaged by the chain at once.

Why that's easy. Shift into the little/little combination. Unless the lower run of your chain goes slack, you're good-to-go.

Is the 42T chainring on a single chainring crank and being replaced by a 39T or is it one ring of a double crank and the 52 or 53T big ring is going to remain unchanged? If it a double, there will be no change in the chain length at all since the big-big combination hasn't been changed.

If you are replacing a single 42T chainring with a single 39T you can, in theory remove 1 half link, but since this is impossible while having the mating ends of the chain still match, the chain must remain the same even in this case.

There are half links for 3/32 chains , but really that is useful with a standard 5 or 6 speed rear cog cluster.

I'm using one on each my 2 bikes with internal gear hubs, and 3/32" chains, but they dont shift sideways..

Agreed. Unless there is a functional reason there's no reason to mess with a properly operating chain for that little a difference.

I thought the half-links (actually a half-half-link) was mostly used to adjust chain length on fixies and single speeds built on frames with vertical dropouts.

Amen.