OK to mix Upper & Lower Crankcase halves on '73-'75 RD350?

[RDnuTZ]

as subject line says I'm wondering about possibility of mixing Upper & Lower Crankcase halves on '73-'75 RD350?

I have a titled 1974 RD350 project/builder I want to get roadworthy this Spring/Summer. It has matching VIN on motor and frame as shown on Title. I also have 3 complete spare lower ends: 1 each '73, '74, '75. Would also like to build up the spare '74 motor, but the top case half has a giant weld repair around the rear motor mount, so I'd like to replace it.

Any reason I can't simply swap the top case from the 1973 or 1975 spare motor onto the 1974 spare motor lower case? Yamaha North America website shows same Crankcase Assy part number for all 3 years: 278-15100-02-00

[m in sc]

ive done it. its fine. they aren't line bored together. set crank in, bolt the 8 m8 nuts on and it will spin free. thats all you need to know.

one side note: very early 73 cases may -not- be tapped out for the 3 screw clutch bearing retainer.  I ran into this last year. I just ran a tap thru the bosses and added it.   

[RDnuTZ]

thanks for the case swap info Mark  My 1973 motor VIN is 351-103-768. Is that a very early one? I don't have production run numbers handy to reference.

[m in sc]

its the 3768th 73 made.. maybe? if it is, its obvious to see but an easy fix. hell, its alredy drilled just not tapped if it IS early. dont sweat it.

these are the ones i tapped for what is now the LC motor, they were blank w no threads, ist time id ever seen that. had the r5 style stepped washer on the main shaft behind the clutch, was weird. R5 cases do NOT have the bosses cast in.

 

[SoCal250]

thanks for the case swap info Mark  My 1973 motor VIN is 351-103-768. Is that a very early one? I don't have production run numbers handy to reference.

Frame & engine numbers started at 351-100101 so yours is #3668 of a run that reportedly went over 24K units in '73. Mine is about 11K units after yours.

[RDnuTZ]

Thanks for the info. I only have the engine from this bike. Same with the 74 Welded case motor and 75 motor. I do have a complete 1973 RD250, Complete 1974 RD350, complete 1977 RD400, and mostly complete extra 1974 RD350 builder. None run currently, but I'm finally able to get building on them after 20 years of storage! I was on a bike buying binge all over SoCal mid 1990s to early 2000s when I got sucked into VMX  Now I'm flipping back the other way as age, health, and lack of local VMX opportunities caught up with me...