Hi Toivo,
Have you considered offsetting your tenon towards the lower face. I am assuming that the lower face is not cracked since the upper face is affected by a drying radial heart check. Drive a tapered peg from the underside so that the peg hole is not completely filled with peg on the upper face. This should help reduce the probably of the peg propagating the crack any further.
Occasionally one will come across situations where pegs need to be driven from inside a building towards the outside for example when a space in a row of houses in a town is being infilled preventing the pegs being driven outside in.
Regards
Ken Hume
