what do I have to fuse a tach down to?

Question

1975 chevy truck, 350 HEI

Answer 1

I would fuse the hot side + 50 1 amp. Tachometers use Milliamps of current and could use a smaller fuse, but the windings in the tach movement are fuses in their own right. The only reason for the fuse is that the input could short to the frame or block and the wire would become the fuse for a short period and could cause a fire. A smaller fuse would work, but 1 amp is fine.
Some folks would get pickier and suggest a fractional fuse, but it is your time. The cost is really not a factor in small amp fuses.