Quote Originally Posted by Bentley Boyd
There used to be an adaptor made by Pentax and several accessory manufacturers that fitted in to the K mount ,and was spring loaded so it stayed in and it adapted to fit M42 screw lenses , it was just a little silver ring you should be able to get hold of one still. If you do locate one make sure it comes with the little tommy bar to remove it otherwise you will have problems. As far as I remember the adaptor may effect a lenses ability to focus on infinity but at least it will be usable to some degree

The K-mount, although designed by Zeiss, took into account of backwards compatibility with M42 lenses via that adapter, so full focussing range is preserved. The original adapter by Pentax was not supplied with a tool for removal; the original instruction manual indicated that you use something like the top of a ball-point pen for the purpose.

While the K-mount throat is a little larger than M42 screw mount, the body thickness is the same, hence the adapter sits inside the throat, which also makes it necessary to take the lens off before removing the adapter. In addition to that, the lens itself needs to have facilities to stop down the diaphragm externally, as the striker pin cannot be pushed in when mounted. When Praktica introduced the B-mount system, the camera body was designed to be a little thinner so the adapter sits on top of the body bayonet, and at the same time has an inner flange which forces any automatic diaphragm lens to work as manual, eliminating the danger of over-exposure which afflicts the Pentax system. Of course, you can fit a Praktica adapter to each M42 lens to make it bayonet mount if you so wish, but with the Pentax arrangement, it was probably designed to frustrate you enough to get K-mount lenses instead.