I measured with a digital caliper from the outside, the thicker portion not in the channel or recess. I'm just gonna recommend my friend not be such a cheap ass and get new friction discs and springs.

Operating worm spiral thing needs to be free & lubed in that left cover & defo needs a good spring [not a useless one like they have] to pull it right back so ya know clearance [is actual clearance] when released. Have had experience with a 360 that had extra end clearance on the bearing fit in the case where i think it'd bent the circlip over from gremlins inside causing damage. Then because the bearing could move--only a mm or two--the helical-beveled? [angled clutch & crank gears] under load & power it would force the mainshaft over that little bit & take up any clutch rod clearance plus a fraction more & make it slip by just lifting it enough to let it slip. I knew what was going on so just gave it extra clearance until i had the motor in bits again [almost weekly lol]. Could feel the lever clearance go away under throttle then return if i backed off & pulled the lever & let it go again. Ok, that's a bed time story it's so long [asleep yet]. & probably not even relevant to yours. My theory on those rings is they serve no useful purpose but can help a worn or glazed clutch to slip more than it would otherwise. They might make for a smoother uptake when releasing the lever. & my theory [only my theory] is they carry oil around & almost try to drag the plates around giving a clutch drag situation as they're sort of V shaped & i think thet carry oil in the V. Bit of a lame theory though lol.
Forgot though, ya need a nice new Lubed & free moving Cable.

Here's pictures of one of my old OEM friction rings, measured from the outside and the inside, with quite a difference. You can get a set of aftermarket friction rings for under $15 with free shipping,, and clutch springs go from $18 to $25 for 5 depending on where you buy them. Great points from RT325 about lubing that worm mechanism in the mag cover as well, and I may look at my spring. I still have the original return spring.

   

Great advice Tom. 

One other thing I've seen is sometiimes people can't figure out adjustment.  They end up with free play at clutch lever but worm gear adjustment is too tight.  Might be a gummy worm gear or cable like RT325 mentioned.  I like to completely disconnect the cable at lever when doing the worm gear adjustment...  verify I have a little free play when pulling cable by hand at engine.  Then connect cable and set lever free play. 

might find a pic or video sometime of re-corking clutch plated. it'll make ya laugh if ya too young to remember or know about it. must've done literally 100's in the "60's. thumb on my right hand is wider than the left slightly i recon caused by that haha.

New plates and springs have arrived. Will be installing tomorrow and I had one thought. Maybe we had the clutch spring bolts too tight? I was recently working on a Brit bike that when you'd put it into gear it'd lurch and stall. Clutch springs we're too tight throwing off free play, lever feel, and wouldn't allow the plates to separate. On the CT1 how tight should those spring bolts be compressed?

No. i was bought up on all the british brands working on them from 1962 in the shop, so i know what you mean. But jap stuff is screwed right home to the spring post.

Yes, what RT325 said.  Yamaha spring bolts have a built in spacer and you snug them down. 

I would take the clutch cover off and make sure there are the right amount of plates and they are in the right order and then make sure the pressure plate is not hanging up on the inner basket
doug

What order do the plates go in on a ct3 please