How to fill large cracks in wooden beam

depends what timber the beam is made from and the type of finish you require. i'm going to guess oak? there are many products available which will be of use but again it depends, there are many types of woodfiller which will take a stain, but be careful to use a water based stain on a water based filler and an oil based stain on an oil based filler. in my experience it's best to mix the filler you intend to use with the stain so in effect, you are making the filler the same colour as the finished beam is intended to be. to be honest i wouldn't fill the cracks as they show age and character. just a good quality varnish and / or stain.

To fill a large crack on an old beam i would use putty but id mix a little of the stain to the putty so it blends in better.....careful when using ready mixed wood fillers as if you smear this around the outside of the crack it may be difficult to remove. Please not you get a dark brown putty for a dark beam and natural for a lighter beam.