The rectifier will heat proportional to the power being used by electrical devices and the battery undergoing charge. This is why is has a heat sink, to get rid of enough heat that the diodes will survive. Electric start draws a great deal from the battery (150 amps). This has to be put back after the engine is running. And, the alternator will give all it has until the recharge task is finished. It provides more with engine RPM. And, it all must go through the rectifier.
Check to make sure that the battery isn't getting above 14.5 Volts when the engine is sustaining RPM above 3-4000. Higher voltages will incurr a higher draw amperage with fixed resistance devices like lighting, etc. A charging battery increments slowly.
The fuse should get warm. But not warmer than you can stand by holding your thumb on it. (engine reved)
If it is warmer than this, then you either have more than 2/3rds it's rating passing through it. Or, the contact clips have some resistance heating going on. Surface oxidation, even if it is transparent, can create heating and this can be transferred to the fuse. Low clip contact pressure can also contribute. You should need a tool to remove the fuse. If you can remove the fuse with simple finger pressure, the clips have lost their retentivity and this increases resistance and contact heating.
Poor connections on the back of the fuse block can also heat, and the heat can conduct to the fuse. Lastly, the fuse clips should "float" so they will align themselves for maximum contact surface area between fuse clip and fuse. A small contact area, can also increase resistance and heating effects of passing large currents through them.
All this assumes you haven't changed the electrical load with higher wattage headlights, signal bulbs, or added some other current needy devices to the electrical system.
When checking your stator, you must subtract the measurement error introduced by the test leads. Subtract the value you read when you simply touch the probe tips together.
Cheers,
