Thermal Fan Clutch

[A1218]

I am replacing the fan clutch and blade on my 1997 Pathfinder. Fan blade has developed several cracks on it due to age.

I see online there are standard duty and heavy duty fan clutches. What’s the difference here and the appropriate option?

Also, how do these thermal fan clutches work when the fan on these engines spin all the time even on a cold engine?

[VStar650CL]

Also, how do these thermal fan clutches work when the fan on these engines spin all the time even on a cold engine?

They're "viscous" clutches, meaning they have a thick silicone liquid inside that couples the driving and driven sides of the clutch. They usually use a temperature-sensitive valve (most often controlled by a bi-metal spring similar to an old-school carburetor choke) which allows more or less "slip" in the clutch depending on the heat load coming off the radiator. That allows the blades to push harder at higher temperatures and less at lower temperatures. The effect on power consumed by the fan is actually pretty dramatic, particularly at highway speeds where forced air is doing most of the cooling and very little fan is needed. This vid has a pretty good demonstration:
https://www.youtube.com/watch?v=UwM4OqsLek4

[mdmellott]

I see online there are standard duty and heavy duty fan clutches. What’s the difference here and the appropriate option?

The viscous clutch description and video shared are excellent explanations of these fan clutches.
As for the differences in standard and heavy duty, typically that has to do with how much the clutch locks up when engaged at higher temperatures. These fans never lock up 100% unless they are broken and seized up. Heavy duty thermal fan clutches will lock up at a high percentage than standard duty thermal fan clutches. Heavy duty may be a good choice in the southwest of the US but in the northeast, and the rest of the country, standard duty is a better choice.

[A1218]

I’ll be sticking with the standard duty then.

Are the aftermarket parts for this okay or should this be OEM Nissan only?

There is a wide selection for it on Rockauto. Four Seasons, Hayden, GPD, SKP, Beck/Arnley, GMB, and FVP.

[Niti QX4]

I would go OEM. It's lasted this long