Afternoon folks
When do you start to get concerned about their play biting/mouthing being something more? Pooch is 13 weeks now and has been pretty bitey since first week we've had him (8 weeks, Border Terrier). He's full on teeth clamped, growling and snarling. We've tried redirecting with toys or treats, I've tried making sharp noises. I've tried gently pinching skin at scruff of his neck. DH has tried gently tapping his bum but gave up quickly on that as it was too close to slapping him. I've tried holding his collar gently and pushing into his cheek to avoid losing a finger.
Occasional he backs off then within seconds launches himself at you.
It's not always over tired either as sometimes after a nice long nap or waking first thing he has a nice stretch, then goes into Piranha mode
like, straight away.
I've today resorted to putting him in his crate for time out which I swore is never do as I'm terrified of him having negative associations with it as generally he loves it in there.
DD 16 is now so sick of it she avoids time by herself with him more because she's scared she'll hurt him shoving him off
All the books/online guidance say try something and stick with it for 6 weeks to create a habit. 6 WEEKS!!! How is that possible? We'll have no limbs left!!! 
It almost feels like aggression! Or is this normal? There's no other signs to indicate teething yet.
Please give me some perspective!!