I just bought this altec lansing 421-8lf from a freind of mine. just wondering what kind of box should i build for this? what kind of crossover should i use or make? i wanted to use this as a subwoofer along with my 4 H&H mid high speakers for outdoor.

I have a pair of brand new ones that I plan to make subs out of some day, my son has a left over dual 15" box from his car subs, it's a sealed box which will probably reduce the 421's efficiency some since it was normally used in a vented bass reflex cabinet. Todd White has a great matrix of the Altec Parms that might help you desihgn something; http://www.greatplainsaudio.com/Thiele-Small.html
:)

Figured out a ported box for 421s a long time ago. I don't remember much except it was dual ported (the large 4" adjustable) and the box was 10 liters, to which you have to add the space of the ports (approximate) and the driver. The stuffing and the port length really have to be done by trial and error as the amount of stuffing determines port length as it is a virtual way to adjust the size of the box.
I have an older 421 with an alnico and a smaller frame and a newer one of the II series with ferrite and heavier frame. They are not exactly the same size and the alnico has an FS a few Hertz lower so the box size might have been a slight compromise, but with the allowances made by the amount of stuffing I guess this really doesn't matter.
The design called for rear ports to maximize the bass.

I have read about using 421 frames with different cones as 421s are music producers and not reproducers. They tend to have larger, stronger magnets which make them good for this and possibly would have the same FS, but I don't know. I think I read about this on a personal site of a guy called Thunder Bob, so you might do a search for him. Also Bill at Great Plains in OK City would know and is the guy to recone 421s with music reproducer cones.

Correction on the 10 liters, I thought about it this morning and it was 10 feet cubed. Pretty damned big which is why I didn't bother with it.

Another site with TS parameters and lots of info on Altecs,

<www.soundpractices.com/altec.html>