I am doing something wrong in my tessellation.
I have a material that is essentially a bunch of tessellated tiles. As Shown Below :
However, if I just move the camera over to show you another angle, where that edge connects with another edge (which has a different texture) - You can see that a very noticable gap appears here :
These to polygons are from the same mesh by the way - that ledge is not a separate mesh that has been brought in separately or anything.
Any ideas? 
Have you tried checking âCrack Free Displacementâ under the tessellation options?
Yeah, I have that ticked - I actually thought that may have been the problem myself and have been back to check it twice to be safe! 
The other material might need to have tessellation and displacement as well⌠or you might need to incorporate both materials into a single material, this can be done via creating material functions for basic materials.
Here is a single mesh that I made that has displacement, it has around 3 materials combined through masks and blending via vertex color. Every material has at least some value applied to displacement.
http://wwwâŚcom/images/env04/-env04-06.jpg
Okay, thanks⌠Will look into that 
**Ive had a different kind of cracks **LeamDelaney its when i move the camera close enough then they appear, i thought it was kina viewport stuff but the renders carry those cracks as well. I have tried your solution of disabling cracks but nope that doesnt really do anything âŚany insights and solutions will be helpful
I have been stucked in the same situation for weeks now guys and it has been impossible to find a solution to this issue. I do love Unreal but I hate when the self proclaimed evangelists leave us with our problems like this when they should know how to fix this stupid issue. Tessellation in Unreal is absolute garbage.
Who have same problem try adjust max displacement settings in material. Should work.
When you displace material A and itâs so many units âupâ from where it ought to be then have that border another material w/o displacement or otherwise a different value for how far up/down might be, that crack is expected (in my experience). Unreal isnât going to feather/blend the two edges, thatâs the humans job to know one side is 10units high (displaced) and the other is 5 units high.
If I had to books of different thicknesses side by side, would I expect life to suddenly smooth out the edge there so that one book is not higher than the other??
You need to account for this disparity in your materials yourself, Unreal isnât going to do it for you. This is a logical issue, not a rendering one.
We had the same thing when using a tesselation material on some buildings we were making, gaps were appearing in the model along the mesh texture seams and it was due to our texturing, basically as the texture wasnât perfectly tiled at the seams the height maps had different height values either side of the seam causing gaps to appearâŚ
