RD250 vs RD350

[soonerbillz]

Is the RD250 motor identical to the RD350 except or bore size?
Can a RD 250 cyber bored out to accept 350 pistons.
I've never messed with them but have that 250 I want to build the motor on.

[m in sc]

no, you cant overbore it to a 350. however, the motor is basically identical except for pistons, heads, cyls, carb brass (including emulsion tubes) AND mor eimportantly, the oil pump, primary gear on crank and clutch. the ratio on the 250/350 primary gearing is different.

that being said, you CAN just bolt on a set of 350 cyls/heads/pistons and rejet and bam, instant 350 . to offset the primary ratio, you can go up 1 tooth on the front sprocket if need be. YES,t he 250 pump will oil just fine ont he 350 topend if you keep the primary ratios as 250. (been there/done it)

also, keep in mind,t he 250s have MUCH larger transfer port ducts due to the smaller bore size. they really arent -that- much slower than a 350, at all. IF the 250 topend is savable, run it, you might be pleasantly surprised. .02

[soonerbillz]

Ok thanks Mark.
Currently the top end is stuck.  But have evaporust sitting in the bores and stud holes.
Praying it's not stack as bad as that other R5 I fought with.

[m in sc]

https://yambits.co.uk/pistons_and_parts_for_yamaha_rd250_a_and_b.html

im not aware of a good cheap source of 250 pistons in the states, but i've ordered a bunch of stuff from yambits and its really always been quite good.

just fyi, cheap but good pistons. id plan on a rebore and just not bothering to save them. ;)

as far a who can rebore them, I cant recommend anyone out there off the top of my head, I use a local guy that is phenomenal.