Reply To: Loose mortise and tenon joints

I can second the comment above about very fine paring, it feels slower, but it’s quicker than a repair or a replacement.
The guide block was developed (if I remember the blog posting / comment correctly), to help his students chop plumb, and in line. Means if you do use the guide (and make sure the chisel stays plumb as you chop), the width of the mortise will be the width of the chisel, and the position of the mortise will be governed by the guide, regardless of what you have marked on the work piece.
Its also worth noting that if you are going to use the guide, that you don’t really need to set up a mortising gauge to mark the tenon, you can use the guide for that as well (on the assumption that both pieces are the same thickness.
In the past I have been able to rescue loose tenons with long shavings. You lay them out flat, apply a film of glue to the upper surface, then wrap them around the tenon like tape. The shavings need to be thin, and you do need to be careful when you do it. Let the glue set and then trim any excess length off, and start to fit
For very loose housed tenons, split off some wood from a scrap piece and glue it to the tenon, or try a lengthwise fox tenon.
If it is a through tenon, and the stock you split has a taper, see how well it might function as a wedge (if you can get two thin wedges down either long side of the through tenon, I suspect only you would know it was there after you finish everything).