75 RD 250 to 350 conversion

[s_busch]

I have a Yamaha '75 RD 250 I got for my first bike last season. Over the July 4th weekend I burned a hole in one of the pistons.
Now that we am doing an engine rebuild, we are considering making it a 350 as it was been a bit challenging finding 250 replacement jugs.
What parts need to be changed it I convert it to a 350?

 

[jpmobius]

A 350 top end is a direct bolt on replacement. The primary drive gears are different, but not a problem so you can keep what you have. You can change the sprocket(s) if you want more top end speed. Technically, the oil pump spins slightly slower than the 350 because of the gearing difference, but should not be an issue. You can add some shims to the pump if this worries you. Likely the pump is already rich as the service after break in was often not done to reduce the oil output. I believe I have 250cc cylinders if you want to keep it a 250 so pm me and I will look.