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?
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Knowing what engine we're talking about would sure go a long way to telling you whether this could happen on your specific engine.– Pᴀᴜʟsᴛᴇʀ2 ♦Sep 10, 2021 at 19:52
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@Paulster2 - This is a general question about any engine which is designed based on friction between crank hub and sprocket.– Nariman AsgharianSep 10, 2021 at 20:30
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2 Answers
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.
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You picked a good example of a poor system: casestudies.atlanticmotorcar.com/… Sep 10, 2021 at 11:27
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1I'm not defending the design, I'm just saying that it exists and I am sure other engine designs use similar systems. The problem noted in the article, however, has to do with the chain itself, not the crankshaft gear attachment. The OEM chains had a tendency to break which causes severe engine damage. It was, however, relatively easy to replace.– jwh20Sep 10, 2021 at 11:39
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You should note that I did not say that that design did not exist... Sep 10, 2021 at 11:41
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@jwh20 - In the engines that are designed based on friction between hub and sprocket, how is it possible to replace a faulty sprocket without disturbing engine timing? If there is no keyway, you can install the sprocket at any angle without knowing the right angle! How do you find the correct angle? By marking? Sep 10, 2021 at 18:44
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1I can only speak for the Mini R56 but there is a pin that is placed in the flywheel that fixes it and then a bracket that fits over the camshafts that fixes them. You then remove and replace the chain. If you need to remove the cams for some other reason then there is a more complicated timing setup that I didn't need to do.– jwh20Sep 10, 2021 at 19:47
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.
answered Sep 10, 2021 at 6:31
