I have recently re-trimmed my RHD 1999 90 seats with Exmoor Trim covers and all went well, except for the passenger front seat back keeps falling forward. I have removed the seat & frame, and it would appear that this is caused by the main spring causing the frame to twist, and this in turn is enough to make the two arcs of "teeth" to not bind sufficiently.

When on the workbench and the seat back is fully down, the seat rails are flat on the bench, but if I raise the seat back to the fully upright, the rail opposite the reclining lever rises to about 8cm above the workbench at the front (due to the "twisting".

The "teeth" themselves are not particularly worn; I didn't have this issue before I re-trimmed the seat; the driver's seat is fine, and I _think_ that I re-assembled the seat frame correctly.

Any ideas how I might resolve this issue?

TIA

Can you not just twist the frame back the other way if that is the problem

Not really; when I bolt the seat down in the cab, the rails are flush to the floor & the twist is no longer visible, but the effect of this is that the seat back teeth don't engage sufficiently to hold the seat back firmly. Unless I have misunderstood your reply?