I’ve got an old Gary Fisher on a 8 speed dual mech brake/shifter setup. The drivetrain and bb are shot so new drivetrain,cassette and chain require replacement. Could I simply use a 9 speed crank with new 8 speed cassette and chain (using existing shifters) or do I need to source a new 8 speed crank? Don’t want to spend much as it’s an old knockabout.
You can practically use what you want for 8/9spd with anything as long as you stay within manufacturer (do what you want with chains).
I’m still running 8spd on my all-mountain bike: hg51 cassette, 9spd chain (sram PC951), 10spd mech (zee), some unknown/vintage shimano 8spd shifter, works component n/w ring (renthal SR4 in the past which are supposedly 9spd only), deore m590 cranks.