The Stovebolt Site Forums The Shop Area Driveline 1967 GMC transmission grinding

Hey everyone. I have a 67 gmc c10 with a 327 engine and 4 speed manual transmission I recently started fixing up. I can’t seem to figure out why the gears keep grinding every time I downshift. It shifts up perfectly. Downshifting from 3 to 4 works ok and gets progressively worse as you go down. I end up having to stop before shifting up again. Does this mean the clutch or clutch brake is going out? Would the whole engine and trans need to come out for a fix? Any help is appreciated.

Thanks

Is the clutch properly adjusted? If it is, then, time to replace clutch/pressure plate.🛠

How do you DOWN shift from 3 to 4? What kind of lubricant are you using? If someone has convinced you to use synthetic lube, the synchronizers probably can't get a grip. Be sure the clutch is adjusted correctly and there's no hesitation or grind when you try to engage a gear from a full stop.

Assuming you're running a SM-420 truck transmission (not a Muncie or a Saginaw car trans) I hope you're aware there's no way to downshift to 1st. while moving- - - -that one's not synchronized.
Jerry

Sorry I meant downshifting from 4 to 3. Going from 3 to 2 is way harder than it should be. I havent changed the trans fluid which could be a limiting factor. What kind should I put in it? It doesn’t seem to hesitate or grind when it starts moving. I will check the clutch for adjustment and see where that leads to.

I think the transmission is a 420 from the looks of it.

Thank you

What are you accustomed to driving?
Transmission might be perfect but your expectations or technique may need adjusting.
My GMC for example shifts nicely, but not nearly as quickly as my VW.

If you match speeds by giving it just the right amount of throttle while passing neutral on the down shift it’ll shift easier, and make shifting possible if the synchronizers are worn. If they’re plumb worn out you can still shift by letting the clutch up in neutral, match speeds, clutch again and complete the gear change, it’s called “double clutching”.

Regular 90 weight mineral oil gear lube (not hypoid lube) is the best choice. Some of the highly touted synthetic gear oils are so slippery the brass blocker rings in the synchronizers can't grab the cone clutch area on the gears, resulting in grinding gears on a downshift. For the synchronizers to work properly, there can't be any power transmitted to the cluster gear by the clutch- - - -it needs to be fully disengaged. There's a lot of inertia in those big heavy gears, so don't expect a 420 to shift like a sports car- - - - -it ain't gonna happen.

I learned to drive big rigs on a B-73 Mack with a 5 X 3 setup with two shift levers- - - - -both crash boxes with no synchronizers. The only time we used the clutch on that rig was for starting and stopping, and grinding a gear was a rare occurrence.
Jerry