I started with the 138 thread and had some ok results but was then reccomended to switch to an 18 round point needle with 69 thread. haven't tried that yet was a bit of tensioning to work out had to back it off quite a bit due to the softness of the leather. make sure you try multiple scrap trials to get it right.
Yes you may need to make the edge wider especially at the corner it's the worst to sew. Try some Barge cement to hold it together so it can't pull apart when you are trying to sew it together.
needle and tension. I ordered some round point #18 from Steve so I could sew thinner stuff. No they aren't designed for it but I have and still use it to sew thin stuff. I have sewn 2 layers of lamb with my class 4. try smaller needle and thread. When I got mine my smallest needle was a #23 that I used with 138 thread. Also play with the tension to pull the knot in to the middle of the leather
Yes, you may be mad for trying it wet forming might do the trick. the Gussets being only 4cm and 8oz may be overkill though. When you have to stitch the center (middle separator and 2 gussets) it's going to be 24oz thick. you could probably use much thinner material for the gussets but I also understand how it is when you get an idea and you want to MAKE it work. Make the bends on each side of your gusset a bit wider than you want them to end up partly because you lose some around the corners. Also with an 8oz gusset it and only 4cm wide it won't want to "accordion" in but then you were only hoping to gain a cm
http://www.sewingmachineoutlet.com/ricetn.htm I've never ordered from them but click on the size you want "277,346, or 415" in the "colors" section click add to cart and then you can choose the size and color.
DoubleC I wish Rancn2arena showed more on his website. I didn't see any buckles but do like small companies that make a good product. Also I see you are in VT. Alot of my family is there, but I wanted to let you know on Craigslist someone in Walden (yes I know it's quite a bit north of you) is selling deer hides in a few shades for $20 a piece.
what is the smallest needle I can get in the 794 system for a cobra 4? I really want to sew some light leather and needles seem to be a real pain to find or figure out I never realized there were so many needle sizes/types
Check Wicket& Craig maybe there English Bridle. You have to call them but they are helpful and they will skive to your thickness and finish the back. I always get nice leather from them http://www.wickett-craig.com/index.php/leathers/description-and-grading
$26 for a tooling belly you want at least 5/6oz http://springfieldleather.com/25850/Belly%2CHermannOak%2C9-10oz%2CEach/
As said before you want veg-tanned leather if you want to tool it.
http://www.buckleguy.com/products/G7341-19-Links--Natural-Brass%2C-Swivel-Bolt-Snap-w%7B47%7DChain%2C-Solid-Brass.html
he has different ones also. Ive always had good luck with him