I’m having some issue with a non-human skeleton I’ve built. I’ve redone the ragdoll completely, but no matter what I try, I have some body parts get sucked inside of the parts they descend from. For instance, the hand will get pulled into the middle of the forearm, and the forearm gets pulled inside the upper arm. The end effect is that my poor little devil character gets black-holed into his own body. I can get around all of the crazy dancing action, but I can’t make the ragdoll stop from collapsing my character. Anyone seen this before? Is there a solution?
Have you edited the Physics Asset (collision sphylinders attached to the skeleton) and told the adjacent limbs not to collide with each other? if you don’t, they will de-penetrate away from each other, causing them to stretch away from where they are supposed to be. This might be what is causing some of them to get sucked in. Maybe they’re actually being pushed in.
Like what @mightyenigma said with the collisions on the constraints (make sure you are in constraint mode) That can cause this problem. But also this can happen from the mass of the physics bodies themselves. You can manually override them and set values for them in the body mode.
nope I’m completely mistaken. just the body. The constraints is where you should set fixed on the linear motion. My Bad >.<
Hey wait a minute. Are you MaxSmoke from Unreal4Ever?
Woah, so you have to set no collision on the Constraints, too? Not just the collision bodies?
Oh okay cool. I don’t actually know. I just dived in and figured this stuff out last night after installing. Glad to know I’m on the right track - but I’m gonna watch the tutorial videos because I am getting crazy elasto-man ricocheting stretchy pawns just wigging out all over the arena. What does it take to keep the body together? Well, I’ll go take this to another thread - different problem. 
Groovy, didn’t know anyone still remembered our mod. Yup, still me, same name, different date. Unfortunately, it’s Just me. And I was the artist on the mod, not a coder, so this has been an uphill struggle. I’ve already done some weird stuff, with alot more weird plans in the future. Here’s what my project looked like last week (there’s been a bunch of changes and fixes since then):
All Linear motion is locked down. I’ve tried it with Enabled Projection and not, with no effect. I actually just went through and tried every checkbox option on the list, in both Body and Constraint Mode, with no effect, positive or negative. And adjusting the weight did nothing.
It is worth noting that I’m bringing over my IMVU model, which have boxes for bones and not bone-bones. They were setup for Physique, but I’ve removed and reweighted for Skin. Well Physique has no issues in IMVU, UE4 doesn’t like it. I’m no fan of Skin, as it’s very kludgy compared to the simple and easy to use Physique. I have a LARGE body of work setup for Physique, so moving to Skin is going to slow me down CONSIDERABLY. And if I have to use Bone-Bones, things are even going to get harder. Every model will have to be rebuilt! 
My “Chicken-Freak” monsters were rebuilt with Bone-Bones, and they don’t get their bones sucked inside of their other bones.
The lack of flexibility in UE4’s importation process is disappointing. From what I can tell, this comes from a large number of oversights and assumptions about the art being built to fit the code, and not the other way around. This lack of respect for artists time is going to slow down people trying to switch to their engine considerably.
Oh well. I guess I’ll soldier on. I’m annoyed, but undeterred.
Im just assuming your using blender, if not, anyone who has this problem and is, this worked for me. If your still looking, i had this problem for a long time. You need to make sure both the skeleton and mesh parented beneath the skeleton have a scale of 1 (so you need to apply its current scale). When exporting as an fbx. The issue seemed to come from the exportation of the file and not UE4. When exporting, under the main tab, there is a button to the right of the scale slider, it should automatically be toggled or dark. You need to un toggle it and set the scale to 1. That fixed the problem for me, I had both stretching bones and bones being sucked into the stomach. Hope this helps!
Late, but is someone has problems, try to add a body to your root bone and constrain the attracted bodies to it
To start out, right-click on your PhAT skeleton tree list and choose the “Select All Bodies” option. then on the top toolbar/ribbon, click the No Collision (looks like a cube with a red X over it) button to tell all the bodies not to collide with each other. That solves most of the problems most of the time. From there you can tell just the non-overlapping parts to collide with each other by CTRL-selecting both of them and clicking the opposite button in the toolbar (has a green checkmark) to tell them to collide with each other.