I asked this question on chat section but no one replied so I had to ask it here. Is it possible for crank timing gear (sprocket) to slip on crank snout? As far as I know, most crank sprockets are fixed on crank snout without keyways. The only thing that holds them firmly in their position is larger diameter of crank snout relative to that of the sprocket. Is this enough to prevent slipping?
I'm not sure I am in agreement with "most" in the other answer, some definitely but I've never seen any stats on this.
I do know that some engines are designed so that the crank timing gear is held in place with friction. Case in point is the Mini Cooper (i.e. R56 model) which looks like this:
Note the red arrow pointing to the timing gear. There are no keyways or splines on this and it's held in place by pressure between the end of the crankshaft and a bushing that is attached by the crank pulley bolt. If the bolt is not tight enough the gear will slip and that's a big problem.
The advantage to this setup, though, is that you can much more easily change the timing chain from the top. You remove the crank bolt and pull out the bushing, then after unbolting the guides you can lift the entire assembly out the top of the engine.
So in answer to your question, YES. In some engine designs the crank timing gear is unkeyed and it CAN, under the right circumstances, move and change the engine's timing.
In the specific case of the Mini R56, the torque on the crank bolt is significant so that this doesn't happen. As I recall it's something like 120 ft-lbs. PLUS another 180 degrees of rotation. Having done this I can tell you that it's quite tight.
Most crank sprockets are fixed to the crank with keyways afaik.
It is also easy to have a press fit such that they won’t move, but then pullers are needed for removal.
An interference fit can be achieved - often by heating one part or cooling the other. You can find youtube videos showing bearings being fitted into holes after submerging in liquid nitrogen - once the assembly reaches equilibrium then the bearing will not rotate in its housing.
So, if a sprocket without keyway is slipping then something is wrong, either someone has made the hole larger or the shaft smaller.