I had a 30 min talk on the phone with a well respected violin maker about cleats. He was convinced that Diamond cleats are inferior to rectangular cleats . He believes that diamonds pop off easier and don’t provide enough stability to the top of a bass and that rectangular cleats keep the crack from hinging better.
Any thoughts?

I would think the proper fitting, placement and gluing of the cleat matters a whole lot more than the actual shape.

Diamonds are superior. There is less stiffening (better for tone) and the grain crosses at a lower angle, making them LESS likely to come loose from seasonal wood movement. Also they are longer, which gives MORE support to the crack. Square or rectangle patches are more likely to cause cracks on new fault lines they create. Sometimes you have to use a combination due to multiple or overlapping cracks.

Getting the crack 100% closed, flush, and tight is paramount. Cleats are re-inforcement, but inforcement comes in the integrity of the original crack repair.

I've seen rectangular and diamond cleats succeed. I've also seen them fail. I like the diamonds for the same reasons ahnold posted...

(here comes ss)

Ain't a diamond just a cockeyed rectangle? :eek:

(here comes ss)
:D

When and after the diamond cleats are tapered, are they rounded down by sanding or are they left with the tapers carved sharp ?
IM sure every one does it differently.

That's up to you. Sanding can work, but take great care to not sand the crap out of the surrounding area of original spruce. My goal is to have minimal tool marks when doing any kind of repair. Sometimes a clean, bevelled cleat looks fine...