RD250

At least here in the US probably the least familiar of the Rd250/350/400 family.
On this page I'll try to share some info about the RD250 and their development from 
RD250 to RD250LC to RZ250

The US RD250 was produced between 1973-75
Here are the port specs for the US model 

The next cylinder redesign the "2R8" cylinders porting specs looked like this. 


(Special Thanks to John Ritter of Vintage Porting for loaning me a 2R8 cylinder)

When compared to the later Non US 2R8 cylinders 1978-79 you find A Lot of differences.               
The first thing you notice is the increased cooling fin area, the fins are now as large as the RD400.
The transfer passages have a much better much more modern tea cup shape..                               
The Ex port has a much improved floor, gone is the deep valley in the Ex port floor.                          
The EX port width was increased to 33.75, the outlet stayed the same at 37mm but they raised the 
outlet RD400 style to take out some of the steep Ex port angle.                                                         
The transfer width and height changed very little and they retained the front to rear main transfer     
port opening taper (to help control short circuiting).                                                                            
They steepend up the boost port angle a bit and they lengthend the intake port about 6mm to          
unshroud the boost port and increased the reedbox angle.                                                                
The rear wall of the main transfer passage (as the air travels up the tunnel) became straight           
instead of curved to increase airflow a bit.                                                                                         
They dropped the intake floor (4mm) like they were doing with the other RD’s at the time                 
As well as thicker sleeves, and the use of RD400 style head gaskets.                                              
 

The UK received the 73-75 RD250 A/B just like we did here in the US .                                           
   But they also received the RD250 C/D, designated the 1A2 which was updated with wider cooling 
      fins and RD400 style head gaskets but the porting was the same as the A/B,                                     
the exhaust port still had a small valley in the ex port floor and thin sleeves.                                 
       The 2R8 got improved porting, thicker sleeves, and Yamaha fixed the valley in the ex port floor.          
The European E/F was essentially an aircooled RD250 with LC porting and ignition, and the        
"New style" kickstart mechanism.                                                                                                    
 The clamed power for the E/F went up from 30HP to 32HP                                                              

The next generation was the RD250LC (Liquid Cooled Non-PowerValve)
The RD250LC made a claimed 34-36HP
The RD250LC porting was essentially the same as the aircooled 2R8 cylinders with the exception
 of the a slightly wider and much more oval shaped ex port, slightly higher transfers by .5mm and 
the ex port outlet diam dropping from 37 to 35mm. The ex port floor was also raised 2mm from 57
to 55 to finally come within a mm of the piston crown at BDC.

And finally the last of the RD/LC/RZ 250 series the RZ250
(Liquid Cooled with Exhaust Power Valves)
  The RZ250 made a claimed  43-48hp 

(Special thanks to Jon Wallis and Julian Farnam for letting me borrow one of his 51L cylinders!!)

Yamaha had learned a lot of lessons and put them into practice in the RZ250. The intake   
was now bridged allowing for a larger intake without being at the expense of piston life      
and the powervalve meant you could have both good low end and better top end. The         
exhaust outlet again dropped in diameter down to 32mm, the main transfers finally lost       
the front to rear taper at the port opening, the auxiliary transfer ports for the first time now   
had “hooks” in them. and the RZ250 unlike it’s bigger brother the 350 also had a single     
large Boyesen port to the inner auxiliary transfer ports. Hp went up by as much as 12hp.