The arbors come in what I'd call 'reel seat lenghts". I've been cutting the arbors down to smaller 3/4" lengths to allow for spacing between them, much like the spacing if I was using masking tape. Is this necessary with the foam arbors?

Not necessary to cut. But not necessary to use an entire length on one seat. Do which ever makes you happiest. I cut the majority of mine down.

Better to have and not need than to need and not have.

I just use them full length now - they're inexpensive, they're light and so I don't take the time to cut them into pieces. If cost is the consideration, then your time cutting them is not going to be cost effective. If a good bond is your goal, then remember that a thin glue line is strongest - you don't want to pour epoxy in between arbors - it only adds weight. It just depends on what goals you're trying to meet, if any.

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Steve,
One thing I can tell you about these arbors that I learned the hard way. Glue the arbors into the seat before you ream to the final ID. MUCH easier that way!

Bill