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I think its a big one 5/16 between the mushroom & pushrod. Mushroon extension will be 5/16 also. Be unlucky if thats been left out & makes ya wonder what else.
Hope not too many bad surprises in store.
But in the acuator where the cable goes there's a ball in there which in theory shouldn't drop out & without looking at mine i'm sure it's 1/4.
I butchered my actuator case & fitted an actuator from an RD400 or something the similar.
They're a 4 start steel worm--as oposed to three start--with a nylon outer worn and a tin outer casing.
Bit of a fiddle to get it in & set right for angle of the dangle--.
There's a post going back a bit of someone doing that operation.
Ok found something. Scroll down this page.
yamaha-enduros.com/index.php/forums/1972...-actuator-mod#117596

I just tried to fire up my 69 dt 250 and it didn’t want to fire up. It showed signs of life so I tried to bump start it. I figured the carb needed a quick blow out to make sure everything was clean. Usually if that is the issue, I can get it to run when I bump it. 
I just needed it to run for long enough to test my clutch after modifying the push actuator. I just wanted to verify that the clutch would engage and disengage. 

So, I pushed it up my street and then back down. As soon as it fired up I knew what the problem was. It was popping and snapping out of the exhaust manifold. Immediately I knew that one of the exhaust manifold studs had pulled out. It had happened a long time ago but I was able to repair it by retapping the hole and using a very small amount of high temp/pressure JB weld. I am now remembering that I think it originally just had bolts. I changed it to studs when I first fixed it. 

It held up for a long time but it now needs a more aggressive approach. I am figuring that it will require a slightly larger bolt or stud which will mean drilling the hole and retapping it. 

So after doing my usual search of the forum I figured I would throw this one out to the group and hopefully start a discussion amongst the geniuses out there and determine what works and what doesn’t. I certainly don’t want to eff-it-up and end up having to replace the barrel. 

So, who among you knows what the correct path is to get a good solid repair?

As always, I thank you for always being so helpful and supportive. 

Pmb

The cable end fits in to a boss that is cast in to the case ahead of the clutch lever actuator arm. On my 74 / 75 and probably the 76 there is no other option for the clutch lever to be operated. The clutch cable follows the Lh down tube and proceeds to go under the motor right by the exhaust pipe.

Did the cable change from pulling under the motor to pulling from the top. Actuator changed sometime but not sure when.

In my opinion, aftermarket or not it still shouldn't slip unless there's a problem causing it.
Like those rubber rings left in place, or for some reason not enough spring tension although unlikely.
Or adjustment is to close to the zero clearance & the helical primary gears are moving the shaft sideways a fraction under power & reducing clearance to minus zero [i'm clutching at straws lol].
Or cable is binding [bin it & replace with a nice new free sliding one].
Actuator is dry & binding. Things like that!!, or add a washer under the springs.
Do plates stack up less than the standard thickness. Is each one thinner.

Here's a random could of pics from ebay showing how the actuator vertical shaft is offset from the trans main shaft, which i guess it has to be to swing & push if that makes sense..
Sorry to keep this going. Was just googling being nosey & thought ok, link it.
photos.app.goo.gl/Uuh9WCHjWNGffQWH7
photos.app.goo.gl/3uNon91dYg5XzMZj9

Hi Mark, just my opinion but i think the taper turn away & around the side of the pushrod so couldn't see fiber plate wear stopping it coming out.
All good. We wait. Cheers.
ps to OP just be careful engaging the tit of the adjuster in the actuator groove.
Think he shows it adjusting & looks like he's turning constantly but in reality when engaged & in about a turn then he is only going a half turn clock & half anticlock which is your total adjustment movement.
He doesn't hook the cable on & set the arm for cable play or optimum pull angle, but good video none the less.
On second thoughts maybe he has set the arm to an alignment casting mark.
Might be where the spring hooks on, not sure
My useless laptop won't play without buffering. I'll watch it later.

Says clutch rod seal but realy means actuator arm seal.

So ya talking #23. I'd lay bike right over flat on it's right side so you can see what ya doing, then unhook cable.
Then adjuster lock nut, then wind adjuster counterclockwise while arm goes up & down until adjuster falls out--& disengages.
Then actuator arm should fall out if you turn it away from the pushrod.
Away will be opposite way to cable pull direction.
Then carefully flick seal out with screwdriver.
Grease the new one & push in.
Assembly should go well but will need to have the arm around to get past [or beside] the pushrod.
Then feel the adjuster engage in the arm as you screw it in.
If you screw in too far & goes tight just back it out so you can get a full movement of adjustment up & down.
Ya only get a half turn of adjustment by lifting or lowering the arm which you will see has a very tapered flat end that bares [bears] on the pushrod.
Should be a casting mark under the case to give an approximate ideal angle for lift but depending on the cable you might need to slightly ignore the mark.
Sorry if i'm shortcutting words.
Correct me anyone if i'm wrong--been 50 years since doing that at work lol.

www.partzilla.com/catalog/yamaha/motorcycle/1974/dt250a/clutch