What could I do

[cone]

You can foam behind the linings but if you wedge and fix at 5 points per side there should not be much movement. If fire safety is an issue use rockwool pack or fire rated foam. The door should fit to the frame with about 3 to 5 mm each side and top, so you will need to have a gap between the door lining and lintel which you can fill. You don't screw the head to the lintel. The architrave should be deep enough to cover the gap, if not you could use a goalpost type architrave with a deeper top piece,on really good work the uprights are cut into the cross piece so there's no gap if it shrinks .

Is that 5 screws into the wall at 5 places each side.
Any I've done before, the screws pull it off plumb at the sides.

 

[Mark18PLL]

If the door size is essential, rip the frame out and fit one the right size, the architrave will cover the error.

If not, buy doors the right size for the frames.

Old joiners would always measure the frame twice and buy doors of the right size once.

Im with misterdog, this is the right way to do it.

 

[baldkev]

I usually measure down from the head 100mm, then 700 down, 1300 down and 100 up from the floor ( this is based on 1980mm doors ). I then measure in 20mm off each side of the lining and mark that off on each height line. This gives 8 fixings per side and always avoids hinges and lock keeps etc. I use folding wedges right above or below each pair of screws. If the stud or blockwork isnt square with the opening, use weges independently, not as folding wedges.
Check your floor for level both across the doorway and the door open position. Assuming all ok, cut the lining uprights to length, allowing for floor coverings, gaps and the trench in the head. Cut head to length, spacing the trenches equally from the overall width ( unless the opening is seriously out of level ).
Put the lining together and slot in place. Gently wedge the top in place, checking the alignment for plastering. pin in the top hinge side with screws, then level down the side and mark the floor so you know where it should line up. Then put the level on the face of the hinge side lining and wedge out the bottom to suit. Make sure your edge mark is in line and pin in the lower 2 screws. Use your level and wedges to do the middle section.
At this point i gently add a touch od pressure to the wedges, then tighten screws and check level. Repeat on latch side, making sure you head is level. Once pinned in, measure the width at top. Middle and bottom on both edges of the lining, should all be the same.....
Next, bone it in.... stand in the room, checking the edge of one door lining against the opposite edge of the other door lining ( you are looking diagonally through the doorway to see both edges ) if it is good, they will line up. If not, work out why.
Now foam.

 

[Jones]

That's a good description from Baldkev. I've only got a diagram to add. Use 5mm screws, probably 100mm and brown plugs. Also if wall is out use separate rather than folding wedges,or shim folding wedges. Expect to spend an hour or so on getting a lining in right.

 

[cone]

That's a good description from Baldkev. I've only got a diagram to add. Use 5mm screws, probably 100mm and brown plugs. Also if wall is out use separate rather than folding wedges,or shim folding wedges. Expect to spend an hour or so on getting a lining in right.View attachment 121344

Would installing the door in the linings on the ground first first, secure the lingings square around the door then remove door then install linings in the opes be ok.

 

[Jones]

Would installing the door in the linings on the ground first first, secure the lingings square around the door then remove door then install linings in the opes be ok.

I can't see any benefit from doing it that way.

 

[baldkev]

Would installing the door in the linings on the ground first first, secure the lingings square around the door then remove door then install linings in the opes be ok.

I know old time chippies who followed my steps up until the hinge side was done, then hang the door and wedge the lining to suit the hung door, then fix off. I tried it once but found it a faff with the door in place. I just do it like described.