Hi all Most are probably not familiar with the yamaha Rs125dx engine.
but I'm fitting a 74 dt175 top end on one.

Upon doing this I noticed the 125 small end con rod is only 16mm wide.
with the 19mm cage

Where the ct3/dt is 18mm wide with the 21mm cage .

Most 175 pistons have a gap of 23-24mm where the bearing and pin sit.

What cage combo would you run?
19mm cage on the 175 piston / 16mm 125 conrod
Or 21mm cage on the 175 piston /16mm 125 conrod. ​​​​​

I don't know anything about this model.  Just to be clear though, on the Enduro models the connecting rod is the identical part with identical dimensions between 125 and 175.

On the Enduros, the small end bearing is wider on the 175 to match the wider opening in the piston. 

Which is exactly what I would do in your case, use the bearing that matches the 175 piston. 

Be interesting to see how you go fitting a 175 cylinder.
I've had the RS125 which was a 4 speed 'rocket' torquee & revved so best of both worlds.
& i've had the RX125 which is 5 speed & less torquee but revs out.
They both have the extra tall piston & an extra 5mm or so in cylinder height although the RS porting is better but i've also had early & later RX cylinder looking different & bridge inlet etc so i don't know any more haha.
But--i fitted an early AG175 cylinder [just a restricted ported CT3 type] & had to add a 5mm base plate [or would have had to change the rod].
Was many many years ago so open to critisism [or correcting].
So interested to read your outcome.

Just rethinking 'that'--i think the piston was longer from the pin 'down' so must be a 5mm longer conrod for the taller cylinder which would be why my 175 came out the top before making a base plate--i think i think!!.

Ok ignore that, just found a piston pic & is taller from the pin 'up' so i dunno anymore lol.
photos.app.goo.gl/5JcYPxHE2gTe1aoi8

Assure needles do not extend past rod small end.


cliff

This is incorrect, the needles do need to extend past the width of the con rod.

I can help you bit need a bit more info 
You have a RS125 conrod that you want to run a DT175 piston on to ? 
What size in the inside of the small End Eye of the RS 125 ? i have it as 20.00mm 

With the DT175 piston you need you need to use a 16x20x22.4 which is a std size small end for a DT175 

16mm is the G-pin size 
20 is the OD of the small end to fit into the rod 
22.4 is the width of the bearing cage 

You need the small end to fit between the piston with little side play, if you use say a 19mm wide cage in the 175 piston it will screw to one side and the bearing will break up ! 

Post #5 is that a typo saying needle "do" need to extend past the rod?. The cage 'does' need to extend to fill the gap to the piston gudgeon boss [not the needles], but needles need to cover as much of the rod as possible [in my humble experience] Cheers. ps, maybe i misread it or misunderstood.

NEEDLES do not(edge loading and destruction)
CAGE does(axial clearance to piston piers)

cliff

RT325

Sorry not very good a trying to Quote a post, not sure this is going to work. 

My reply was to post #4 "Assure needles do not extend past rod small end" 

You can have a small end bearing that over hangs the small end eye of the conrod, it does not cause any issues at all and is normal in 99% of engines. 

What you dont want is a small end that has too much float side to side in the piston, you need the widest small end you can fit between the G-pin Bosses.

It the case of the DT125 V DT175. They both use the same width conrod, but the small end of the 125 is 19.50mm wide V the 175 which is 22.40mm wide. If you fit the DT175 piston with the 125 small end, the bearing will float side to side and fail. 

Hope that makes sense 

All good & thanks. Have a good Xmas. Ps not sure if anyone's had a little end bearing breakup but for some reason back in "72 & either my dt2mx or might've been my bosses rt2mx the bearing cage broke up & did it more than once. That was a new bike but had done a few races & pulled down for rings or piston kit & found 'that'. Must've stayed in place or I'd be telling you of a mess of damage. Probably the thin area between rollers that let go. Can't remember now but might've been just a bad batch on the assembly line & also as replacement. Must've improved the cage thickness or something after that as was cured. Not a good thing to happen though. Sorry keep editing & adding in, hope readable.