Hello guys, I noticed that the small transfer port (marked in red) is approximately 2mm lower than exhaust and large transfer ports in my 45200 cylinder (1974 125DT). I checked that on two different cylinders and it appears that both (one of them NOS) feature the same abnormality. What is the reason for that? When the piston is at BDC, the extra 2mm reach into the piston so, in my eyes, that does not make any sense.

Thank you.
Mario
 

Is it open to area under piston with piston at TDC?  Could be some pressure balancing trickery?

In any case, really doesn't matter too much...  a lot of 2 strokes use the piston as the transfer port wall... 

Here‘s some pics. You can tell the bottom line of the small port is clearly hiding behind the piston at BDC. At TDC there is nothing visible from the small port underneath.

Pretty common on yamaha two strokes of the era, my 360 is the same. Having the port lower than the piston will produce a slight upward bias in the aim of the port. Most of the flow goes through the main transfers, these little guys just help direct the scavenging.

Does it make sense to work on the piston to free up that hidden 2mm from the small port? Or would it bring difficulties with detonation at TDC due to the exposed squish at that area?

Modifying the piston in that way would open the port significantly sooner as well.  You don't want that. 

Its not a problem as is.  Certainly intentional on Yamaha's part. 

What‘s the contra of the port being open longer? (Open sooner and staying open longer due to the piston reworked).

I just fitted my RT2MX cylinder on my RT2 motor & i'm certain that was the case with the rear transfers. [too late to look again now] I've seen it before & just thought--wonder why!!. Yamaha must've had a plan. Be interesting to see the factory flow testing the ports to see how they interact with each other. What Turbodan says makes good sense to me.

@italjet

Problem #1 is that you don't want these rear transfers opening before all the rest.  More commonly these days when you see staggered porting the rear transfers and boost port are actually opened later than the main transfers.  The difference is also much less than 2mm.  The first ports to open see the highest pressure, to prevent mixture heading for the exhaust port you need the main transfers directing the flow rearward to properly scavenge the cylinder. 

Problem #2 is that you don't want too much transfer duration for the same reason you don't want too much exhaust duration.  I don't know what RPM you're trying to run this thing at but without a very good pipe and large carburetor you would be best off matching all of the transfer ports to the same height.  Measure all of them, find the highest one and match the others to it.  Excessive transfer duration will rob low-mid power without any gain on top.

I really wouldn't worry about this little step at the bottom of the port.  The majority of the transfer flow is going through the much larger main transfer port, and of the small amount of flow being delivered through the aux transfer, most of that mass is moving through the top half of the port.  It is likely that Yamaha wanted the step to induce a certain amount of turbulence to slow down the transfer stream and allow it to join up with the main transfer flow up the back and over the top.

I noticed the complete manual explains the reason for the addition of the “auxiliary” ports if those are what you’re referring to. Seems this is the basis for the  torque induction Principle, better top end range and cooling for the piston leading to longer engine life. I didn’t read through it carefully as I was looking for carb info and had just read this string.