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6 speed RD400 close ratio transmission in a 6 speed RD350

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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:

--- Quote from: SUPERTUNE on November 27, 2019, 11:31:06 AM ---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

--- End quote ---
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.

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