6 speed RD400 close ratio transmission in a 6 speed RD350

[SUPERTUNE]

RD350 6 speed transmissions have terrible 4th,5th ratio and higher ratio overdrive 6th gear. (l'll come and edit later for the ratio's)
When modifying 350 engine with intake upgrades, porting and chambers... the 350's have trouble staying in the powerband and fall off between upper gear shifts and 6th is a oversized cruising gear.

Pic of the RD400 shafts.
When looking at RD400 transmissions the 400 gears have a slash cut line right through the center top of the teeth, look close...




There is 2 ways to do this.

One way:
Is to just change out the gears from a 400 shaft to the 350 shafts, but a press with some fixture tubing to press Off/On 5th gear from the output shaft. Any general 2 ton press will work.

If you have a RD350 late model with the 7 plate clutch, just need a 400 donor tranny and you use all the re-moveable gears off the input shaft, 5 gears.
As stated you will need to press off 5th gear (it's on the shaft next to the sprocket flange) off the 400 output shaft once all the other 5 gears are removed by taking off circlips and laying out the circlips, shims and gears in order.

Press off the 5th gear (24T) on the 350 output shaft




Then press 400 5th gear (25T) on the 350 shaft.






Use all of the 400 gears from the output shaft on the 350 output shaft or technically only the other 2 gears 4th and 6th as 1st 2nd and 3rd are the same on the 400 output shaft as the 350 gears.

Input shaft for 1st has a built in gear for both the 350 and 400, but the splines are different for the clutch hub.
6 wide spline for a 400, 12 fine spline for a 350.
The reason you have to replace 5 gears on the input shaft, is the 2 slider gears (2nd and 3rd slider gears), do have the same ratio as the 350 gears, but gear engagement dogs are machined smaller O.D. to fit the smaller closer ratio 400 gears and RD350 slider gears (2nd and 3rd) don't fit in.

2nd way:

You can drop a complete 400 tranny in also, but have to use a 400 drive sprocket with 6 spline, a 400 6 spline inner hub and pressure plate with all of the 400 springs and clutch spring bolts, first thrust washer against the input double row bearing (thicker than a 350) as they're all different. You also have to grind out the casting ribs on the inside of the 350 clutch cover to clear the 400 clutch pressure plate hub when clutch is pulled in, it will hit the ribbing inside the cover.

Both RD350 and RD400 7 plate models can and do use the same steel plates and fibers other than with all hot rod RD's I build I use the YZ250 fiber plates, and throw the rubber cushion dampners in the trash (same as the FZR100 fiber conversion) to gain more fiber plate surface area to have a better holding power than a stock RD fiber plate.

I also will up the front sprocket from a 15T to a 16T if riding a lot at highway speeds. Good time to do a #520 chain size conversion, for way less rotating weight than the stock #530 size and pick up even more acceleration.

I will come back, edit and add pic's soon.

Chuck

[kar1zma]

Very informative , thanks chuck

waiting for the pics

[motosapien]

Those are the ribs that needs to be filed down

[SUPERTUNE]

Sorry S, I was out of town and didn't look here...
You need to now need to grind more further out, the cover turns in at top, bottom and to the rear.
I grind almost flush at the outer portion of the cover.
Grind the stop nub some more too.
I have some pic's I have to upload to my host from my phone.
Chuck

[motosapien]

Sorry S, I was out of town and didn't look here...
You need to now need to grind more further out, the cover turns in at top, bottom and to the rear.
I grind almost flush at the outer portion of the cover.
Grind the stop nub some more too.
I have some pic's I have to upload to my host from my phone.
Chuck

I ground the small rib on the 6 clutch bolts/screws to make it flush with the above cover mod and mine doesn't rub against the cover.

[m in sc]

CHUCK, QUESTION.  can you just double gasket the clutch cover?

[Czakky]

I've got my clutch together (finger tight) and cover on with no gasket. I have removed the material from the grid stabbed the kick starter a few times and nothing touched.

I was thinking though, it should be tested with the clutch disengaged (lever in), Right?

[motosapien]

I've got my clutch together (finger tight) and cover on with no gasket. I have removed the material from the grid stabbed the kick starter a few times and nothing touched.

I was thinking though, it should be tested with the clutch disengaged (lever in), Right?

Yes

[SUPERTUNE]

They always hit at full disengagement.
C

[rodneya]

If you do the complete 400 trans and clutch swap into a 350, do you use the 400 or the 350 primary gear on the end of the crank?

[motosapien]

400

Sent from my SM-G981U1 using Tapatalk

[Czakky]

  you sure? I didn't I don't think....

[Yamaha 179]

The number of teeth on the primary gear are the same for both engines, 23T, and either gear will work, however the RD 400s all have the groove machined into the back for the O ring to prevent oil leaks where the early RD 350s did not have that feature.  Some of the 350 primary's leaked oil pretty badly and caused a fair amount of smoke from the right side pipe if you did not use Yama-bond to seal the gear in place.  So, use the 400 if available.
Lyn Garland

[rodneya]

The number of teeth on the primary gear are the same for both engines, 23T, and either gear will work, however the RD 400s all have the groove machined into the back for the O ring to prevent oil leaks where the early RD 350s did not have that feature.  Some of the 350 primary's leaked oil pretty badly and caused a fair amount of smoke from the right side pipe if you did not use Yama-bond to seal the gear in place.  So, use the 400 if available.
Lyn Garland

Thanks Lyn
For some reason I thought the 400 had a different ratio, but that may have been for a 250.
Ill use the 400 gear, and that way the gear lash will still be correct.

[Greaser Greg]

The number of teeth on the primary gear are the same for both engines, 23T, and either gear will work, however the RD 400s all have the groove machined into the back for the O ring to prevent oil leaks where the early RD 350s did not have that feature.  Some of the 350 primary's leaked oil pretty badly and caused a fair amount of smoke from the right side pipe if you did not use Yama-bond to seal the gear in place.  So, use the 400 if available.
Lyn Garland

Drifting the thread a little here, but will the Yamabond help even on a stock R5 that has the shaft o-ring with the keyway nub? More specifically, is it recommended to use sealant and the o-ring, and where exactly? Behind the gear as it goes home to the seated position?