WTB: Drive Shaft for K75

[Radioshack1312]

Looking for a drive shaft for my 1989 K75. Gotta be in better condition than the one I've currently got, which shouldn't be too hard.

I'm located in NYC but willing to travel in the tri state area to pick up, otherwise will pay shipping. Venmo or Zelle preferred, cash of in person.

Thanks,
Ben

[Past-my-Prime]

I'd sell you this one but I'm pretty sure yours is better.

I was wondering about re-splining these?


fucked if I can figure out how to show photos!!

Look in my gallery if you want to see a bad driveshaft spline! (2019 photos)

[frankenduck]

These guys do spline replacement:

https://kpartsholland.com/products/k75-k100-rebuild-driveshaft-20-tooth

Or you can just buy new splines:

https://kpartsholland.com/products/replacement-spline-for-drive-shaft-monolever-only-20-spline

[Radioshack1312]

Oh that's a really good resource frankenduck, thanks! I think I may send out my stock drive shaft for that repair and use some drive shaft of dubious condition from here or eBay.

Which brings me to my next wondering, there's some wear on the final drive as well, is putting a fresh drive shaft on there akin to replacing a chain but not the cogs?

[frankenduck]

Disclaimer: The following is my opinion and may not be fact.

I believe that the final draft input splines are harder than the drive shaft splines which is why the shaft splines wear faster.

However, when/if the DS splines completely strip they will trash the FD input splines as a lovely parting gift.

Whatever DS you're using, be sure to lube the splines well. I'd use this goo:
https://www.beemershop.com/product/ts-60-spline-lube.html

[The Mighty Gryphon]

As the Duck has mentioned, the drive shaft splines are softer than the splines on the final drive.  This is so the (relatively) cheaper drive shaft will wear before the more expensive(and harder to repair) final drive.

The initial spline wear is the result of rubbing action between the drive shaft and the final drive caused by the change in shaft length due to the linear action of the universal joint at the transmission end of the shaft.  While this motion is very small( ~ 1mm ) it occurs every rotation of the drive shaft(1000's of times a minute at highway speeds).  This rubbing is the reason spline lubrication is so important.  Since the shaft splines are softer than the final drive splines, the wear is focused on the drive shaft.

The real damage starts happening when the splines are visibly worn.  At this point, the back and forth rubbing action is combined with impact between the splines as the clearance between them opens up.  Despite the lubrication, this hammering starts to wear the harder final drive splines.  Lubrication has no effect on this hammering impact.  Since the magnitude of the impact increases with the clearance between the splines when wear begins to appear it is important to get the best drive shaft available to get the closest fit.

[frankenduck]

Ted Porter sells a Z20 drive shaft with a rebuildable U-joint. $469 vs. current BMW price of $750.
https://www.beemershop.com/product/ei-k75k100-driveshaft-20-tooth.html

This site sells a compound for repairing splines:
https://appliedmaintenance.com/machinable-repair-compound.html

Here's the instructions:
https://appliedmaintenance.com/spline-shaft-repair.html

I've never used it but it's an interesting approach to spline repair.

Spline wear occurs when metal slides against metal under load. When the monolever final drive moves up and down the splines move back and forth so a lot of spline wear occurs when you're accelerating and the final drive rotates up. You can actually measure the deflection if you remove the final drive and rotate the swing arm up and down.