Datsun Roadster Forum-311s.org

Although once warmed up after some driving, I’m finding my gear shifts are somewhat stiff. I’m running a heavier shift knob to help. Thinking of trying out a Bottle of friction modifier. Anyone have thoughts on how much I should add to the tranny? Trans hold about 2 qts + so should I purge about half the fluid before adding? Any experience along these lines would be appreciated. Is there something else out there thats better?

That additive is for the friction disks to friction plates that slip normally on auto trans, your problem, likely the syncros not working as they should is from the tapers on the speed gears being polished /glazed and the brass blocker rings won’t grab the gear to bring it to the matched speed so this makes it even slippyer which will make it worse it's slippery than reg hydro oil ( it treats the metal )
I see it is manual Trans fluid , newer blocker rings have friction material in the blocker ring that grab the speed gear taper and the modifier is recommend for those Trans .
Yes what you need is less slipping ,and more modifier , more it slips
Techs rebuilding would sand the tapers with 400 paper ,a new speed gear taper is like a new brake rotor

So,I will not see any benefit in shifting from this product? Anyone else care to elaborate? I’ve heard of MTL
-90 that might help.

MT-90 in the 5-speed since the 1980s and no regrets.

The old GM syncro mesh was VG at fixing slowing old Trans shifts ,it was thin milky , didn't feel slippery resembled a yohoo drink the new stuff is different. Mitsu in Chrysler Trans used non detergent 30w oil

rwmann wrote: Sat Oct 19, 2024 9:09 am MT-90 in the 5-speed since the 1980s and no regrets.

Just curious, do you have steel or brass synchros? Did it improve the shifting?

Thanks!

Semi related... my trans shifted much better with the Swepco oil than any I've had since. It was accidentally dumped when I had some work done by some Roadster buddies or I would have reused it. Since then I've tried Valvoline, Lucas, and Royal Purple, and the downshift from 3rd to 2nd will scratch unless I give it time. The shifts also seemed smoother before with the the Swepco (work done did not involve gears). I tried adding the recommended amount of Motorkote to the current Royal Purple and it shifted better for a short time but it continues. I plan to get some swepco for the Roadster for Christmas. Might be next spring before I can report what happens though...

mikeb wrote: Sat Oct 19, 2024 9:31 pm

rwmann wrote: Sat Oct 19, 2024 9:09 am MT-90 in the 5-speed since the 1980s and no regrets.

Just curious, do you have steel or brass synchros? Did it improve the shifting?

Thanks!

FS5C71A with steel synchros. No muss, no fuss. It never gave me any problems in daily commuting use, or more recently in occasional driver mode, which I suppose is another way of saying ‘improved shifting’.

Are you running straight MT90 or?

100% MT90

A test a trans tech does if someone wants his manual Trans rebuilt is this .
Besides just driving it ofcourse
With it going down the road 35-40 mph ,shift out of 4th no clutch then put pressure on it to 3rd ,if new in great shape it will bring the 3rd speed gear up to the output shaft speed often it goes in ,same is done on 2nd & 1st ...with no power applied it won't damage the sycro sleeve or speed gear coggs it sounds like something whirling spinning up in speed ,if it does not then the syncros arent catching the gear ,a good tech might do it for you for $$

Whew! I would not let that tech get near my trans! Not that it isn't a fine way to do it, but I feel mine is too fragile, he is going to get BOOM coming from mine, and I will be walking home! I have been double-clutching into 2nd from both directions (Up & Down) since Day 1.
Hah! Anyway, I tried RedLine MT-90 way back when, and its viscosity at start up or when warm was too low for my seals, it immediately applied a nice, red layer of undercoating to the car in a triangular shape going backward!. Old school rust-proofing!

Gosh darn now that you mentioned it I’ve heard that from someone years ago too. I don’t need a leaky trans. Maybe I’ll just let it go cause once warmed up it’s fine. Thanks Fergus.

I'm curious if anyone has tried VP Racing Classic API GL4 SAE 80W90 gear oil ?
The product description indicates it can be used for manual transmissions containing yellow metal (bronze,brass,copper..) components.
Comments?

Cheers
Ray B.

I suppose it depends on condition of the seals, or luck, but I’ve never had Redline MT-90 leak from the FS5C71A, since first used in the 1980s. Nor any Redline engine oil or axle lube leaks.