Why will my tenon not fit the mortise?
Too tight, too loose, will not go home, or goes home crooked — four different faults with four different causes, and which surface to correct in each case.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 3 min
Short answer
Chalk or pencil the tenon cheeks, push the joint together until it stops, withdraw, and correct only where the marks transferred. Too tight is almost always the cheeks near the shoulder. Will not go home is usually waste in the bottom of the mortise or a tenon cut too long. Crooked is a shoulder that is not square, not the tenon at all.
A mortise and tenon that will not fit is normally being corrected in the wrong place. The instinct is to pare the tenon everywhere until it goes in, which produces a joint that slides together sweetly and has no glue line left. Finding out where it is actually rubbing takes thirty seconds and saves the joint.
Step by step
- Find the interference before removing anything.Rub chalk, pencil or a candle over both tenon cheeks, push the joint together as far as it will go, and pull it apart. The bare patches are the only places that need work.
- Too tight across the width: pare the cheeks, not the mortise.The cheeks are long grain and pare cleanly. Take a shaving at a time with a wide chisel or a shoulder plane, keeping the cheek flat rather than rounding it.
- Tight only near the shoulder: the cheek is not parallel.The saw drifted as it went in. Pare the thick end back until the cheek is flat along its length; a straightedge along the cheek shows it immediately.
- Will not go fully home: check the tenon length and the mortise bottom.A tenon should be a millimetre or two shorter than the mortise is deep so it cannot bottom out, and the mortise must be clear of chips. Poke the bottom with a narrow chisel and clear it.
- Shoulders will not close although the tenon is in: the shoulder is not square.Check with a square across the shoulder line. Pare the high side back to the knife line — this is the surface that shows, so it has to be right.
- The joint goes home but the frame is out of square: one shoulder is longer than the other.Shoulders on opposite faces must be exactly in line, or the piece is pulled askew. Mark both from a single knifed line carried right round.
- Too loose: pack the cheek rather than remake the joint.Glue a shaving of the same species to one cheek, let it cure, and pare it to fit. A veneer or a slice off a plane shaving works. Do not rely on glue to fill the space — a thick glue line is a weak glue line.
- Aim for a fit that goes in by firm hand pressure.You should be able to push it together and pull it apart with your hands, and it should feel snug all the way. Anything needing a mallet dry will split the mortise wall once glue swells the wood.
Tips
- Cut the mortise first and fit the tenon to it. It is far easier to adjust a tenon than to widen a mortise accurately.
- A haunched tenon — where a short stub of the tenon fills a groove or the top of the mortise — is used at the top of a frame to stop the rail twisting while keeping the mortise from breaking out at the end of the stile.
- Chamfer the end of the tenon slightly. It picks up the mortise mouth without crushing the edges on every trial fit.
Common mistakes
- Paring the whole tenon to get it started — The first millimetre of the mortise is often the only tight part. Removing material all along leaves a joint with no grip and a glue line too thick to be strong.
- Forcing a tight dry fit with a mallet — Water in the glue swells the compressed fibres. A joint that needed hammering dry can split the mortise cheek during glue-up, when nothing can be done about it.
Questions people ask
How tight should a mortise and tenon actually be?
Snug enough to hold together dry when the frame is stood on end, loose enough to push apart with your hands. If it is loose enough to fall apart, or tight enough to need a mallet, correct it before the glue comes out.
Can I fill a slack mortise and tenon with epoxy?
It will hold, and on a repair to an old chair leg it is sometimes the pragmatic answer. On new work it is a poor substitute for a fitted joint, and it makes any future repair much harder because it is not reversible.