For example, on a spincast reel, if the manufacturer states a spincast reel use 10 lb mono line at 125 yards, how heavy of mono line can you go above that, typically, 12 lb., 14 lb., or higher, and it will still function well?

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Not one reply...hmm, losing the touch I never had. I went with 12 lb mono for catfish and carp on an Shakespeare Uglycast. It came with 10 lb., but we'll see!

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The heavier you go in line the bigger spool you need to prevent problems.

If you get the line on wrong then you're going to have problems no matter what the size.

I practiced a bit...it appears to be okay! Thanks...

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I would hook a lure to a fence and stretch 50-75 yards of it.

There is no right answer to your question as lines come in differant diamoters and may still be 12lb etc.

Hmm, I'd think there was at least an approximate answer...if a reel manufacturer recommends 10 lb. mono, and I want to go a big heavier, how much heavier is too much, 12, 15, 20?

As stated, I went with 12 lb. just to add some strength for catfishing or even carp...no big deal!

Thanks for the responses.

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I would say 12 is ok, 15 is on the big side, and 20 is too much.

You will run into a point where the reel doesn't have the ability to hold enough line to function well for casting etc. Also smaller reels have smaller diameter spools and closer tolerances that function better with limper/lower diameter lines. If you spool 20 lb on an ultralight you probably are going to have issues.