Why does rust on driveshaft mean you need a new one?

[RCSConstruction]

Our cars/trucks have exposed u-joints and last 100's of thousands of miles. Why is it that when you get a little leak in the bellows, and rust forms, the shops all want to replace it?
Why is a VP driveshaft so different than a Chevy?

 

[Fun Times]

It's the inside internals of the u-joint that develop water intrusion from "soaking" in water over a period of time due to resting within a capsule designed U-joint bellows.

Trucks typically just splash through water but hardly ever submerged in water.. If they do then it would be best to replace the truck ones as well.

Also bear in mind that typically the U-joints on a stern drive tend to be moved/turned at sharper angles than a truck drive shaft.. Plus they're turning at higher RPM's. The damage cost is high when they come apart at speed sometimes breaking the bell housing and possibly other related components within the area.

 

[jerryjerry05]

Usually the drive shaft isn't replaced but the U-joint assy. is.
​The assy. contains the front yoke where a seal rests.
If the yokes surface is rusted then the seal won't hold oil and the water can get into the drive.
​You can replace just the yoke.
​Depending on the amount of rust?? the u-joints should be replaced.
​Surface rust?? or badly corroded?
​The whole assy. is expensive, the pieces aren't.
​They sell you an assy. because it's way easier to replace that.
The labor to change the u-joints brings the cost up as it's not that easy unless you have the right tools.
​I bought a u-joint press from Snap-On one of my better purchases.

 

[Scott Danforth]

a bit of rust on the input yoke assembly and it will take out the input shaft seal on the upper gear box. when that happens, you start loosing gear lube in the outdrive and eventually water gets pulled back in from your leaking bellows.

the parts are expensive.

the maintenance to keep the bellows in good shape an the u-joints pristine is priceless