Pre 75 Yamaha Trans Conversions

[Tahitian_Red]

Trying to figure out all the Close Ratio transmissions that will work in my 71 to 72 RT-1/2 lower ends?  I missed out on a DT-2MX transmission recently, so I'm looking for any that might work.  Thanks!

[vandy010]

a lot of the earlier cogs will swap over.
not always the case with the shafts.
but cogs/gears generally will swap quite happilly from one shaft to another always being mindfull that both gears (say 2nd as an example,), one from each shaft, go together and you keep checking the shimming. although i've not had a problem with shimming.
there is a difference in shifting forks if you choose to use the slightly later YZ250C gears.
i've had DT/RT1,MX250A,YZ250C gearsets side by side on the bench to verify.
ratio,s + beer + calculator = headphuck
have fun with it all...

[Tahitian_Red]

Vandy,

Will a complete trans (not just gear swapping) from an MX250/360 swap into my RT-1 cases?  I thought Firko said once that the RT2MX gearbox would swap into DT-1/RT-1 cases.

 

[vandy010]

Been a while and cant remember exactly but more than lkely yes.

[firko]

Jay, the RT2MX trans slips straight in because they share the same cases with the RT1. The later case MX250/360 trans can be swapped in but work's needed as Vandy said. I went through the ' ratio,s + beer + calculator = headphuck' process on my bench when I was building the DT1 From Hell but that was back in the early 90's and I've long forgotten the details. I abandoned the research anyway when I found a rusty old YZ250 bottom end in a bike wreckers. I'm sure there's someone on this forum that knows the shaft/gear relationship between early and late gearboxes.....perhaps Davey Crockett or Iain

[Tahitian_Red]

Firko,

So....RT-1/2MX and YZ250/360A transmissions will bolt right in, but the MX250/360 will not?  Does it matter if you get the trans from a 250 or does it have to come from a 360?

[crossedup2]

Tahitian

Just done the swap, MX360 gear set into RT2 bottom end. The MX360 has a longer clutch shaft. You need the MX250 gear set. So mine is actually running the original 1 gear and close ratios for the rest. (I have ordered that MX250 clutch shaft for the next tear down.)

The only change you need to do is use the original 4th gear cog that slides using the shift fork. The original MX gear set has a larger area fork slot. This was done before beers!!!!! Nearly started cutting grinding bit realised that they are exactly the same part (size and gear teeth). This way you use the original RT1 fork as the MX for in very much different. MX360 is the closer set with the longer shaft.



Good luck with it and PM me if you need more information.

Peter B

[firko]

    So....RT-1/2MX and YZ250/360A transmissions will bolt right in, but the MX250/360 will not?  Does it matter if you get the trans from a 250 or does it have to come from a 360?

Jay, the DT2MX/RT2MX and YZ250/360 A and B transmissions bolt straight into your RT1. The DT250MX and RT360MX have the same ratios and are identical so either will do the trick and I'mreasonably sure that the YZ A/B 250 and 360 also share the same ratios. Crossedup2 has answered the second part of your question and from memory that's exactly what Vandy and I worked out but couldn't remember . The 360 has a longer clutch shaft to allow for the balancer (I think!). I seem to recall the ratio situation being discussed here in the past and apparently the MX250 ratio spread is much better than that of the DT2MX, making the swap worthwhile. I've got one more spare RT1 engine that needs an overhaul so if I can locate an MX250 trans I may have a go at the conversion myself.
 

[Freakshow]

are you swaping in both gear shafts ?

[Tahitian_Red]

So...If I swap in a complete YZ250B gear set and shafts I should good to go? 

[Freakshow]

tha same case so yes.

[firko]

So...If I swap in a complete YZ250B gear set and shafts I should good to go? 

The whole shebang goes straight in. That's exactly what I fitted to my DT1 motor.....along with the clutch/primary drive and some other stuff

[Tahitian_Red]

How well will a '75 MX250B gear set work in the RT-1?  The only YZ250/360 A/B gear sets I've found are pricey. 

[vandy010]

How well will a '75 MX250B gear set work in the RT-1?  The only YZ250/360 A/B gear sets I've found are pricey. 

off the top of my head i can't say for sure but from a vague memory, the shafts are different but the cogs will swap over to the RT shaft but i believe the shift forks are different too, which means the cog that has the machined area for the shift fork, well that machined area is a different diameter.
if that makes sense.
so i'd guess (in my best weather man voice)
theres a possible slight chance

[Tahitian_Red]

Well I found a 73-74 MX250 gear set. 

So, reviewing the posts it looks like I only need to replace 4th gear (both shafts?) with the one from the RT-1 shaft.