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A friend asked me to build three 8-string tenor ukuleles, he's supplying the Koa. I have built several acoustic guitars, many ukes, and a few cigar box guitars but not an 8-string uke. Do I use mandolin tuners, individual uke tuners (Grover)? I assume I use tenor ukulele plans. Do I re-enforce the neck? What are the nut specks-string separation and the separation and spread at the saddle?

I know, a lot of questions here. Anyone build an 8-string tenor uke that can point me to where I might find the info? Have not found plans anywhere for 8-string tenor ukulele.

Use whatever tuning machines you feel comfortable with. Of course Mando would be a natural with 4 on a plate, or individual would be fine.

Since 8-strings are usually 17" scale, Tenors would be a natural starting point.

Personally, I reinforce all my necks. I doubt if most production ones are reinforced, even 6's and 8's.

For the string spacing within a course, I would spec out one that you like, and go from there.

I've built a 6, but strung it like 3/4 of an 8 (octave g, c, e, double high a). Not sure if you'll find plans for an 8. Not unlike the spacing (which would come with plans), I would spec one out that you like. Kamaka has made 8's for a long time. For me, I upped the bracing a bit, widened the fretboard a bit, stretched out the headstock a bit.

Personally, I'm not an 8-string fan, the high g, high c, double high a make it a very treble instrument. I've not really heard any good sounding 8's that I wouldn't replace with a 4 or a 6. Oddly, I have made Tiples, and prefer them exponentially over 8's.

Not sure if this helps, but I thought I'd take a stab in response.

I did my taropatch uke with 1.5" wide nut. The strings were spaced about 1/8th inch apart for each course. (I just copied the spacing from a mandolin)
I used a carbon fiber reinforcement in the neck (15" scale). I don't think you NEED a reinforcement but its easy insurance. Tuners are up to you......lighter is better
I did 3 fan braces with a popsicle stick brace on each side of the sound hole.

There's no real "standard" for taropatch ukes because they're rare. Seems like Martin made the most of them so I took my cue's from that model. Your friend might be doubling the world's population of 8-string ukes with this order

I have built several 8 string tenors...koa/spruce, koa/redwood (and I play mostly 8 string).....

Tuners; Have been using Grover 4 on a plate....
Nut width/Saddle; Your preference....nut around 35-40mm, saddle around 50mm.
Neck; I use either 1/8 x 3/8 carbon rod, or laminate black/koa/black in neck.
Bracing; Up to you...I X brace, but many out there are fan. Depends on the sound you're looking for.

Key to playability for me....LOW action at nut (gotta press 8 strings down on 1st fret), and close pairing of strings. Finally, I like some real estate between bottom a string and edge of fretboard to keep string from sliding off in upper positions....hope this helps....can't seem to post a pic....oh well...good luck!!

Thanks all. This helps a lot! Nice builds powdrell!