Smoke box door are held with the dart type of fixing.
Do you mean that

Hello David,

Here are the firebox doors.

Here is picture showing the smokebox door.

The (detached) smokebox door is central to this picture and has the 'dart', or device for securing it closed lying on it.

Hopefully those pics will assist you in identifying at which end of the boiler your problem lies.

From your description I suspect that your issue is with the firebox doors, which will often be 'stiff' until they become a little worn. If this is so a moderator may be able to amend your thread title on your behalf. Is your loco from a kit? If, so there may be some 'fixes' if this is a common issue. (Note: from what I have seen all the kits need additional works of various types to 'sort' them). The slides that the doors run-in may need some (very gentle) fettling or adjustment if they are simply 'sticky' or too stiff to use easily, and/or the slot in the operating lever that allows coordinated opening may need smoothing. The doors themselves may benefit from smoothing where they run on or on the sliders. Maybe the bottom door slider needs a slot adding behind it so coal-dust can fall-through and not 'clog' the runner if your issue arises when running.

There are owners of both kit-built and 'scratch-built' Britannias on Model Engineering Clearing House who may be able to assist. The author of the recent Britannia kit rebuild series in Model Engineer Magazine is an active member there and also drops in on this site occasionally so he may spot your posting.

Regards,

Phil

Edited By Weary on 13/11/2022 12:28:48