Hi!

The capacitor values are a range because the caps are made with
a large tolerance. Even if you got one marked exactly 580 mfd, it
could actually be much smaller or much larger. I don't think
Mallory has made capacitors for many,many years.

Funny you'd mention that. I bought an old furnace fan assembly ($10)
and found that not only was the motor rusty inside, but the start cap
was bad. The motor cleaned up well and that got it starting a lot
better, but it was still stubborn at times.

So I popped the cap out and looked at it. Turned out it was a Mallory
capacitor. I took another look at it and said "that's been a long
time". What I took as a date code suggested 1983 or 84 (don't remember
which). I'm sure that little motor started up lots of times between
then and now.

One trick we used to do in a emergency is to get two DC electrolytic
capacitors of twice the needed AC capacity and connect them
in series, either both + leads or both - leads together and wire the
remaining leads to the motor to replace a single AC capacitor.

I'm going to have to remember that! (Or at least remember to try it
and see the next time a capacitor drops out.)

I'd have thought that finding a replacement capacitor for my fan would
have been easy. (Find one at the hardware store, right?) Nope. Struck
out at various hardware stores, Farm and Fleet (now, come on!) and
only had any luck at a little hole-in-the-wall motor repair shop.

$7 later I had a slightly larger "Rotom" ("motor" spelled backwards--
clever, huh?) capacitor and everything was fine once again. I was
fortunate in that it did fit the capacitor compartment perfectly.

If the OP can't find an exact replacement, mounting it elsewhere on
the motor should be acceptable.

William