Good day to the biggest yamaha enduro fan base ever. 
I pose a question today that I hope someone here can shed a light on.
Is there a service notice that covers the increase in size for the conrod big end pin from 24mm O.D. to 25mm ?
Makes a difference when ordering a conrod service kit as I have just recently learned.
Also be nice to know what the engine no is that the change was implemented on. That may be impossible to know, but a base to start with might be established amongst the group. I have one of those engines with 25mm pin in the big end.
I hope to hear from one or more of the senior folks on here, who serviced these old bikes, that may know something about this subject,
Have a great day guys and carry on rebuilding the greatest enduros ever. JMO

The way i understand it is : 68-71 DT1 24mm, 72-73 DT2 & 3 most likely 25mm due to change in rods on reed valve 250 engines. 74 DT250 and up 25mm for sure. 

I don't know of a service bulletin...but I've always known there are two sizes. 

When I get an order for rod parts, I always ask them to measure the pin first. I don't rely on the year or model. Things could have been swapped over the years.

Measure first. Everything has to be mated to that dimension.


 

"DT1F" is not a model, just a VIN prefix that was used for several model years. As Skydancer says, the change seems to have happened during the "reed valve" years but I suppose it's possible to find the 25mm pin in an earlier model that had the crank replaced?

The Service Bulletin is confusing because it says the update is for the "DT2/DT3" and gives an ending number of "DT1F-153463" for the switch to the 25mm pin. That could be because a new DT2/DT3 crank assembly would come with a 25mm pin or it could mean any DT2/DT3 might have the 25mm pin from the factory?

Do you have a DT3 with a VIN above 153463?

360's have the 25mm pin by memory. Easier for them to keep it that way on the 250 too I guess. With the 25mm pin the rollers are smaller diameter as the Conrod big end 'eye' doesn't change?? Think it stay as a 214 rod?? Correct me!!.
Might have to correct myself--i keep seeing 431 rod. Just ignore me, i guess you need a 130mm rod for a piston port 250.

Mark, I have two DT3 engines.
         The pin in question is from engine no. DT1F-159200.
         DT3 Engine # 2 vin is DT1F-164884. I have not split the cases on this motor yet. 
         My 3rd engine is a DT2 with vin no. DT1F-125788 and is not split yet either.
The good news is the vendor I got my rod kit from with 24mm pin is sending me the kit I need with 25mm pin.
He knows about the two sizes but not when/where it started in the vin no. sequence. Thankyou Skydancer for enlightenment !
Also makes sense that if crank was switched at some point it could be either one.
So as Deet says, measure before ordering your kit. Saves time and dollars. 
Not being aware of this change in pin sizes was a stumbling block for me getting the correct kit.

Thanks to all who shed light on this topic for me.
More knowledge going forward with the next motor rebuild is a Great thing.
 

Also.... what does this tell you about buying crank halves?

Ya'll may have better eyesight than me... but I doubt that I could tell the difference between a photo of a 25mm hole and a 24mm hole.

Better ask for measurements.

With the 250 64mm stroke the pin breaks into the circle the rod thrust washer bares against. So if your eyesight is good you'll see it breaking in by half a mm more--that me out then haha can't see very well.
I'd better go find a pic i might be wrong again.
Ok i meant where the main bearing buts up to not conrod washer.
Says "79 DT250 but same stroke.
photos.app.goo.gl/vtjDRj36aD4NCXDD8

Ok i'm making a pigs ear of it all.
Time to go & do something useful.
But my parting shot is--
RT3 crank then DT3 crank suposedly both with a 25mm pin by "73.
photos.app.goo.gl/oqbXsLZkzZR6wEGR7
photos.app.goo.gl/2uytrfnZZYjJt4t1A
Can see the difference where it pokes through the crank wheel.
Mind you that didn't help comparing 24mm to 25mm on 250.!!

Steve,

Sounds like you might be able to verify the Service Bulletin info I posted (assuming parts haven't been swapped over the years)

Your two DT3 engines are past the cutoff of DT1F-153463 and should have 25mm pins. The DT2 is lower than the cutoff number and should have a 24mm pin.

But again, always better to measure before ordering.