What’s the maximum thickness the Singer 29K71 can handle

[JRodriguez]

Hi everyone, I’m selling a SINGER 29K71 on behalf of a retired cobbler. I’m a shoemaker in Australia and I only have used my 29K71 for leather used in shoemaking, probably maximum 3mm thick. A person wants to buy it but they’re asking about the maximum thickness that this machine can stitch. The information I found on the net says 8mm. I tried to stitch two pieces of veg tan leather together (8mm) total and the leather doesn’t even move. The needle is going into the same position all the time. The machine stitches nicely on a lighter weight leather (2mm) but not with 8mm. Can someone’s share their experiences please?

[AlZilla]

Here's a manual that says the foot rises 3/8" of an inch, so less then that: 

https://leatherworker.net/forum/applications/core/interface/file/attachment.php?id=149218