Invisible red parts

Hi,see those red faces on the mesh? those are invisible faces i founded a solution how to fix it,by flipping the faces so its blue again

the problem is,if i flipped the red faces it will be blue(visible)in the inside of the exhaust but red(invisible) outside of it this causes the red faces to be invisible in roblox studio but what i want is for all parts/faces to be blue and visible

i could add a solidify modifier but even if i changed the threshold of the modifier it will make the model shading unrealistic although all the parts are blue and visible

so the question is there any ways to make all faces blue(visible) without adding solidify modifier?

Invisible… and you don’t have backface culling activated for this material?

how to activate backface culling

Red and blue in blender shows the face orientation here in eevee. With culling you only see them from one side. You flipped them and now you have a problem in roblox, so why did you? Or in roblox ther is culling and you dont want it??

This is how to turn backface culling on or off:

If the checkbox is set then all faces with normals facing away from the camera are NOT drawn.