Technical > Transmission Forum
Grinding into 3rd Gear...
Bruce:
--- Quote from: kinggeorge13 on March 07, 2017, 11:29:40 AM --- So I've been reading up lots on this.
--- End quote ---
The problem with taking the advice of the armchair experts is that 99.99% of them have never been inside a gearbox to see the effects of what they condemn or what they crow about.
The Bus guys swear you must use GL-4 oil because that's what their 40 year old manual says. Of course it says that, GL-5 didn't exist then. Then they say if you use GL-5, it will instantly eat your synchros.
On the other hand, what makes an oil meet GL-5's standards is the extreme pressure rating. In other words, how well it prevents metal to metal contact under high loads. The pressure rating of GL-5 is higher than that of GL-4
When I go inside a gearbox, if I see all the synchros gone, blame it on GL-5. If the gears are all worn out, blame it on GL-4's inferior extreme pressure rating. What do you think I see in almost every gearbox, and what has never been seen by anyone?
I see plenty of metal to metal wear, while I've never seen the so-called corroded synchros.
Here's some specific info for your 77 Bus. If you were parting out a Bus like yours and you wanted to sell your core gearbox, it is currently worth $800C as a core in California due to the high demand by the off road guys. But before you get that loot, the trans shop will pull off your bell housing to check the R&P. That's because the #1 failure in an 091 trans is worn out R&P teeth. They're worn out because so many guys out there are using gear oil that's not good enough at preventing metal to metal contact.
Still not convinced? There's a compromise out there. Many GL-4 rated oils do meet or exceed the pressure rating for a GL-5 oil. These oils say right on the label, Meets GL-4 and GL-5. Get that one and save your R&P.
All this talk about oil is moot. Your 3rd gear synchro is broken.
kinggeorge13:
I was happy until I read your last sentence. That's my fear as well as I suspect you are correct but it's (almost) free for me to change the transmission fluid and play with the clutch adjustments and if that takes me to where we expect me to end up with a still-grinding into 3rd gear, I'll have to find someone who can fix it.......wildly assuming that is fixable with parts available.....
To answer a couple other questions from other posters, I don't have any knowledge of anyone changing the transmission fluid previously. The guy I bought it from only had it a year or two and there were a stack of old receipts in there (replaced engine with a rebuilt engine, then replacing both carbs, etc), there was nothing I saw quickly about changing the transmission fluid. The owner back around 2000 did do a ton of service with it and lots of oil changes, tuneups and a dizzying amount of parts replaced but it would not surprise me that nothing done with the transmission fluid since then.... The mileage is completely unknown except that the engine replacement happened around 1997.
Thanks everyone!
-George
owdlvr:
Oh there is no doubt your third gear syncro is bad. Even the best fluid out there won't fix it, but it might make it liveable.
jim martin:
Switched from LAT gear oil to this last 3 and 4 gear change.
I was never happy with the way the box shifted hot or cold.
Smooth shifting under full load , excellent product
http://www.torcousa.com/technology/RTF.pdf
kinggeorge13:
--- Quote from: owdlvr on March 07, 2017, 01:34:41 PM ---For fluid, I'd recommend Motul Gear300. Though I can't say anything about its ability to assist your 3rd gear syncro, I can confidently say it helped with the same brass syncros in my old Audi rally car transmission. Motul Dealer Finder: https://www.motul.com/ca/en-us/resellers?q=vancouver&activities%5B1%5D=1
Otherwise, for a GL-4 specific oil use Redline MT-90: http://driversedgeautosport.com/redline_MT90.html, available at Lordco or Driver's Edge Autosport.
Don't use Amsoil...though I'm sure someone will post up about how you should only use it. But transmission fluids are like politics, everyone thinks they're right...
--- End quote ---
from George: I was looking at the Redline MT-90 75W90 GL-4 over at Mopac in Langley yesterday as I wanted to do my tranny oil change today but I see that it says on the site "Not for use in differentials with hypoid gears" Doesn't that mean I cannot use it in my 1970s (1974-1979 are what I have) bus transmissions?
Thanks,
-George
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