The brand new oil I put in the tranny was Maxima MTL which is a wet clutch oil (see photo) and I have been using this in all my vintage 2 strokes for many years now. I split the cases on this bike so everything was clean as a whistle when I put it back together.
Now that I see that the needle bearing washer (thrust bearing) comes with the washer (my  "2nd washer") it now makes sense and confirms that I have installed everything correctly.
All five of the ebay bought frictions plates (see photo) are 3.9mm thick (not 4 or 3) and the recessed clutch hub nut is installed with the recessed side facing inward.

Is there a big difference between aftermarket and OEM friction plates? How about heavy duty plates? Heavy duty springs?




 

Oh yeah, I did not install the rubber rings.

Sounds silly because it's such a simple thing, but since all else looks/sounds good, have you double checked the clutch cable tension? I've been running what are probably the same ebay friction plates, EBC or whatever brand, for a couple thousand miles in my CT2 with no issues, any slipping I had was from out of spec springs, new oem springs solved the issue for me - but you said your springs are in spec so probably not the issue.

 
 
 
As promised.

Looks like the same nut as the sprocket nut with the recess.

I did have the recessed nut installed correctly (recess facing inward). Thank you for taking photos for me.

DARINM, The cable tension is good.
The Ebay friction plates are a black material while the OEM ones are a natural fiber color. Since the oil residue all over the clutch parts is black (and the oil drained from the motor) one can assume that I've been smoking the plates pretty good during only 10 minutes of use. I also noticed that the raised surface area of these plates (little raised rectangles all along the surface that first come into contact with the steel plates) is significantly smaller than the surface area of the worn OEM plates I replaced. It's also strange that Yamaha uses the same plates in both the AT and the CT.

The clutch didn't slip when I first ran it as a barn find (sucking tranny oil thru blown crank seals). Now that it has a fresh bore and a new piston it eats friction plates.

I've seen guys do a lot of weird stuff to get their clutch to function, like playing with the adjustment and adding an extra friction plate or adding washers to saggy springs. At this point, the only solution (that I can think of) is to pony up and buy new OEM plates and OEM springs. I'm just concerned that if the problem isn't the plates and/or springs I'll just be smoking a set of $100 plates just to eliminate them as the possible cause of my slipping clutch.......

Or, as my local engine machinist once told me, "Your over thinking it."

Could think more with this thread; www.yamaha-enduros.com/index.php/forums/...lutch-plates?start=0

Same plates for the clutch because the AT and CT bottom end is the same.

How about the push rod adjustment? The cable can loose yet push rod itself too tight and is worth checking on the off chance you have not already double checked it.

DARINM,
I'm not sure what you mean by, " ...push rod adjustment? The cable can <be> loose yet push rod itself too tight  "?

That was a great thread (link) you posted. Unfortunately, Willlgood never came back to confirm how the YZ125 friction plates performed under power and, although JayB confirmed that the Barnett plates fit, JayB  didn't confirm that the Barnett plates worked. I love how they are thinking "outside the box" though. When OEM parts aren't available it's great to find alternates that can (may) work.

It would be nice to have a definitive result. No slipping issues on mine so no reason to test out any other plates...yet.

Anyhow, clutch rod adjustment, page 61 in the manual, section J. If it wasn't backed out enough, it'll certainly cause slipping. Probably done with engine reassembly but worth double checking if you have not yet. May as well pull the rod and roll it on a flat surface if that has not been done. (page 60 in the manual).

www.yamaha-enduros.com/index.php/service...-and-parts-catalogue