I have a mh 6’10 fast action rod that is rated 3/4-1/2 oz. is a 3/8oz floating crank bait too light? I’m using 12 pound mono line.

Spinning or baitcasting?  For baitcasting, it all depends on the rod, reel, luck and the wind.

No but a fast action rod is not good for crankbaits.

 

Allen

You could probably cast it, but between the difficulty of doing so and the higher possibility of missing/loosing fish, I wouldn't recommend it.  A big part of the equation when fishing cranks is casting distance and with that rod/lure combination, you'll be limiting that major element.

36 minutes ago, Munkin said:

No but a fast action rod is not good for crankbaits.

 

Allen

Agreed. All my main jerk/crankbaiting is done on a M/M Casting rod with 20# Power Pro. Only time I switch to the M/F Spinning rod with 12# FC is LIGHT jerkbaits - 2 treble Rapalas.

3/4-1/2? Guessing that's either supposed to be 1/4-1/2 or 3/4-1 1/2oz. The former will work perfectly with a 3/8oz, the latter most likely won't load properly to cast a 3/8oz.