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Chevrolet C70 transmission syncronizer

Printed From: Unofficial Allis
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Forum Name: Shops, Barns, Varmints, and Trucks
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=207598
Printed Date: 31 Jul 2025 at 11:32am
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Topic: Chevrolet C70 transmission syncronizer

Posted By: thendrix
Subject: Chevrolet C70 transmission syncronizer
Date Posted: 28 Jul 2025 at 7:53pm

Looking pretty serious at a mid 80s Chevy C70 that belongs to a friend. Believe it's a 427 and Eaton 5+2. According to the owner it pops out of 2nd on occasion. He's fairly certain it's a syncronizer. It's currently a dump truck. At some point it might become another spreader truck but for now will stay a dump. How big of a job is a syncronizer in these transmissions? Would it be a better idea to just leave it considering it's a farm truck and not a daily work truck? He's asking $5500 from me and it's a solid truck that runs pretty good. Air brakes and (I believe) a 14 ft bed. Not sure what brand. He's been using it as a site truck for a couple of years. Says he has $5500 in it and that's what he wants out of it.

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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan

Replies:

Posted By: DMiller
Date Posted: 29 Jul 2025 at 2:40am

Synchro will not be the problem, couple pointers, Forks in these wear as the right hand tends to sit on the shifter and applies pressure that allows the sliding hub to not fully engage similar to our non synchro Allis boxes, next the 'Teeth' on the actual gear are worn to a bevel and allowing to slip out from not really ever being engaged due to wear. Typical of these.

Posted By: thendrix
Date Posted: 29 Jul 2025 at 8:07am

Thanks Dave. I figured you'd probably have some experience with these. Is it enough to worry with or just let it be? I start off in 2L in my spreader truck which is the same drivetrain. Other than that I don't use 2nd. Correct me if I'm wrong but as long as I hold the shifter in 2nd it shouldn't come out? If it was 4th or 5th that would be a different story

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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan

Posted By: Ed (Ont)
Date Posted: 29 Jul 2025 at 6:35pm

Seems like a fair price to me if the truck is reasonably good. I would get it then send trans to rebuilder. Or buy reman unit although those are getting kind of rare.

Posted By: PaulB
Date Posted: 29 Jul 2025 at 7:44pm

If you had asked me that question 20 years ago, I'd say pull the transmission and either take it to a rebuilder or tear it apart yourself and put bearings and all the syncronizers in.

Today I have a different view, Pull the manual and toss it on the scrap pile an find an Allison from a school bus where they brought the bus to put the Cummins into a Ford Pickup and will let you have the Allison to get it out of their way

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If it was fun to pull in LOW gear, I could have a John Deere.
Real pullers don't have speed limits.
If you can't make it GO... make it SHINY

Posted By: thendrix
Date Posted: 29 Jul 2025 at 8:07pm

Ed, from 20ft it's a damn nice truck especially for that price. Haven't really looked at it closer than that. And I'm pretty good friends with the seller and I trust him. I'll probably come home with it either way. If nothing else I'll use it for a minute and flip it.

Paul, I hadn't considered that. May have to look into it. Reckon what would be involved in adapting an Allison to a mid to late 80s 427?

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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan

Posted By: Coke-in-MN
Date Posted: 30 Jul 2025 at 10:01am

My GMC tandem dump has a RT 613 trans in it , with the 427 , might find one of them cheaper than rebuilding your 5 speed . Somewhere out back I have a 5 speed and 4 speed I took out of another GMC dump truck - how good unsure as I bought it after it got hit by a train - wanted the engine and box hoist for other truck .
On my White with 5 x 4 It jumped out of gear , i replaced the slider collar and the gear itself to solve the problem - no sincro - just sliding coupling between gears .

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Faith isn't a jump in the dark. It is a walk in the light. Faith is not guessing; it is knowing something.
"Challenges are what make life interesting; overcoming them is what makes life meaningful."

Posted By: DMiller
Date Posted: 30 Jul 2025 at 6:42pm

613s were improved to RTO6613s, mount same and just as easy to use.

Alisons were good trans as well but engines worked at peak rpm and SANG like canaries with their butts pinched. Had a AT540 in the C70 I built grain box on, Gutless. The later 1,2 & 3000 series proved reasonably reliable.